## Rate of light absorption by aquatic plants

The rate of photosynthesis by an aquatic plant must ultimately be limited by (although it is not always simply proportional to) the rate at which the higher plant leaf, or multicellular algal thallus, or individual phytoplankton cell or colony, is absorbing quanta from the underwater light field. In the case of the leaf or the algal thallus, the rate of absorption of quanta of a given wavelength incident at a particular angle on a particular element of tissue surface is equal to E(1, 0, f).8s.A(l, 0, f), the product of the irradiance at that angle, the area of the element, and the absorptance for that wavelength and that angle. The total rate of absorption of light of that wavelength by the whole leaf or thallus is the sum of this product over all angles of incident light for each element of surface, and over all the elements that constitute the total area of the leaf or thallus.

It is clear that the crucial optical property of the tissue is the absorp-tance (the fraction of incident light absorbed) rather than the absorbance. Because of the variation in optical path through the tissue with angle, the absorptance of the leaf or thallus at any point will in fact vary somewhat with the angle of incidence of the light. The effective absorptance of the tissue at a given point for the total light of a given wavelength incident at all angles is a function of the angular distribution of the light field, and so it is not, strictly speaking, possible to attribute a particular absorptance to the tissue for underwater light of a given wavelength independent of the radiance distribution of that light. The absorptance is in fact usually determined with the measuring beam at right angles to the plane of the photosynthetic tissue. This is considered to provide at least an approximate measure of the absorptance that the tissue will present to the light incident on it within the water. Due to variations in thickness, and/or chloroplast number and pigment composition, there can be variation in absorptance from place to place within a leaf or thallus. This is particularly likely to be the case with long algal thalli, different parts of which are normally exposed to different light environments, e.g. because the lower parts are shaded. The increase in photon pathlength resulting from multiple scattering within the tissue can accentuate absorption by multicellu-lar plants.

Figure 9.3 shows the 90 ° absorptance spectrum for an aquatic higher plant; corresponding spectra for various kinds of multicellular algae are shown in Fig. 10.6a, b and c. The differences between the spectra are in part due to differences in concentration of chloroplast pigments per unit area, and this can of course vary markedly within any algal class as well as from one class to another. Some of the differences in the shape of the spectra are, however, attributable to the different types of pigment present. The relatively greater absorption in the 500 to 560 nm region in the brown alga compared to the green alga and higher plant is due to the presence of fucoxanthin in the former. The broad peak at 520 to 570 nm in the spectrum of the red alga is due to the presence of the biliprotein, phycoerythrin.

In the case of phytoplankton, the rate of absorption by an individual cell or colony of light quanta of a given wavelength coming from a given

Fig. 9.3 Absorption spectrum of the leaf of a freshwater macrophyte (Kirk, unpublished). The spectrum was measured on a piece of leaf of Vallisneria spiralis (Hydrocharitaceae), free of epiphytic growth, from Lake Ginninderra, ACT, Australia, with the sample cell close to the photomultiplier: the spectrum has been corrected for scattering.

Fig. 9.3 Absorption spectrum of the leaf of a freshwater macrophyte (Kirk, unpublished). The spectrum was measured on a piece of leaf of Vallisneria spiralis (Hydrocharitaceae), free of epiphytic growth, from Lake Ginninderra, ACT, Australia, with the sample cell close to the photomultiplier: the spectrum has been corrected for scattering.

direction, is equal to E(1, 6, f).sp(6, f).Ap(l, 6, f), the product of the irradiance in that direction, the cross-sectional area (in the specified direction) of the cell or colony and the absorptance of the particle in its particular orientation with respect to the light stream. Since the cells or colonies are randomly oriented, each will present a somewhat different projected area, and absorptance, to light flowing in the specified direction. The average rate of absorption of this light per particle is E(l,9, f).spAp. It will be recalled that spAp is the average absorption cross-section of the particles (§9.2). It follows from the random orientation of the cells or colonies that they have the same average absorption cross-section for light at all directions. We can therefore validly attribute an average absorption cross-section to a phytoplankton population, regardless of the angular distribution of the underwater light field. It is in fact possible to attribute an average absorptance to the individual particles in a plankton population, but this is not a useful thing to do, since it is the product of absorptance and cross-sectional area (which vary together, with orientation) rather than absorptance alone, which determines the rate of collection of quanta from a particular light stream.

What, in the present context, we wish to know about a phytoplankton population is the average absorption cross-section of the cells or colonies composing the population, at all wavelengths in the photosynthetic range. To carry out measurements on individual cells or colonies, although possible, is technically difficult and does not give results of high accuracy. We must therefore rely on spectroscopic measurements carried out on suspensions of phytoplankton. The concentrations at which phytoplankton normally occur are too low for accurate absorption measurements. It is therefore necessary to prepare more concentrated suspensions by filtration or centrifugation, followed by resuspension in a smaller volume.

Given a reasonably concentrated suspension of phytoplankton, by what sort of measurement can we determine the average absorption cross-section? Despite the importance of absorptance of the individual cell or colony in determining the absorption cross-section, measurements of the absorptance of the whole suspension tell us relatively little about the absorption properties of the individual particles in the suspension. The absorptance spectrum of the suspension changes shape as the phyto-plankton concentration changes, and at very high concentrations tends to become a straight line, with Asus«1.0 throughout the spectrum (total absorption of light at all wavelengths). The absorptance spectrum of the suspension, however, is of relevance if, for some reason, we need to know the rate of light absorption by the whole suspension - for example, in laboratory studies of photosynthetic efficiency.

To determine the average cross-section per individual free-floating particle, whether cell or colony, in the suspension, what we in fact measure is the absorbance of the suspension, Dsus. Figure 9.4 shows the absorbance spectra of suspensions of three planktonic algae: Chlorella (green), Navicula (a diatom) and Synechocystis (a blue-green). The absorb-ance, in a l cm pathlength, of a suspension of particles, as we saw earlier (eqn 9.2, §9.2), is equal to 0.434 nspAp, where n is the number of particles per ml, and spAp is the average absorption cross-section per particle. Thus we may obtain the values of spAp throughout the photosynthetic range for any phytoplankton population by preparing a suitably concentrated suspension, measuring the absorbance spectrum and n, and applying the relation

at a series of wavelengths from 400 to 700 nm. Since the pathlength is 1 cm then n, as well as being the number of cells per ml, is also the number of particles per cm2 in the path of the measuring beam. Thus, in eqns 9.2 and 9.5, N or n can be taken to mean the number of particles per unit area,

Fig. 9.4 Absorbance spectra of cultured cells of three species of planktonic algae measured using an integrating sphere (after Latimer and Rabinowitch, 1959). (a) Chlorella pyrenoidosa (green). (b) Navicula minima (diatom). (c) Synechocystis sp. (blue-green).

provided only that the units in which area is expressed are the same as those in which absorption cross-section (spAp) is expressed, i.e. with a given number of particles per unit area, the absorbance is the same no matter through what pathlength they are distributed.

The average rate of absorption of light of wavelength 1, per individual phytoplankton cell or colony at depth z m, is E0 (1, z). spAp (1), where E0(1, z) is the scalar irradiance of light of that wavelength at that depth and spAp(X) is the average absorption cross-section in m2. The rate of light absorption (in W, or quanta s-1) per horizontal m2 by all the phytoplankton within a thin layer of thickness Dz m, at depth z m is given by

where N is the number of phytoplankton cells or colonies per m3.

In the field of aquatic primary production, phytoplankton concentration is more commonly expressed in terms of mg chlorophyll am-3, than cells or colonies m-3. Since, from eqn 9.1

where ap(1) is the absorption coefficient due to phytoplankton, we can write

where [Chl] is the concentration of phytoplankton chlorophyll a in mg m-3, and af *(1) is the specific absorption coefficient of the phytoplankton per mg chlorophyll a m-3: af *(1) has the units m2mg chlorophyll a-1.

In any given waveband, if the total absorption coefficient due to all components of the medium is at(1), then the proportion of the total absorbed energy which is captured by phytoplankton is ap(1)/ at(1). A set of values for the specific absorption coefficient of marine phytoplankton across the photosynthesis spectral region may be found in Fig. 3.9 (Chapter 3).

A useful concept when considering light capture by phytoplankton is the effective absorption coefficient of the phytoplankton population existing at a given depth for the light field at that depth, across the whole photosynthetic spectrum.940 It may be thought of as a weighted average absorption coefficient of the phytoplankton for PAR, taking into account the actual spectral distribution of PAR at the depth in question, and is defined by

where E0(1, z) is the scalar irradiance per unit bandwidth (nm- ) at wavelength 1 and depth z m. We can also define a specific effective absorption coefficient of the phytoplankton, af*(z), for PAR as the value of ap(z) for phytoplankton at 1 mgchl am-3, and having the units m2mgchl a-1.

Even if the concentration and nature of the phytoplankton remain the same, the values of the effective, and specific effective, absorption coefficients for PAR do vary with depth, in accordance with the change in spectral distribution of PAR. In the ocean, where there is generally little dissolved yellow colour, the light field with increasing depth becomes increasingly confined to the blue-green (400-550 nm) spectral region (§6.2, Fig. 6.4). Since this is where phytoplankton have their major absorption peak (Fig. 3.9), the value of af*{z) in such waters increases with depth, by up to 50 or even 100% within the euphotic zone.728,940 In the Pacific Ocean southeast of Japan, Kishino et al. (1986) found âf *(z) to increase rapidly with depth from 0.022m2mg chl a-1 at the surface to 0.044 at 30 m. In the green waters of highly productive upwelling regions, af*(z) can decrease with depth.940

The photomicrograph in Fig. 9.5, of a mixed sample from a Tasmanian coastal site, shows what at least some of the phytoplankton cells that absorb energy from the underwater light field actually look like.