Absorption characteristics of selected oxygenic photoautotrophs

To appreciate the spectral properties of irradiance on Proxima Centauri b it is necessary to calculate how extant terrestrial oxygenic photoautotrophs would perform under the spectral signature on Proxima Centauri b. Figure 1 shows a comparison of the in vivo normalized absorption spectrum of the cyanobacteria Synechococcus R-2 PCC7942, P. hollandica and the unusual cyanobacterium A. marina; and also amongst the eukaryotic algae, C. vulgaris, Rhodomonas sp. and the diatom P. tricornutum. All the spectra of these oxygenic photosynthetic organisms have been normalized onto the blue Chl (Soret Band) a or Chl d peak fixed at an Absorbance (A) of 1 [A = 2−log₁₀ (% Trans)]. The in solvent red peak of Chl a is at about 665 nm but in vivo is at about 675–685 nm depending on the organism (Fig. 1). The respective blue and red peaks for the photosynthetic organisms used in the present study were: blue peaks, Rhodomonas, A ₄₃₇; Phaeodactylum, A ₄₄₀; Prochlorothrix, A ₄₃₅, Synechococcus, A ₄₃₇; Chlorella, A ₄₃₇; Acaryochloris, A ₄₅₇, for comparison the red or Qx peaks were Rhodomonas, A ₆₇₅; Phaeodactylum, A ₆₇₅; Prochlorothrix, A ₆₇₆, Synechococcus, A ₆₈₀; Chlorella, A ₆₈₁; Acaryochloris, A ₇₀₅. The spectrum for Chlorella is redrawn from Ritchie & Runcie (Reference Ritchie and Runcie2013).

Fig. 1.

Comparison of the normalized absorption spectra of selected oxygenic photoautotrophs. These include the cyanobacterium Acaryochloris which uses Chl d as its primary Chl and cyanobacteria Prochlorothrix, Synechococcus and the eukaryotic algae Chlorella (Chlorophyta), Rhodomonas (Cryptophyta) and the diatom Phaeodactylum which use Chl a. All the spectra of these oxygenic photosynthetic organisms have been normalized onto the blue Chl a or d peak (Soret band) fixed at an absorbance of 1 (10% transmission).

All six oxygenic organisms can use blue light (400–500 nm) and the in vivo blue peaks of Chl a and d are very similar. Prochlorothrix, Synechococcus, Acaryochloris and Chlorella cannot effectively utilize green light (500–575 nm) whereas Rhodomonas is able to use green light because of the presence of phycoerythrin (orange/red) light-harvesting pigment. The high levels of carotenoids, especially fucoxanthin, in Phaeodactylum also give it an advantage in green light (500–574 nm). Phycocyanin (blue) pigments of Synechococcus effectively absorb yellow and orange light (575–649 nm); these pigments are also present in both Prochlorothrix and Acaryochloris but in lesser amounts and so are less significant. All the photo-oxygenic organisms, using Chl a as their primary photosynthetic pigment, strongly absorb red light (660–699 nm). The Chl d of Acaryochloris strongly absorbs in the far-red (700–749 nm) and since it also contains some Chl a Acaryochloris also absorbs in the 660–699 nm range (Miyashita et al. Reference Miyashita, Ikemoto, Kurano, Miyachi and Chihara2003). Cyanobacteria that contain Chl f would perform similarly, except that they are able, in addition, to up regulate unique phycobiliproteins that absorb in the far-red/NIR (NIR>750 nm) (Gan et al. Reference Gan, Zhang, Rockwell, Martin, Lagarias and Bryant2014).

Absorption spectra of selected anoxygenic photosynthetic bacteria

Figure 2 shows the spectral absorption properties of a selection of five photosynthetic bacteria with differing pigmentation: three purple non-sulphur bacteria: A. marina, R. palustris and B. viridis; one purple sulphur bacterium: T. tepidum and one green sulphur bacterium: C. (Chlorobium) tepidum. They have been scaled against the in vivo Blue (Soret) band maxima set to an absorbance (A _λ) of 1 [10% transmission, A _λ = 2-Log₁₀(T _λ%)]. The wavelengths of the blue absorption maxima are: Afifella, 370 nm, Rhodopseudomonas, 376 nm, Blastochloris, 400 nm, Thermochromatium, 377 nm and Chlorobaculum, 459 nm. Afifella has a single IR in vivo maximum at 834 nm whereas Rhodopseudomonas has double IR maxima at 807 and 868 nm and Thermochromatium also has double IR maxima at 805 and 872 nm. Blastochloris, which uses BChl b as its primary photosynthetic pigment, has a single IR peak (at 1017 nm) that is more red-shifted than any other known photosynthetic organism (Segura et al. Reference Segura, Kasting, Meadows, Cohen, Scalo, Crisp, Butler and Tinetti2005). Chlorobaculum has a prominent peak at 748 nm attributable to BChl c in vivo, which is the most abundant, but accessory, BChl in the cell and a small ‘knee’ at about 810 nm due to BChl a which is the primary BChl even though it is much less abundant. Photosynthetic bacteria, using accessory photosynthetic pigments, are all able to use a substantial proportion of irradiance in the 400–700 nm range and both BChl a and b type photosynthetic bacteria are able to absorb orange/red light at about 600 nm (Fig. 2) due to a secondary Soret absorption band of both BChl a and b (Hellingwerf et al. Reference Hellingwerf, de Vrij and Konings1982). In vivo spectral shape and peaks of BChls are typically very different to in solvent and so in solvent spectra should not be used to deduce photosynthetic behaviour of photosynthetic bacteria (Segura et al. Reference Segura, Kasting, Meadows, Cohen, Scalo, Crisp, Butler and Tinetti2005; cf. Komatsu et al. Reference Komatsu, Umemura, Shoji, Kayanuma, Yabana and Shiraishi2015).

Fig. 2.

Comparison of the normalized absorption spectra of the photosynthetic bacteria, Afifella, Rhodopseudomonas, Thermochromatium, Blastochloris and Chlorobaculum. These should be compared with those of the oxygenic photosynthetic organisms in Fig. 3. All the spectra of these anoxygenic photosynthetic organisms have been normalized onto the blue BChl (Soret band) fixed at an absorbance of 1. The spectra are based on scans of laboratory grown cells suspended in 60% sucrose.

TES of the sun and Proxima Centauri b

The irradiance of the Sun at the TOA was calculated using SMARTS (2011) (Fig. 3) and converted into quantum units using Planck's Law. The spectrum for Proxima Centauri has been calculated at the orbit of Proxima Centauri b and is based on the TES of the star. The spectra are very different reflecting the large difference in effective temperature (Sol, Type G2 V – 5780 K) and Proxima Centauri (Type M5.5 V – 3050 K). The stellar atmosphere of Proxima Centauri absorbs light strongly and so the TOA spectrum has much stronger features than the Sun and there are absorption bands of critical importance to photosynthesis. The region where the in vivo red peak of Chl a is located (660–700 nm, see Fig. 1) is strongly depleted and the region near the 761 nm oxygen absorption peak is also strongly depleted. Note that in IR wavelengths (>900 nm) Proxima Centauri bat TOA would experience irradiance higher than Earth.

Fig. 3.

The total emission spectra of the Sun (in μmol photon m⁻² s⁻¹ nm⁻¹) at the top of the atmosphere (TOA) of Earth based on the SMARTS (2011) software for the Equator at noon equinox compared with the recently measured TES of Proxima Centauri b at TOA. Compared to the sun, Proxima Centauri b produces very little visible light (400–749 nm useable by oxygenic photosynthetic organisms but produces comparable irradiance in the far-red IR range. The cool temperature of Proxima Centauri b results in strong absorption bands in its stellar atmosphere. Strong depletion in the range 650 to 699 nm would disadvantage Chl a-based oxygenic photosynthesis.

Planetary atmospheric absorption and the light spectra at the surfaces of Earth and Proxima Centauri b

The solar spectrum reaching the ground was calculated for the Equator at noon equinox (SMARTS 2011) and compared with the TOA at each wavelength in order to calculate an atmospheric absorption ratio for each wavelength (see the Supplementary Atmospheric Absorption File) (Fig. 3, Table 1). When Proxima Centauri is in its usual quiescent state (Anglada-Escudé et al. Reference Anglada-Escudé, Tuomi, Gerlach, Barnes, Heller and Jenkins2013, Reference Anglada-Escudé2016), UV-A and UV-B would be virtually absent from the TOA emission curves for Proxima Centauri b (see Fig. 3) and so the surface of the planets would receive very little UV irradiance regardless of the nature of its atmosphere. Better measurements of frequency and extent of UV flaring are needed to more fully assess the UV-A & UV-B environment for Proxima Centauri b (Davenport et al. Reference Davenport, Kipping, Sasselov, Matthews and Cameron2016; Fig. 4 in Ribas et al. Reference Ribas, Gregg, Boyajian and Bolmont2017). Based on current evidence, the episodic flares (every 10 days or so) would increase UB-A & UV-B by about an order of magnitude above current TOA levels of the Earth. For comparison, the irradiance (300–1200 nm) reaching the ground on Earth with the Sun directly overhead is included on Fig. 4 for both an oxic atmosphere and an anoxic but Earth-like atmosphere. Using the absorption properties of an Earth-like oxic and anoxic atmosphere the calculated irradiance on the surface of Proxima Centauri b would have little blue (400–499 nm), green (500–574 nm), orange (575–650 nm), red or far-red light (650–699 & 700–749 nm): overall PPFD irradiance is about 3% that on Earth (Fig. 4, Table 1, PPFD ≈63 µmol photon m⁻² s⁻¹). There is more than twice as much useable irradiance for Chl d-type oxygenic organisms able to use 700–749 nm light (132 µmol photon m⁻² s⁻¹, Table 1). This would greatly favour Chl d containing organisms and those Chl a organisms able to use far-red light (Fig. 4). The large amounts of far red and NIR (700–799 nm) and IR (I & II) would benefit anoxygenic organisms (Figs. 2 and 4). On the surface of Proxima Centauri b the irradiance potentially usable by oxygenic organisms is very low but total photons potentially usable by anoxygenic photosynthesis approaches that available on Earth and well above the optimum photon fluxes for photosynthetic bacteria (Ritchie Reference Ritchie2013; Ritchie & Runcie Reference Ritchie and Runcie2013; Ritchie & Mekjinda Reference Ritchie and Mekjinda2015). On both planets H₂O strongly absorbs irradiance in the range 900–970 and 1100–1150 nm hence the atmospheric IR absorption valleys in Fig. 4 compared with Fig. 3.

Fig. 4.

The ratio of the TOA at each wavelength and the irradiance reaching the ground for the Earth with the current oxic atmosphere and for an Earth-like but anoxic atmosphere were used to calculate the irradiance reaching the surface of Proxima Centauri b with an Earth-like oxygenic or anoxygenic atmosphere. An Earth-like but anoxic atmosphere would have very little effect on the irradiance reaching the Earth's surface in the range of wavelengths useable by oxygenic or anoxygenic organisms. The conspicuous differences are the increase in UV light reaching the Earth's surface and the conspicuous 761 nm O₂ absorption band present with an oxygenic atmosphere. In the case of Proxima Centauri b the only conspicuous difference made by an oxic versus anoxic Earth-like atmosphere is the presence of the 761 nm absorption band in the case of an oxic atmosphere.

Table 1.

Surface irradiance of Earth and Proxima Centauri b. Photon irradiance wavebands of importance for photosynthesis on the Earth and Proxima Centauri b for a planet with Earth-like Oxic Atmosphere and for an Earth-like but Anoxic Atmosphere

Absorption of light by mats of oxygenic photosynthetic organisms on the surface of Earth and predictions for Proxima Centauri b

The Appendix outlines a model for photosynthesis both for mats composed of oxygenic photoautotrophs on the Earth and on Proxima Centauri b. We concentrate on microbial mats in this study because they are the first known photosynthetic structures on Earth (≈3.5 Gyr; Tice & Lowe Reference Tice and Lowe2004; Schopf Reference Schopf2011; Djokic et al. Reference Djokic, Van Kranendonk, Campbell, Walter and Ward2017) and persist to this day and photosynthesis on a surface is straightforward to deal with compared with plankton in water columns, where side and backscattering and progressive absorption by water are major problems for estimating irradiance (Falkowski & Raven Reference Falkowski and Raven2007; Kirk Reference Kirk2011). Combining the data on the light regime on the surface of the Earth and Proxima Centauri b (Fig. 4, Table 1) with the light absorption characteristics of the oxygenic photosynthetic organisms included in the present study (Fig. 1) provides a comparative estimation of the capacity for growth of these various organisms. The 400–700 nm range is used by all oxygenic photosynthetic organisms with Chl a as their primary photosynthetic pigment and the 400–749 nm range is useable by Acaryochloris which uses the far-red absorbing Chl d as its primary photosynthetic pigment (Miyashita et al. Reference Miyashita, Ikemoto, Kurano, Miyachi and Chihara2003; Larkum Reference Larkum and Rengler2008; Chen & Scheer Reference Chen and Scheer2013; Schliep et al. Reference Schliep, Cavigliasso, Quinnell, Stranger and Larkum2013). To better interpret the importance of various pigments for oxygenic photosynthesis the visible spectrum was divided up into various bands based on what irradiances are most important to known oxygenic photosynthetic organisms: blue (400–499 nm), green (500–574 nm), yellow/orange (575–649 nm) and red (650–699 nm) (Tables 2–5). In addition, far-red (700–749 nm) is useful to Acaryochloris because of the absorption properties of Chl d and a few exotic chlorophytes such as the endolithic Ostreobium, which is able to use 700–749 nm light even though it has only Chl a + b (Wilhelm & Jakob Reference Wilhelm and Jakob2006).

Table 2.

Irradiance useable by photooxygenic organisms on the Earth with the modern Oxygenic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for solar radiation on Earth with an oxygenated atmosphere. The values are based on the observationally known solar spectrum. The percentage absorption is shown in brackets.

Table 3.

Irradiance useable by photooxygenic organisms on an Earth with an Anoxic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for solar radiation on Earth with an Anoxic atmosphere. The values are based on the observationally known solar spectrum. The percentage absorption is shown in brackets.

Table 4.

Useable irradiance for Photooxygenic organisms on Proxima Centauri b with an Earth-like Oxygenic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for stellar radiation on Proxima Centauri b with an oxic atmosphere. The values are based on a detailed TES of Proxima Centauri b. The percentage absorption is shown in brackets.

Table 5.

Irradiance useable by Oxygenic organisms on Proxima Centauri b with an Earth-like but Anoxic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for stellar radiation on Proxima Centauri b with an anoxic atmosphere. The values are based on a detailed TES of Proxima Centauri b. The percentage absorption is shown in brackets.

Earth

Table 2 shows the in vivo absorption of a mat of each of the six oxygenic photosynthetic organisms with a blue peak absorbance of 2 (1% transmission) of irradiance from the Sun at sea level at the Earth's equator at noon on an equinox. We defined a ‘standard mat’ or idealized model mat as a reference case as having 1% transmission because such a mat would be essentially optically black at the blue peak of Chlorophyll absorption. All six microbial mats would be able to use nearly all blue and red light for photosynthesis but other useable irradiance in the range 500–649 nm depends on their specific accessory pigment composition. Table 2 shows the total absorption for blue (400–499 nm), green (500–574 nm), yellow–orange (575–649 nm), red (650–699 nm) and far-red (700–749 nm) light compared with the number of photons available in each wavelength window for the ground-level irradiance. Where an organism is known not to have a suitable accessory pigment the box is left blank (Falkowski & Raven Reference Falkowski and Raven2007). All the organisms with large amounts of accessory pigments are able to use about 55–69% of the irradiance in the 400–699 nm window usually thought of as the limits of photosynthetically useful radiation for organisms with Chl a as their primary photosynthetic pigment (Synechococcus, Prochlorothrix, Rhodomonas and Phaeodactylum). The cryptophyte, Rhodomonas and the orthodox cyanobacterium, Synechococcus are the most efficient users of 400–749 nm visible light for oxygenic photosynthesis. The green alga, Chlorella, which has the same pigment array as terrestrial vascular plants (Chl a, Chl b + carotenoids), is able to use only about 43% of the 400–699 nm window. The far-red absorption band of Acaryochloris (700–749 nm) enables the alga to use about the same amount of light as the red absorption band of Chlorella and so the irradiance actually photosynthetically useable by Acaryochloris in the range 400–749 nm is about 33%. It turns out that Acaryochloris is the least able to use 400–749 nm light for oxygenic photosynthesis despite its exotic photosynthetic mechanism because it is poorly endowed with blue and green absorbing carotenoids and phycobilin pigments (Fig. 1).

A similar set of calculations was done for an Earth with an Earth-like but anoxic atmosphere (Table 3). There is very little difference between the results for Tables 2 and 3. Aerobic and anaerobic atmospheres are essentially equivalent for oxygenic photosynthetic organisms. The importance of absorption of red/far-red light by Chl a (650–749 nm) and appears to be overestimated. In all the algae used in this study more than 2/3 of the photons that would be absorbed by photosynthetic algal/microbial mats are blue and green light (400–574 nm) not red light (Tables 2 and 3).

Proxima Centauri b

Irradiance is severely depleted on Proxima Centauri b in the blue and green parts of the 400–749 nm range compared with irradiance on the surface of the Earth (Table 1). Tables 4 and 5 summarize the irradiance conditions for Proxima Centauri b for oxygenic organisms on a planet with an Earth-like oxic atmosphere and for an Earth-like but anoxic atmosphere. In the range of wavelengths used by oxygenic photosynthetic organisms there is very little difference in the available irradiance. All the oxygenic photoautotrophs would be able to use nearly all available light in the blue part of the spectrum. Tables 4 and 5 show absorption by layers of oxygenic photoautotrophs with a blue absorption peak of 2 (1% transmission) on Proxima Centauri b. The planet receives only about 3% (≈63 µmol photon m⁻² s⁻¹) of the 400–699 nm maximum irradiance experienced on Earth (Tables 1–3) and the spectrum provides very little blue, green and yellow light. Despite the very large difference in total irradiance in the visible range on Earth and Proxima Centauri b the proportion of total light within the 400–749 nm range useful to oxygenic organisms is very close to a constant. Thus, Synechococcus is able to use about 65% of the available light from the Sun and Proxima Centauri, Prochlorothrix 49–55%, Chlorella 43%, Rhodomonas 63–68% and Phaeodactylum 58–62%. The Chl d organism Acaryochloris would be able to use 33–37% of the light available from 400–749 nm. Although light on Proxima Centauri b is depleted of 650–699 nm light because of stellar absorption of light in this part of the spectrum it does not greatly affect the proportion of useable light in the 400–699 nm or 400–749 nm range. The matter of most importance is that the overall amount of useable irradiance in the range useable by oxygenic photosynthetic organisms is very low compared with the Earth (≈3–5%) or 63 to 132 µmol quanta m⁻² s⁻¹. Algae are grown routinely in culture rooms under similar irradiance and so the ground-level irradiance on Proxima Centauri b is no barrier to the survival of oxygenic photosynthetic organisms on the planet.

Absorption of light by mats of anoxygenic photosynthetic bacteria on the surfaces of Earth and Proxima Centauri b

Photosynthesis of mats composed of anoxygenic bacteria can be estimated using a similar procedure to that used above for oxygenic photosynthetic organisms on Earth and Proxima Centauri b. Combining the data on the light regime on the surface of the Earth and Proxima Centauri b (Fig. 4, Table 1) with the light absorption characteristics of the anoxygenic oxygenic photosynthetic organisms included in the present study (Fig. 2) provides a comparative estimation of the capacity for growth of these various organisms. Photosynthetic bacteria can use violet light (350–399 nm). The visible range of wavelengths (400–749) nm can also be used for photosynthesis and the defining feature of photosynthetic bacteria is that they have BChls, which enable them to use IR light. In Tables 6–9, the spectrum has been divided up into various bands based on what irradiances are most important to the photosynthetic bacteria selected for the present study: violet (350–399 nm), blue (400–499 nm), green (500–574 nm), yellow/orange (575–649 nm) and red (650–699 nm). Photosynthetic bacteria, using accessory photosynthetic pigments, are all able to use a substantial proportion of irradiance in the 400–700 nm range and both BChl a and b are able to absorb orange/red light at about 600 nm (Figs. 2 and 4) (Hellingwerf et al. Reference Hellingwerf, de Vrij and Konings1982). In addition, far-red (700–749 nm) and NIR (750–799 nm) is useful to Chlorobaculum because of the absorption properties of BChl c. The BChl a containing Chlorobaculum (BChl a + c) and Afifella, Rhodopseudomonas, Thermochromatium can use IR(I) (μmol photon m⁻² s⁻¹, 800–949 nm light). Blastochloris has BChl b as its primary photosynthetic pigment and is exceptional: it can use IR light up to 1100 nm (Fig. 2) and so can use a second window unavailable to other photosynthetic bacteria [Fig. 4, IR(II) (μmol photon m⁻² s⁻¹, 950–1100 nm].

Table 6.

Irradiance useable by photosynthetic bacteria on an Earth with the modern Oxygen Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of photosynthetic bacterial cells with an absorbance of 2.0 (1% transmission) at their Respective Blue Absorbance Peaks for Solar Radiation on Earth with an oxygenated atmosphere. Based on the observationally known solar spectrum. The percentage absorption is shown in brackets.

Table 7.

Irradiance useable by photosynthetic bacteria on an Earth with an Anoxic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of photosynthetic bacterial cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for solar radiation on Earth with an oxygenated atmosphere. Based on the observationally known solar spectrum. The percentage absorption is shown in brackets.

Table 8.

Irradiance useable by photosynthetic bacteria on Proxima Centauri b with an Oxygen Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of photosynthetic bacterial cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for solar radiation on Proxima Centauri b with an oxygenated atmosphere. The values are based on a detailed TES of Proxima Centauri b. The percentage absorption is shown in brackets.

Table 9.

Irradiance useable by photosynthetic bacteria on Proxima Centauri b with an Earth-like but Anoxic Atmosphere

Photosynthetically useable irradiance in different wavebands for a cell layer or thin suspension of photosynthetic bacterial cells with an absorbance of 2.0 (1% transmission) at their respective blue absorbance peaks for solar radiation on Proxima Centauri b with an anoxic atmosphere. The values are based on a detailed TES of Proxima Centauri b. The percentage absorption is shown in brackets.

Tables 6 and 7 show estimates of the useable irradiance for the five anoxygenic photosynthetic bacteria included in the present study for Earth with an oxic and anoxic atmosphere. They are based on the absorption curves in Fig. 2 and the surface irradiance values shown in Fig. 4. Table 6 shows a comparison of absorption of sunlight by layers of a mat of photosynthetic bacteria (Afifella, Rhodopseudomonas, Thermochromatium, Blastochloris and Chlorobaculum) for an oxic Earth atmosphere. Afifella, Rhodopseudomonas, Thermochromatium, Blastochloris and Chlorobaculum could all use violet light (350–399 nm) not generally useable by oxygenic photosynthetic organisms. All the photosynthetic bacteria can use blue (400–499 nm), green (500–574 nm) and orange (575–649 nm) light far better (72 to 80%) than the oxygenic photosynthetic organisms tabulated in Tables 2 and 3. None of the BChl-based photosynthetic bacteria included in this study can use red light (650–699 nm) for photosynthesis. However, in vivo BChl a and BChl b both have a minor peak at about 590 nm (Fig. 2) so some photosynthetic bacteria can use and grow on orange light not useable by most oxygenic organisms (Hellingwerf et al. Reference Hellingwerf, de Vrij and Konings1982). In the case of Chlorobaculum its secondary pigment BChl c strongly absorbs Far Red light (700–749 nm) and NIR light (750–800 nm) with a peak at about 748 nm, but direct absorption by its primary BChl a pigment at about 800 nm is very small. Note that the in vivo absorption peaks for BChl a and BChl b are on opposite sides of the strong atmospheric H₂O absorption peak from 930 to 970 nm (Figs. 2 and 4). BChl a organisms can use 55–60% of the light in the range 350–950 nm, Blastochloris can use about 47% of the light available in the range 350–1100 nm. In Table 7, calculations are made for the use of irradiance by photosynthetic bacteria for Earth with an Earth-like but anoxic atmosphere. The results are almost identical as for Table 6. The presence of an oxic or anoxic Earth atmosphere does not significantly affect the irradiance available for anoxygenic photosynthetic bacteria. Photosynthetic bacteria not only have a much wider range of useable irradiance than oxygenic organisms, but also can utilize the 400–749 nm range better than oxygenic organisms.

Table 8 shows useable irradiance for layers of Afifella, Rhodopseudomonas and Thermochromatium, Blastochloris and Chlorobaculum with a blue absorbance peak of 2 (1% transmission, 99% absorption) on Proxima Centauri b with an oxygenic atmosphere. Almost no violet light is available on the surface of Proxima Centauri b (0.38 µmol quanta m⁻² s⁻¹) but essentially all would be absorbed by a microbial mat of any of the photosynthetic bacteria included in this study. All five photosynthetic bacteria would also be able to use nearly all (>90%) the blue light (400–499 nm) and large proportions of green and yellow/orange light. The photosynthetic bacteria would be able to use 40–50% of the light in the 400–699 nm range, which is comparable with but perhaps slightly lower than for photosynthetic bacteria under Earth-type irradiance (Tables 6 and 7). Table 9 shows the similar calculations for the scenario where Proxima Centauri b has an anoxic but Earth-like atmosphere. As in the case of oxygenic photosynthetic organisms (Tables 6 and 7) for an oxic versus an anoxic Earth there are no significant differences in the photosynthetic quality of light available for anoxygenic photosynthetic bacteria on Proxima Centauri b whether or not the atmosphere contains oxygen.

The critical difference between Earth and Proxima Centauri b is that the irradiance available in the 400–699 and 400–749 nm wavelength ranges is only 3–5% that found on Earth. In sharp contrast, far-red/NIR (700–799 nm), IR(I) and IR(II) available on Proxima Centauri b are much more comparable with that found on Earth (Table 1). Photosynthetic bacteria with BChl a-based mechanisms would be use about 40–50% of total irradiance in the range 350–950 nm on Proxima Centauri b (≈700–749 µmol quanta m⁻² s⁻¹). Blastochloris with its BChl b-based photosynthetic mechanism can use 38% of light in the range 350–1100 nm (≈1550 µmol quanta m⁻² s⁻¹). In terms of photic resources available photosynthetic bacteria are at a clear advantage over oxygenic photosynthetic organisms on the surface of Proxima Centauri b.

Irradiance under water

As is well documented, most red and far-red light is eliminated in the first 1 m of water (Falkowski & Raven Reference Falkowski and Raven2007; Kirk Reference Kirk2011), but it is less well known that UV-A, UV-B and UV-C also disappear with depth, with absorbance coefficients of ≈0.1–1 m⁻¹ for wavelengths from 400 to 280 nm (and higher than 1 m⁻¹ for UV-C <280 nm). Absorbance data for 280–300 nm (Smith & Baker Reference Smith and Baker1981) and 300–749 nm consensus absorbance data from Ritchie (Reference Ritchie2013) were used to calculate irradiance at different depths of water on the various planets in this study. As a result of red and violet–blue light absorption in very deep clear water (100 m) the irradiance peak is at about 400 nm. Figure 5 shows the absorption of solar irradiance useable by oxygenic photosynthesis (400–699 and 400–749 nm) with depth in deep, very transparent, water on an oxic Earth. In shallow water the longer wavelengths are rapidly eliminated (Fig. 5) and wavelengths shorter than 400 nm are also removed with the shorter wavelengths removed first. As the irradiance becomes more monochromatic, with a peak at about 400 nm, the attenuation of irradiance versus depth approaches Beer's law. In the case of Proxima Centauri b the irradiance at the planet surface is already very low (Fig. 6) and heavily red-shifted and so the irradiance reaching the ground on Proxima Centauri b has poor water penetrating properties (400–699 nm, 63 µmol quanta m⁻² s⁻¹, 400–749 nm, 132 µmol quanta m⁻² s⁻¹. The oxygenic photosynthetic compensation point for Earth (net photosynthesis above respiration is zero) is at about 10 µmol quanta m⁻² s⁻¹ in oceanic water or at about 200 m (Falkowski & Raven Reference Falkowski and Raven2007; Kirk Reference Kirk2011): the equivalent compensation point would be reached on Proxima Centauri b at only about 10 m depth (Fig. 7). Oxic or anoxic atmospheric conditions would make no significant effect upon these results and the compensation depth for Chl d containing organisms is little different to that of Chl a organisms because the far-red (700–749 nm) is so rapidly eliminated by passage through water (Figs. 5–7). An ocean on Proxima Centauri b, as proposed by Turbet et al. (Reference Turbet, Leconte, Selsis, Bolmont, Forget, Ribas, Raymond and Anglada-Escudé2016), would be able to support very limited oxygenic photosynthesis because the surface irradiance is so low and the oxygenic euphotic zone would be only a maximum of about 10 m thick.

Fig. 5.

Decrease in irradiance versus depth in deep water for Earth with an oxic atmosphere. For an anoxic atmosphere the results are little different except that on an Earth with an anoxic atmosphere there would be no strong absorption band at 761 nm. Near IR (750–799 nm) and IR (I) (800–949 nm) and IR II (950–1100 nm) is largely eliminated in less than 0.3 m of water and completely eliminated in 1 m of water. Most far-red light and then red light is absorbed in very shallow water and so the irradiance spectrum changes rapidly with depth. As depth increases longer wavelengths are progressively eliminated: IR (II) → IR (I) → near IR → far red → red → orange → green. Violet light is also progressively eliminated in deeper water leaving blue light centred around a wavelength of 400 nm.

Fig. 6.

Absorption of photosynthetic irradiance with depth in deep very transparent water on Proxima Centauri b. As in the case for Earth (Fig. 5) for Proxima Centauri b an anoxic atmosphere the results are little different except that for an oxic atmosphere there is a strong O₂ absorption band at 761 nm. Compared with Earth there is very little light in the range useable by oxygenic photosynthesis (400 to 749 nm) at the surface of the planet. IR light (NIR, 750–799, IR(I), 800 to 949 nm and IR(II), 950 to 1100 nm) are all eliminated or severely limited under only 0.3 m of water. By 1 m all NIR, red and far-red light absorbable by Chl a and BChls has disappeared. Photosynthetically useable irradiance progressively disappears with depth from the red end of the spectrum and the irradiance peak moves towards the blue end of the spectrum. UV-A and UV-B also disappear with depth starting at the shortest wavelengths. As a result of these two processes total photosynthetically useable irradiance for oxygenic organisms is reduced to a blue peak with less than 10 µmol quanta m⁻² s⁻¹ in only about 10 m of water.

Fig. 7.

Absorption of solar irradiance useable by oxygenic photosynthesis (400–699 and 400–749 nm) with depth in deep very transparent water on an oxic Earth. Fig. 7 also shows estimates of oxygenic photosynthesis of algal mats on Earth and on Proxima Centauri b with depth of clear water using an average proportion of useable irradiance from Tables 10 and 12 for Earth and Proxima Centauri b respectively. In shallow water the longer wavelengths are rapidly eliminated (Fig. 5) and wavelengths shorter than 400 nm are removed with the shorter wavelengths removed first. As the irradiance becomes more monochromatic with a peak at about 400 nm the attenuation of irradiance versus depth approaches Beer's law. On Proxima Centauri b the irradiance at the planet surface is already very low (Fig. 6) and heavily red-shifted. The irradiance reaching the ground on Proxima Centauri b has poor water penetrating properties. Oxic or anoxic atmospheric conditions would make no significant effect upon these results. Primary productivity of an algal mat was calculated using equation (10). The oxygenic photosynthetic compensation point for Earth is at about 10 µmol quanta m⁻² s⁻¹ in oceanic water or at about 200 m: the equivalent compensation point would be reached on Proxima Centauri b at only about 10 m depth and productivity is very low underwater. The photic zone reaches down to about 200 m on Earth and the plot of productivity versus irradiance slowly intercepts with Irradiance versus depth because photosynthesis is not directly proportional to irradiance over most of the range of depth.

Table 10.

Estimates of possible primary production by oxygenic photosynthesis as mg C m⁻² s⁻¹ and per h on the surface of Proxima Centauri b compared with Earth using the same model

E _opt is the optimum irradiance, Abt is the absorptance of the algal mat, Q is the quantum efficiency. Primary production estimates are shown for Earth and Proxima Centauri b and the percentage of the production found for a mat on earth is expressed as a percentage.

It is possible that Proxima Centauri b has an anoxic ecology similar to the early Earth. Figure 8 compares the absorption of irradiance useable by photosynthetic bacteria (350–1100 nm) with depth in deep very transparent water on an anoxic Earth and anoxic Proxima Centauri b. Most NIR, IR(I) and IR(II) light are eliminated in the first 0.3 m of water (Fig. 6). By 1 m all NIR, red and far-red light absorbable by BChls has disappeared and with increasing depth the spectrum becomes more and more monochromatic with a peak at about 400 nm. Photosynthetic bacteria can grow at much lower irradiances than oxygenic organisms and so the compensation irradiance has been taken as 1 µmol quanta m⁻² s⁻¹ (Parkin & Brock Reference Parkin and Brock1980; Burke & Burton Reference Burke and Burton1988) Photosynthetic bacteria should be able to exist photoautotrophically on Earth at oceanic depths as much as 400 m (although this appears to be undocumented). On Proxima Centauri b the 1 µmol quanta m⁻² s⁻¹ euphotic zone threshold would be reached in only 60 m of water.

Fig. 8.

Absorption of irradiance useable by photosynthetic bacteria (350–1100 nm) with depth in deep very transparent water on an anoxic Earth and anoxic Proxima Centauri b. Fig. 8 also shows estimates of anoxygenic primary production for a mat of photosynthetic bacteria with an RC-2 photosystem at various depths of clear water on Earth and on Proxima Centauri b. By 1 m all NIR, red and far-red light absorbable by BChls has disappeared. The irradiance on the surface of Proxima Centauri b is of very poor quality with regards to its ability to penetrate water NIR, IR(I) and IR(II) irradiance are quickly eliminated in 0.3 to 1 m of water and the amount of deeply penetrating Violet (350–399 nm) and Blue (400–499 nm) light available at the surface is very low (Table 1, Fig. 6). Primary production was estimated using equation (11) using parameters from Tables 11 and 13, including on a mean value for the proportion of useable irradiance by the four RC-2 organisms. Photosynthetic bacteria can grow at much lower irradiances than oxygenic organisms and so the compensation irradiance has been taken as 1 µmol quanta m⁻² s⁻¹. On that criterion, photosynthetic bacteria should be able to exist photoautotrophically on Earth at depths as much as 400 m but below ≈50 m depth productivity is essentially directly proportional to irradiance. Productivity on Proxima Centauri b is limited by light: the compensation depth is only at about 60 m and production is directly proportional to irradiance except at the near surface.

Table 11.

Estimates of possible primary production by anoxygenic photosynthesis as mg C m⁻² s⁻¹ and per h on the surface of Proxima Centauri b compared with Earth using the same model as for Table 10

E _opt is the optimum irradiance, Abt is the absorptance of the algal mat, Q is the quantum efficiency. Primary production estimates are shown for Earth and Proxima Centauri b and the percentage of the production found for a mat on earth is expressed as a percentage.

Photosynthesis under shallow water

Figures 5 and 6 show the elimination of IR irradiance with depth in shallow water for the Earth and for Proxima Centauri b. By 1 m all NIR, red and far-red light absorbable by Chl a and BChls has disappeared on both Earth and Proxima Centauri b. Violet light is also progressively eliminated with depth but not as drastically as far-red and IR light and there is no significant attenuation of violet and blue light under only 1 m of water. Table 10 is an analysis of the different photosynthetically useable wavebands of light on Earth under 1 m of water for oxygenic photosynthetic organisms. These values should be compared with surface irradiance data shown Figs. 4–6, Table 1). On the Earth surface 400–699 nm light totals about 2200 µmol photons m⁻² s⁻¹, but under 1 m of water this is reduced to 1616 µmol photons m⁻² s⁻¹, however nearly all of this loss is as red light (650–699 nm) and far-red light (700–749 nm). There is very little loss of blue or green light. Synechococcus, Prochloroxthrix, Rhodomonas and Phaeodactylum are all able to use most of the available light, Chlorella and Acaryochloris are more dependent on red/far-red light than the other oxygenic photosynthetic organisms and so are at some disadvantage underwater.

Table 11 shows an analysis of the different photosynthetically useable wavebands of light on Proxima Centauri b under 1 m of water for oxygenic photosynthesis (compare with surface irradiance Figs. 4 and 6, Table 1). On the planet surface 400–700 nm light totals only about 64 µmol photons m⁻² s⁻¹ but under 1 m of water this is reduced to 36 µmol photons m⁻² s⁻¹. Nearly all of this loss (≈50%) is as red light (650–699 nm) and far-red light (700–749 nm). There is very little loss of the small amounts of blue or green light available on the planet. All the oxygenic organisms are disadvantaged by the loss of much of the red and far-red light because it is a much higher proportion of photosynthetically useable irradiance on Proxima Centauri b than on Earth. Chlorella and Acaryochloris are more dependent on red/far-red light than the other oxygenic photosynthetic organisms and so are disadvantaged underwater because much of the light available from a red dwarf is easily attenuated by water.

Tables 12 and 13 show that 1 m of water effectively eliminates the far-red, IR(I) and IR(II) radiation useable by anoxygenic photosynthetic organisms on both Earth and Proxima Centauri b. This effectively leaves them only the visible light resources also used by oxygenic organisms although they can also use violet light. On Earth photosynthetic bacteria can use well over 80% of the light available under 1 m of water and the total irradiance is high (1616 µmol photons m⁻² s⁻¹) (Table 12). This irradiance is well above the saturation point for photosynthetic bacteria and so effectively under 1 m of water they are not light limited. On the surface of Proxima Centauri b only about 63–65 µmol photons m⁻² s⁻¹ is available in the wavelength range 350–699 nm but 700 (350–949 nm) to 1500 (350–1100 nm) μmol photons m⁻² s⁻¹ is available to photosynthetic bacteria depending on whether they use BChl a or b as their primary photosynthetic pigment. One meter of water eliminates at least 90% of the available photons for anoxygenic photosynthesis leaving only 37 to 41 µmol photons m⁻² s⁻¹ available for photosynthesis. Photosynthetic bacteria would be disadvantaged underwater on Proxima Centauri b.

Table 12.

Estimates of possible primary production by oxygenic photosynthesis as mg C m⁻² s⁻¹ and per h under 1 m of water on Proxima Centauri b compared with Earth using the same model

E _opt is the optimum irradiance, Abt is the absorptance of the algal mat, Q is the quantum efficiency. Primary production estimates are shown for Earth and Proxima Centauri b and the percentage of the production found for a mat on earth is expressed as a percentage.

Table 13.

Estimates of possible primary production by anoxygenic photosynthesis as mg C m⁻² s⁻¹ and per h under 1 m of water on Proxima Centauri b compared with Earth using the same model as for Table 12

E _opt is the optimum irradiance, Abt is the absorptance of the algal mat, Q is the quantum efficiency. Primary production estimates are shown for Earth and Proxima Centauri b and the percentage of the production found for a mat on earth is expressed as a percentage.

Estimations of potential primary production

Equations (10)–(12) in the Appendix have been used to make estimates of potential primary productivity for oxygenic photosynthetic organisms [quantum number (γ) = 9], photosynthetic bacteria using RC-2 with a γ ≈ 17 and Chlorobium-type photosynthetic bacteria with a γ ≈ 9.7 respectively (see Appendix). These equations make minimal assumptions about the photosynthetic mechanisms and only require estimates of the optimum irradiance (E _opt), the absorptance (Abs) of the algal or microbial mat and the quantum efficiency (Q) (McCree Reference McCree1972) and calculations of the proportion of useable light in the photosynthetically useable range. Total photosynthesis of an opaque algal or microbial mat that is thick enough to absorb virtually all incident light is a slowly saturating exponential curve.

Table 10 shows estimates of potential primary activity of oxygenic algal mats using equation (10) on Earth and on Proxima Centauri b on the surfaces of the planets. A representative E _opt value of 200 µmol photons m⁻² s⁻¹ was used based upon Falkowski et al. (Reference Falkowski, Greene, Kolber, Baker and Bowyer1994); Falkowski & Raven (Reference Falkowski and Raven2007) and Ritchie (Reference Ritchie2008). Absorptances are taken as 0.95, based on reflectance–absorptance–transmission (RAT) measurements of algae filtered onto glass fibre discs (Ritchie Reference Ritchie2014; Ritchie & Runcie Reference Ritchie and Runcie2014). Quantum yield (Q) is taken as 0.9 (McCree Reference McCree1972). Estimates of useable irradiance in the 400–700 nm windows for Synechococcus, Prochlorothrix, Chlorella, Rhodomonas and Phaeodactylum and useable irradiance for Acaryochloris (400–750 nm) are taken from Tables 2–5). Estimates of primary production of mats on Earth of the representative oxygenic organisms included in the present study cover only a small range (in gC m⁻² h⁻¹) from 0.809 (Acaryochloris) to 0.820 (Synechococcus, Rhodomonas and Phaeodactylum). Table 10 also includes similar calculations for Proxima Centauri b (in Italics). The 400–699 nm and 400–749 nm light available is very low (≈ 3%) that found on Earth but the non-linear response of photosynthesis to irradiance results in much higher potential productivity than might be expected. Productivity estimates (gC m⁻² h⁻¹) vary from 0.10 (Chlorella) to 0.149 (Rhodomonas) and Acaryochloris has the highest estimate 0.178 due to the higher amount of light available because Chl d allows it to use far-red light. Overall productivity would be about 17% that found on Earth.

Table 11 shows estimates of potential primary activity of photosynthetic bacterial mats using equations (11) and (12) on Earth (in Bold) and on Proxima Centauri b (in Italics). A representative E _opt value of 200 µmol photons m⁻² s⁻¹ and absorptance of 0.7 were used which was based on RAT measurements of algae filtered onto glass fibre discs (Ritchie Reference Ritchie2013; Ritchie & Runcie Reference Ritchie and Runcie2013; Ritchie & Mekjinda Reference Ritchie and Mekjinda2015). The quantum efficiency (Q) of rhodopseudomonads is known to be 0.95 or higher (see Ritchie Reference Ritchie2013; Ritchie & Runcie Reference Ritchie and Runcie2013; Ritchie & Mekjinda Reference Ritchie and Mekjinda2015). Estimates of the proportion of useable irradiance in the BChl a window (350–949 nm) for Afifella, Rhodopseudomonas, Thermochromatium and Chlorobaculum were taken from and useable irradiance for the BChl b organism, Blastochloris (350–1100 nm) are all taken from Tables 6–9. On the surface of the Earth much higher amounts of irradiance are available to organisms that can use violet, visible and IR light than oxygenic photosynthetic organism. Estimates of primary production of mats of BChl a – containing anoxygenic organisms included in the present study (Afifella, Rhodopseudomonas and Thermochromatium) are all about 0.338 gC m⁻² h⁻¹. The productivity of Blastochloris is also about 0.338 gC m⁻² h⁻¹ despite the wider range of wavelengths useable by Blastochloris. Chlorobaculum would be capable of much higher productivities (0.592 gC m⁻² h⁻¹, approaching those of oxygenic organisms) because Chlorobaculum uses 9.7 photons to fix one CO₂ compared with ≈17 required by RC-2 photosynthetic bacteria. Table 11 also includes similar calculations for Proxima Centauri b (in Italics). The 350–949 nm and 350–1100 nm irradiance is lower than on Earth (≈19%), but is much higher than the optimum irradiance of the photosynthetic bacteria (≈200 µmol photon m⁻² s⁻¹). Productivity estimates (gC m⁻² h⁻¹) vary from 0.278 (Rhodopseudomonas) to 0.321 (Blastochloris) for RC-2 type photosynthesizers. Chlorobaculum has the highest estimate (0.442 gC m⁻² h⁻¹) because it uses the RC-1 type photosynthetic mechanism. Productivity of photosynthetic bacteria on the surface of Proxima Centauri b would be nearly as high (≈85%) as those found on Earth and importantly would be greater than achievable by oxygenic photosynthesis (≈0.14 gC m⁻² h⁻¹).

Underwater, the red and far-red light useable by oxygenic organisms is rapidly attenuated (Figs. 5 and 6) and so it is useful to estimate productivity under a standardized aquatic condition (1 m) and compare them with productivity estimates on the surface of Earth and Proxima Centauri b. Using the data in Tables 10 and 11, it is possible to estimate productivity under 1 m of water. Table 12 compares productivity under 1 m of water on Earth (in Bold) and on Proxima Centauri b (in Italics) making the same assumptions about E _opt, Abt and Q as for Table 10 but using the values for total potential useable irradiance and proportion of useable irradiance taking pigment absorption into account. Under 1 m of water on Earth about 25% of 400–699 nm and 400–749 nm light is lost, the bulk of it red and far-red light, losses are heavier (≈50%) on Proxima Centauri b because of the low temperature of the star and the red shift in its emission maxima. On Earth primary productivity under 1 m of water is barely affected by the small loss of total irradiance or the change in spectral properties of the incident light (Table 10: Earth Surface ≈0.82 gC m⁻² h⁻¹versus Table 12: 1 m Underwater ≈0.81 gC m⁻² h⁻¹). On Proxima Centauri b loss of light is more severe and the effects of attenuation of red and far-red light have more serious consequences (Table 5: Proxima Centauri b Surface ≈0.14 gC m⁻² h⁻¹versus Table 12: 1 m Underwater ≈0.068 gC m⁻² h⁻¹). Aquatic conditions adversely affect oxygenic primary productivity far more severely (≈50%) on Proxima Centauri b than on Earth.

In Table 13, the consequences of aquatic conditions upon productivity of anoxygenic photosynthetic conditions using similar reasoning as used to prepare Table 12. One (1) m of water on Proxima Centauri b is sufficient to completely eliminate infra-red light (Fig. 6) and so would be expected to have severe effects on anoxygenic primary productivity. Surprisingly, on Earth anoxygenic primary productivity by RC-2 organisms under 1 m of water is unaffected by the complete loss of IR light or the large change in spectral properties of the incident light (Table 11: Earth Surface ≈0.338 gC m⁻² h⁻¹ versus Table 13: 1 m Underwater ≈0.338 gC m⁻² h⁻¹). The same result is found for Chlorobaculum, which uses an RC-1 mechanism (Table 11: Earth Surface ≈0.592 gC m⁻² h⁻¹versus Table 13: 1 m Underwater ≈0.592 gC m⁻² h⁻¹). The reason is that the total useable irradiance available 1 m underwater is still well above the optimum irradiance (E _opt) [equations (11) and (12)]. This reflects the photonic nature of photosynthesis: a blue photon can do no more photochemistry than an infra-red photon. In contrast, on Proxima Centauri b only 1 m of water drastically reduces the number of photons available for anoxygenic photosynthesis (by ≈95%). Available light fell from well above optimum irradiance to well below optimum irradiance. On Proxima Centauri b anoxygenic primary productivity by RC-2 organisms under 1 m of water is drastically affected by the complete loss of IR light (Table 11: Earth Surface ≈0.29 gC m⁻² h⁻¹versus Table 13: 1 m Underwater ≈0.0369 gC m⁻² h⁻¹). The same result is found for Chlorobaculum, which uses an RC-1 mechanism (Table 11: Earth surface ≈0.442 gC m⁻² h⁻¹versus Table 13: 1 m Underwater ≈0.0672 gC m⁻² h⁻¹).

Primary production under water

Figure 7 also shows estimates of oxygenic photosynthesis of algal mats on Earth and on Proxima Centauri b with depth using an average proportion of useable irradiance from Tables 2 and 3 for Earth and Tables 4 and 5 for Proxima Centauri b. The photic zone reaches about 200 m on Earth and the plot of productivity versus irradiance slowly intercepts with Irradiance versus depth because photosynthesis is not directly proportional to irradiance over most of the range of depth. The current record depth is 268 m (Littler et al. Reference Littler, Littler, Blair and Norris1985, Reference Littler, Littler, Blair and Norris1986). Finds are common to depths of 200 m. Questions arose whether oxygenic photosynthesis could actually occur at such depths but Runcie et al. (Reference Runcie, Gurgel and Mcdermid2008) proved that algae are photosynthetically active in situ at such depths. Productivity on Proxima Centauri b underwater is limited by light. The compensation depth is only at about 10 m and production is both low and falls off very quickly with depth.

Figure 8 includes a similar calculation for mats made up of RC-2-type anoxygenic photosynthetic organisms based on a mean value for the proportion of useable irradiance by the four RC-2 organisms (Tables 6–9). On Earth below about 50 m the irradiance is essentially monochromatic blue light and productivity is directly proportional to irradiance down to a theoretical compensation depth of nearly 400 m. On Proxima Centauri b large amounts of irradiance is available at the surface but in water the IR light of benefit to photosynthetic bacteria is eliminated in the first 1 m of water. The compensation depth is at about 60 m and because useable irradiance is so low underwater on Proxima Centauri b. Anoxygenic photosynthesis is directly proportional to irradiance at depths below 1 m. Productivity by anoxygenic bacteria in aquatic environments on Proxima Centauri b will be very low compared with anoxygenic photosynthesis on the planet surface and lower than oxygenic photosynthetic organisms underwater (Fig. 8, Table 13): 1 m Underwater ≈0.0684 gC m⁻² h⁻¹).