Sensitivity to ultraviolet (UV) radiation (UV-A, λ = 315-400 nm; plus UV-B, λ = 280-315 nm) of zygotes of the brown alga Fucus serratus L. (Phaeophyta) has been assessed through effects on growth of developing germlings. Different stages of development were distinguished by considering 5 h periods of time after fertilisation. Both the stage of the zygote and the UV radiation condition significantly affected growth of developing germlings. The negative response of growth rate of early stages of the zygotes to UV radiation seemed to be caused by UV-B rather than UV-A radiation, as the lowest relative growth rates were always estimated for germlings developed from zygotes irradiated with UV-B radiation. As regards the stage of the zygote, those germlings that developed from zygotes irradiated at 5-10 h after fertilisation showed the strongest inhibition of growth compared with the other stages. These results point to polarisation as the most UV-sensitive process during the first 24 h of the development of the zygote. A non-linear relationship between the developmental stage of the zygote and the sensitivity to UV radiation is suggested.

The brown macroalga Laminaria saccharina exhibits a type of HCO3− utilization that could be almost completely inhibited either by proton buffers or by acetazolamide, an inhibitor of extracellularly operating carbonic anhydrase. This means of HCO3− utilization featured properties similar to direct HCO3− uptake in that photosynthetic rates were proportional to the HCO3− concentration of the seawater over a wide pH range (pH 7·0–9·5). Despite this, it must be characterized as carbonic anhydrase-catalysed external HCO3− dehydration and not as direct HCO3− uptake. A mechanism is suggested involving a CO2-concentrating capability located at the cell membrane. This mechanism, which might be common in brown algae, is suggested to have an adaptational advantage in colder regions of the sea (as compared with the direct HCO3− uptake of green macroalgae). This means of HCO3− utilization is inhibited even by fairly low concentrations of buffer, with consequences for the interpretation of earlier experimental studies on L. saccharina (and possibly other brown algae). These consequences relate both to ecology (e.g. determination of inorganic C affinity) and physiology (e.g. assessing mechanisms for inorganic C uptake).

The effects of full-spectrum solar radiation and of solar radiation deprived of total u.v. radiation (λ<395 nm) on the endemic Mediterranean red alga Rissoella verruculosa (Bertoloni) J. Agardh were studied in situ in early summer, from sunrise to sunset. Photosynthetic performance, pigment content and the activities of enzymes related to nutrient uptake, were monitored under both radiation conditions throughout a daylight period.

The doses of solar radiation, measured on the day during which the experiments were carried out (24 June 1997), were 9228·25, 1109·70 and 13·03 kJ m−2 for PAR (λ=400–700 nm), u.v.-A (λ=315–400 nm) and u.v.-B (λ=280–315 nm), respectively. Under these conditions, a clear daily variation in photosynthetic performance was found. However, no significant differences were detected between the two radiation conditions. At noon, strong sunlight impaired O2 evolution by 75%, but complete recovery occurred during the afternoon, reaching similar values to those measured in the early morning. By contrast, photoinhibition of optimal quantum yield (Fv/Fm), and the relative electron transport rate was followed by only a slight recovery during the afternoon. The rate of photosynthesis in air (simulating emersion conditions), estimated by CO2 exchange, showed a negative balance at noon, which was accentuated in plants exposed to the full-solar spectrum.

Significant changes in the activities of nitrate reductase and carbonic anhydrase were found throughout the day under both radiation conditions. In thalli receiving solar radiation deprived of u.v., total CA activity decreased throughout the day. However, in thalli exposed to full-spectrum solar radiation, the activity of the enzyme tended to increase in the afternoon, correlating with an increase in NR activity.

Growth rates in terms of area expansion per 30 min were measured in the red alga Porphyra leucosticta under different light/dark (LD) cycles, irradiance levels and light qualities. Twelve hours per day under white light (35 μmol m−2 s−1) promoted a dramatic and rapid increase in thallus expansion during the first part of the light period followed by low values during the remaining light and dark phase. Light/dark regimes of 8[ratio ]8 h, 6[ratio ]6 h, 3[ratio ]3 h and 1·5[ratio ]1·5 h resulted in a discontinuous growth pattern where thallus expansion was restricted to the light phase while no growth occurred in the dark phase, and there was even a significant thallus contraction just after light-off. Growth rates in terms of percentage area increased with irradiance at LD 12[ratio ]12. This was due to: (1) the phase of high growth rate following the initial ‘morning peak’ extending from the first 6 h of the light phase to cover almost the entire light phase, and (2) an increase in growth rate during the second half of the night phase. Saturation was reached at an irradiance of 125 μmol m−2 s−1. An irradiance of 500 μmol m−2 s−1 decreased thallus expansion rate during the light and dark culture phases relative to the other irradiances used. Light quality also caused changes in growth rate: the maximum value was reached under yellow light followed by red; both of these were higher than the growth rate under white light. Blue light (35 μmol m−2 s−1) caused a decrease of more than 50% in thallus expansion compared with white or red light due to a very dramatic decrease in the ‘morning peak’ of thallus expansion and very low values in the rest of the light and dark phases. Increasing the percentage of blue light in different white light sources also decreased thallus expansion. It is suggested that light influences the growth of Porphyra in three ways: as an external signal matching thallus expansion with light and dark cycles; as an energy source promoting thallus expansion with increasing irradiance; and, at a morphogenetic level, inhibiting growth in blue light.