Phytoplankton

The absorption of light by the photosynthetic pigments - chlorophylls, carotenoids and biliproteins - of the phytoplankton contributes to the attenuation of PAR with depth. Indeed in productive waters, the algae may be present in concentrations such that by self-shading they limit their own growth.

Light absorption by algal cells grown in laboratory culture has received a great deal of attention because of the use of such cultures as experimental material in fundamental photosynthesis research. What we need, however, is information on the light absorption properties of phytoplank-ton populations as they occur in nature since we cannot assume that the natural populations have absorption spectra identical to those of the same species grown in culture. The total concentration of pigments and the relative amounts of the different pigments can all be affected by environmental variables such as nitrogen concentration in the medium, light intensity and spectral distribution etc. Ultimately what is required is not only the shape of the absorption spectrum, but the actual values of the absorption coefficients due to phytoplankton in the original water body. Data on phytoplankton populations consisting mainly of single known species would be particularly valuable.

Many studies of the absorption spectra of natural populations of marine phytoplankton have now been carried out (refs.164,688,729,804,916,962,1487,

52,1270,275,259,1439,353,1306,328,952,1359,241,1405,1438,563,1171,1218 are a representa tive sample). Almost invariably in recent years - because of the low concentrations of cells present in the water - collection and measurement of the spectrum on a filter has been the method of choice. To correct for non-algal absorption, the spectrum of the material remaining after decolourization of the algae with methanol or hypochlorite (see above) can be measured, and the phytoplankton spectrum obtained of by difference. The filter method does, however, as we noted earlier, require the somewhat problematic estimation of a large absorption amplification factor. As we have noted earlier, the freeze-transfer technique of Allali et al. (1995), although somewhat more demanding, does avoid this problem. Also, as discussed above for total particulates, measurements with an integrating cavity or reflective tube absorption meter, on the unfiltered water sample and on the filtrate, would avoid these optical problems.

Iturriaga and Siegel (1989) have used their technique of microspectro-photometry of individual cells to determine both the shape of the spectrum, and the in situ absorption coefficients, for natural phytoplankton populations in the Sargasso Sea (Fig. 3.7). Neumiiller et al. (2002) have measured the spectra of individual phytoplankton cells from laboratory cultures using an epifluorescence microscope equipped with a spectro-graph and a charge-coupled device (CCD) array detector. This method could presumably, like that of Iturriaga and Siegel, be applied to natural phytoplankton populations.

In the case of productive marine or fresh waters, it is feasible to measure absorption spectra of phytoplankton populations in suspension - if necessary, after a preliminary concentration step - using an integrating sphere, or opal glass,516,701,873 and calculate the in situ absorption coefficients directly. Figure 3.6f shows the spectrum (integrating sphere method) of the particulate fraction from Burrinjuck Dam, a eutrophic impoundment in New South Wales, Australia, at a time when it had a mixed bloom of Melosira sp. (a diatom) and Anacystis cyanea (a blue-green alga). The fraction consisted mainly of algal biomass and so the spectrum provides approximate values (somewhat too high in the blue due to the presence of some tripton) for the in situ absorption coefficients due to phytoplankton. Figure 3.8 shows the particulate fraction spectrum - again largely due to phytoplankton - of the estuarine water of Lake King (Gippsland Lakes, Australia).

A quite different route to the determination of in situ phytoplankton absorption has been taken by Bidigare et al. (1987) and Smith et al. (1989). Making the plausible assumption that essentially all the unde-graded photosynthetic pigments in the water column originate in living cells, they carried out a complete pigment analysis of the total particulate fraction, using high-performance liquid chromatography (HPLC); then, using literature data on the spectral properties of pigment-protein complexes, they calculated the absorption coefficients due to phytoplankton.

It is possible to estimate the absorption coefficient of the medium at a given wavelength from the vertical attenuation coefficient for irradiance

Fig. 3.8 Comparison of the spectral absorption properties of the different fractions in an estuarine water from southeast Australia - Lake King, Victoria (Kirk, unpublished data). Phytoplankton were present at a level corresponding to 3.6 mg chlorophyll am~3 and the turbidity of the water was ~1.0 NTU.

Fig. 3.8 Comparison of the spectral absorption properties of the different fractions in an estuarine water from southeast Australia - Lake King, Victoria (Kirk, unpublished data). Phytoplankton were present at a level corresponding to 3.6 mg chlorophyll am~3 and the turbidity of the water was ~1.0 NTU.

at that wavelength. By carrying out such calculations for pairs of stations (in the Atlantic off Northwest Africa) where the scattering coefficients were about the same but the phytoplankton population varied, Morel and Prieur (1977) were able to arrive at an absorption spectrum for the natural phytoplankton population present. The absorption coefficients corresponding to 1 mg chlorophyll am~3 are plotted against wavelength in Fig. 3.9. The amount of light harvested by the phytoplankton component of the aquatic medium depends not only on the total amounts of the photosynthetic pigments present, but also on the size and shape of the algal cells or colonies within which the pigments are located. This subject is dealt with later (§9.3).

Absorption spectra of phytoplankton are normally measured only over the range corresponding to photosynthetically active radiation (PAR,

Fig. 3.9 Specific absorption coefficient (in situ), corresponding to 1mg chlorophyll am~3, for oceanic phytoplankton (after Morel and Prieur, 1977).
Fig. 3.10 Mycosporine-glycine structure.

~400-700 nm). Absorption in the near-UV, below 400 nm, even though it contributes little to photosynthesis, is nevertheless significant for primary production since it is energy in this waveband that is particularly effective at causing photo-inhibition (§10.1). A protective response that phytoplankton have evolved over the eons of geological time is to synthesize sunscreens in the form of UV-absorbing amino acids known as mycosporine-like amino acids, commonly abbreviated as MAAs. They have absorption peaks in the 300 to 400 nm range. Shinorine and mycosporine-glycine are typical examples, both found in algae: the structure of the latter compound is shown in Fig. 3.10. Figure 3.11 shows the absorption spectrum of a phytoplankton sample collected at 10 m depth over the Chatham Rise, East of New Zealand, in the South Pacific Ocean: the large peak between 300 and 360 nm is plausibly attributable to MAAs. Llewellyn and Harbour (2003) found that in phytoplankton populations in the English Channel, MAAs were present year round, but

Wavelength, nm

Fig. 3.11 Absorption spectrum of a natural oceanic phytoplankton population, showing a peak between 300 and 360 nm in the UV, plausibly attributable to the presence of mycosporine-like amino acids. Sample taken at 10 m depth over the Chatham Rise, South Pacific Ocean (Shooter et al, 1998). The spectrum was measured on a glass fibre filter, and has been corrected for the contribution of inanimate particulate matter. The curve is for the specific absorption coefficient (m2mgchl a-1), assuming a value of 0.02 m2mgchl a-1 at 675 nm.

Wavelength, nm

Fig. 3.11 Absorption spectrum of a natural oceanic phytoplankton population, showing a peak between 300 and 360 nm in the UV, plausibly attributable to the presence of mycosporine-like amino acids. Sample taken at 10 m depth over the Chatham Rise, South Pacific Ocean (Shooter et al, 1998). The spectrum was measured on a glass fibre filter, and has been corrected for the contribution of inanimate particulate matter. The curve is for the specific absorption coefficient (m2mgchl a-1), assuming a value of 0.02 m2mgchl a-1 at 675 nm.

concentrations increased rapidly in the spring and summer to levels (maximum 8.5 mgm-3) exceeding those of chlorophyll a (maximum 3.6 mgm-3). Absorption spectra of phytoplankton from the 0 to 50 m layer in the Tropical North Atlantic off Bermuda, collected in the summer show a strong UV absorption peak centred on ~320 nm, typical of MAAs, which is virtually absent in the winter population.960 The spectrum of a sample of the toxic dinoflagellate, Gymnodinium catenatum, collected during a bloom of that species in the Huon estuary, Tasmania, had a substantial peak in the UV (300-400 nm) believed to be due to MAAs.241 By contrast, a sample of the phytoplankton collected at a time when diatom species were dominant, had a much smaller peak in this region.

0 0

Post a comment