Seaweeds require inorganic carbon, water, light, and various mineral ions for photosynthesis and growth. This chapter will examine the mechanisms of uptake, the nutrient requirements, and the metabolic roles of essential nutrients (excluding C, H, and O). The importance of nutrient uptake and growth kinetics will be discussed in terms of their effects on chemical composition, growth, development, and distribution of macroalgae. Particular emphasis will be placed on nitrogen, because it is the element most frequently limiting to seaweed growth. Even though seaweeds are larger than phytoplankton and usually are attached to a substratum, their nutritional requirements are very similar, and therefore some discussion of phytoplankton nutrition is also included when little or no information exists for seaweeds.

Nutrient requirements

Essential elements

The development of defined culture media for growing algae axenically has allowed the testing of a variety of elements to determine which are essential. The criteria to define an absolute requirement for an element were established by Arnon and Stout (1939):

1. deficiency of the element makes it impossible for the alga to grow or complete its vegetative or reproductive cycle.

2. cannot be replaced by another element.

3. effect is direct and is not due to interaction with (e.g., detoxification of) other, nonessential elements, stimulation of epiflora, or the like (Levitt 1969).

C, H, O, N, P, Mg, Cu, Mn, Zn, and Mo are considered to be required by all algae (O'Kelley 1974; DeBoer 1981); S, K, and Ca are required by all algae, but can be partially replaced by other elements; Na, Co, V, Se, Si, Cl, B, and I are required only by some algae.

The collection of light energy for photosynthesis by aquatic plants is, as we have seen, carried out by the photosynthetic pigments. We have already examined the spectral absorption capabilities of each of the different classes of pigment. We shall now consider the light-harvesting properties of the complete photosynthetic system, with particular reference to the dependence of these on the particular combination of pigments present and, in the case of phytoplankton, on the size and shape of the cells or colonies.

Absorption spectra of photosynthetic systems

We might measure the absorption spectrum of, say, phytoplankton or a multicellular algal thallus, for a number of different reasons. We might seek information on what pigments are present. We might wish to compare the spectral position and shape of an in vivo absorption peak with those of the same peak in the isolated pigment with a view to assessing the extent to which the absorption properties are modified by binding to protein. We might want to know to what extent an alga is equipped to efficiently harvest light from the underwater radiation field in which it lives.

An absorption spectrum is the variation of some measure of light absorption by a system with wavelength. Light absorption might be expressed in terms of the absorptance, A, the percent absorption (100 A), the absorption coefficient, a, the absorbance, D (where D = − logl0(l − A)), or some other function such as the first derivative of the absorbance. The particular light absorption parameter chosen will depend on the purpose of the absorption spectrum.

Having discussed how the solar radiation gets through the water surface, we shall now consider what happens to it within the water. Sooner or later most of the photons are absorbed: how this happens, and which components of the medium are responsible, form the subjects of this chapter.

The absorption process

The energy of a molecule can be considered to be part rotational, part vibrational, part electronic. A molecule can only have one of a discrete series of energy values. Energy increments corresponding to changes in a molecule's electronic energy are large, those corresponding to changes in vibrational energy are intermediate in size, and those corresponding to changes in rotational energy are small. This is indicated diagrammatically in Fig. 3.1. When molecules collide with each other in the liquid or gaseous state, or are in contact with each other in the solid state, there can be transfer of rotational or vibrational energy between molecules and this is accompanied by transitions from one rotational or vibrational energy level to another within each molecule.

Molecules can obtain energy from radiation as well as from other molecules. When a photon passes within the vicinity of a molecule, there is a finite probability that it will be captured by that molecule, i.e. be absorbed. If the photon is captured, then the energy of the molecule must increase by an amount corresponding to the energy of the photon. If the photon is of long wavelength (> 20 μm), in the far infrared/microwave region of the spectrum, then its energy is low and its absorption can only cause a transition in the energy of the molecule from one rotational energy level to another.

We have seen that most of the solar photons which enter the water are absorbed. Many of these photons – most, in some waters – undergo scattering one or more times before they are absorbed. Scattering does not by itself remove light – a scattered photon is still available for photosynthesis. The effect of scattering is to impede the vertical penetration of light. It makes the photons follow a zig-zag path as they ricochet from one scattering particle to the next. This increases the total pathlength which the photons must follow in traversing a certain depth, and so increases the probability of their being captured by one of the absorbing components of the medium. In addition, some of the photons are actually scattered back in an upwards direction. Thus the effect of scattering is to intensify the vertical attenuation of the light.

In this chapter we shall consider the nature of the scattering process and the scattering properties of natural waters.

The scattering process

What do we mean by scattering? We say that a photon is scattered when it interacts with some component of the medium in such a way that it is caused to diverge from its original path. There are two kinds of scattering to be considered – density fluctuation scattering and particle scattering.

Density fluctuation scattering

In understanding the basis of density fluctuation scattering in liquids, it is helpful to begin with a consideration of molecular, or Rayleigh, scattering by gases such as air.

Introduction

The purpose of the first part of this book is to describe and explain the behaviour of light in natural waters. The word ‘light’ in common parlance refers to radiation in that segment of the electromagnetic spectrum – about 400 to 700 nm – to which the human eye is sensitive. Our prime concern is not with vision but with photosynthesis. Nevertheless, by a convenient coincidence, the waveband within which plants can photosynthesize corresponds approximately to that of human vision and so we may legitimately refer to the particular kind of solar radiation with which we are concerned simply as ‘light’.

Optics is that part of Physics which deals with light. Since the behaviour of light is greatly affected by the nature of the medium through which it is passing, there are different branches of optics dealing with different kinds of physical systems. The relations between the different branches of the subject and of optics to fundamental physical theory, are outlined diagrammatically in Fig. 1.1. Hydrologic optics is concerned with the behaviour of light in aquatic media. It can be subdivided into limnological and oceanographic optics according to whether fresh, inland or salty, marine waters are under consideration. Hydrologic optics has, however, up to now been mainly oceanographic in its orientation.

The nature of light

Electromagnetic energy occurs in indivisible units referred to as quanta or photons. Thus a beam of sunlight in air consists of a continual stream of photons travelling at 3 x 108 m s−1.

Four things are required for plant growth: energy in the form of solar radiation; inorganic carbon in the form of carbon dioxide or bicarbonate ions; mineral nutrients; and water. Those plants which, in the course of evolution, have remained in, or have returned to, the aquatic environment have one major advantage over their terrestrial counterparts: namely, that water – lack of which so often limits productivity in the terrestrial biosphere – is for them present in abundance; but for this a price must be paid. The medium – air – in which terrestrial plants carry on photosynthesis offers, within the sort of depth that plant canopies occupy, no significant obstacle to the penetration of light. The medium – water – in which aquatic plants occur, in contrast, both absorbs and scatters light. For the phytoplankton and the macrophytes in lakes and rivers, coastal and oceanic waters, both the intensity and spectral quality of the light vary markedly with depth. In all but the shallowest waters, light availability is a limiting factor for primary production by the aquatic ecosystem. The aquatic plants must compete for solar radiation not only with each other (as terrestrial plants must also do), but also with all the other light-absorbing components of the aquatic medium. This has led, in the course of evolution, to the acquisition by each of the major groups of algae of characteristic arrays of lightharvesting pigments which are of great biochemical interest, and also of major significance for an understanding both of the adaptation of the algae to their ecological niche and of the phylogeny and taxonomy of the different algal groups.

The rate of photosynthesis achieved by a phytoplankton cell or aquatic macrophyte depends on the rate of capture of quanta from the light field. This is determined by the light absorption properties of the photosynthetic biomass, which we have considered in some detail, and by the intensity and spectral quality of the field. The rate of photosynthesis is not, however, simply proportional to the rate of capture of photons. The efficiency with which the photosynthetic apparatus can make use of the absorbed energy to fix CO2 varies from one plant cell to another and within a given cell as its physiological state changes. Light quanta may be collected by the pigments faster than the electron carriers and enzymes can make use of them. In particularly high light intensities the excess absorbed energy can inactivate the photosynthetic system. The relation between the rate of photosynthesis and the characteristics of the incident light is thus not a simple one: we shall examine it in this chapter.

In order to study the effects of light intensity and spectral quality on photosynthesis, suitable quantitative procedures for determining the photosynthetic rate per unit biomass must be used. Detailed descriptions of such methods for use in the field or the laboratory may be found elsewhere, and so they will only be briefly mentioned here. Photosynthesis can be measured in terms of either carbon dioxide fixed or oxygen released. Because of the stoichiometry of the overall photosynthetic process (§8.5), approximately one O2 molecule is liberated for every molecule of CO2 fixed.

The behaviour of sunlight in water, and the role which light plays in controlling the productivity, and influencing the biological composition, of aquatic ecosystems have been important areas of scientific study for more than a century, and it was to meet the perceived need for a text bringing together the physical and biological aspects of the subject, that Light and photosynthesis in aquatic ecosystems was written. The book was well received, and is in use not only by research workers but also in university courses. In the eleven years since the first edition, interest in the topic has become, if anything, even greater than it was was before. This may be partly attributed to concern about global warming, and the realization that to understand the important role the ocean plays in the global carbon cycle, we need to improve both our understanding and our quantitative assessment of marine primary production. An additional, but related, reason is the great interest that has been aroused in the feasibility of remote sensing of oceanic primary productivity from space. The potentialities were just becoming apparent with the early CZCS pictures when the first edition was written. The continuing stream of further CZCS studies in the ensuing years, enormously enlarging our understanding of oceanic phytoplankton distribution, and the announced intention by space agencies around the world to put new and improved ocean scanners into space, have made this a particularly active and exciting field within oceanography.

In any water body some of the light which penetrates the water is caused, by scattering within the water, to pass up through the surface again. Of this emergent flux, 90% originates within the depth (equal to l/Kd) in which downward irradiance falls to 37% (l/e) of the subsurface value. It can be regarded as a sample derived from the underwater light field, and so by studying it with appropriate detection instruments above the surface, information about that field and therefore about the optically significant components of the medium, can be obtained. There is not much point in having detection instruments just above the surface: they would be more useful below. If, however, this emerging radiant flux can be studied by remote sensing instruments, located a considerable distance above the surface, in an aeroplane or space satellite, then the considerable advantage is gained that information about the underwater environment over a large area can be obtained in a short time. This makes it possible to acquire a synoptic view of a large aquatic ecosystem, with a fraction of the time and effort that would be involved in carrying out measurements over the same area from a surface vessel.

It will readily be appreciated, however, that a price must be paid. Measurements of the emergent flux, from a great distance, cannot be as accurate, or yield as much information, as measurements carried out within the water itself. We shall now consider the kinds of measurements that can be made, the correction procedures that must be carried out, and the nature of the information that may be obtained.

In Part I, we considered the underwater light climate: the particular characteristics that it has in different types of natural water bodies, the scattering and absorption processes that take place in the aquatic medium, and the ways that these operate upon the incident light stream to produce the kinds of underwater light field that we observe. Now, in Part II, we turn our attention to the utilization of this underwater light for photosynthesis by aquatic plants. We begin, in this chapter, by asking: with what intracellular structures, from the level of organelles down to that of molecules, do aquatic plants harvest radiant energy from the underwater light field and convert it to chemical energy?

Chloroplasts

In eukaryotic plants, photosynthesis is carried out by the organelles known as chloroplasts, the best-known members of the great class of related and interconvertible organelles known as plastids. Detailed accounts of these organelles may be found in Kirk & Tilney-Bassett (1978) and Staehelin (1986): we shall here content ourselves with a rather brief treatment.

The chloroplasts contain the pigments which capture the light, the electron carriers which use the absorbed energy to generate reducing power in the form of NADPH2 and biochemical energy in the form of ATP, and the enzymes which use the NADPH2 and the ATP to convert CO2 and water to carbohydrate. The pigments and electron carriers are contained in a specialized type of membrane known as the thylakoid.

Having considered the photosynthetic response of aquatic plants to light of different intensities and spectral qualities, we shall now examine how the availability of light influences where, when and how much photosynthesis takes place in aquatic ecosystems, and also the extent to which other parameters of the environment can limit photosynthesis. Aquatic production ecology is an enormous field: a comprehensive account will not therefore be attempted. Rather, the broad principles governing the controlling influence of light and other parameters will be outlined and illustrated by examples. More detailed accounts and extensive bibliographies can be found in the books on phytoplankton ecology by Reynolds (1984), Harris (1986) and Fogg & Thake (1987), and the symposium proceedings edited by Platt & Li (1986) and Falkowski & Woodhead (1992). The essay by Fogg (1991) on ‘The phytoplanktonic way of life’ provides a particularly valuable overview of the multifarious interactions between the phytopl ankton and its environment.

Circulation and depth

We saw in the previous chapter that except under very still conditions with virtually no wind or waves, there is always circulation of water in the upper layer. We also saw that this can be an advantage to the phytoplankton insofar as, by ensuring that they are not exposed to the intense light just below the surface for very long, they avoid photoinhibition. This circulation can, however, also be a disadvantage to the phytoplankton if, in the lower reaches of the mixed layer, the light intensity is too low for net photosynthesis to be achieved.

Of the factors which limit the rate of primary production in aquatic ecosystems – light, nutrients, carbon dioxide, temperature – that which shows the most extreme variation within the aquatic medium is light. As we have seen (Chapter 6) the irradiance decreases with depth from intensities which are so high as to be damaging down to levels which cannot support photosynthesis, and the spectral distribution of the light also changes markedly. We have also seen that at any given depth the intensity and spectral quality of the light vary greatly in accordance with the optical properties of the water. Furthermore, to a much greater extent than the other limiting factors, light availability varies with time: both within the day – from darkness to the full noon Sun, and as clouds pass across the Sun – and with the seasons during the course of the year.

In this chapter we shall consider the ways in which the aquatic flora is adapted to this variability of the light climate.

Aquatic plant distribution in relation to light quality

As we saw in Chapters 8 and 9, there are major differences between the main taxonomic groups of aquatic plants with respect to the kinds of photosynthetic pigment present and, as a consequence, major differences in the absorption spectra. Given the variation in intensity and spectral quality of the light field in the aquatic environment, we may reasonably suppose that for any given location within a water body there will be certain species which are well equipped to exploit the particular prevailing light field and others which are not.