Effects of variation in cellcolony size and shape

We have seen in the previous section that the absorption coefficients and hence the light-harvesting efficiency of the photosynthetic pigments are lower when they are segregated into packages than when they are uniformly dispersed. The kind of packages within which the photosynthetic pigments occur in the aquatic biosphere vary greatly in size, shape and internal pigment concentration, however, and so we need some more general rules to assist our understanding of the light-intercepting capabilities of these different forms.

We already have two rules:

(ii) Where sp is the projected area of the particle (cell or colony) in the direction of the light beam, and Ap is the particle absorptance, then at constant cell/colony size and shape (constant sp), when Ap is increased (by raising the intracellular pigment concentration or altering the wavelength), asus/asol decreases.

It is possible also to deduce the following:

(iii) At constant total pigment and biomass in the system, when both sp and Ap are increased (by decreasing the number and thus increasing the size of the cells or colonies, without changing their shape), asus/ asol decreases.

(iv) At constant total pigment in the system, when biomass is increased (by increasing cell/colony number or volume, without changing the shape), asus/asol increases.

(v) At constant total pigment and biomass in the system, and constant cell/colony volume, as the shape becomes more extended, e.g. more elongated, so asus/asol increases.

In both (iv) and (v) the diminution in absorptance associated with the dilution of the pigment into more biomass, or the stretching-out of the cells/colonies, is proportionately less than the increase in projected area and so, overall, the absorption cross-section spAp increases.

Having examined the underlying mechanism, and the general rules governing the expression, of the package effect we shall now consider, in quantitative terms, just how much it can influence the light-harvesting capacity of planktonic algae. A convenient parameter to look at is the absorption cross-section of a given amount of algal biomass organized into packages of different sizes and shapes.695 Figure 9.2a shows the absorption cross-section at wavelengths from 350 to 700 nm calculated for model randomly oriented blue-green algal colonies of various geometrical forms: the volume of biomass is 100000 mm3 in each case, and the pigment concentration is constant at 2% (of the dry mass) chlorophyll a. It can be seen that the least efficient arrangement for light collection is the large, spherical colony, 58 mm in diameter (lowest curve). Matters are improved somewhat if the spheres are elongated into prolate spheroids, 230 x 29 mm. A much greater increase in efficiency is achieved if the spheres are transformed into long, thin, cylindrical filaments, 3500 mm long and 6 mm in diameter. Only a marginal further increase in light-harvesting capacity is brought about if the filaments are chopped up into about 900 pieces, each piece being rounded up into a sphere of 6 mm diameter (uppermost curve). The advantages of the more extended package are much more evident at strongly absorbed, than weakly absorbed, wavelengths. For example, the ratio of the absorption cross-section of the thin cylinder to that of the large sphere is 3.82 at 435 nm but only 1.16 at 695 nm. A given change in shape cannot increase the absorption cross-section by more (proportionately) than it increases the average projected area. As the light absorption by the cell is intensified (i.e. as Ap approaches 1) so the effect of a given geometrical change on absorption cross-section tends to become identical to the effect on average projected area.

As another quantitative illustration of the significance of the package effect, Fig. 9.2b shows how the specific absorption coefficient per mg chlorophyll a present in the form of a suspension of spherical cells or colonies, at its red peak, decreases as the diameter of the cells/colonies increases.

Algal Suspension Absorbance

Fig. 9.2 Effect of size and shape on light absorption properties of phyto-plankton. (a) The absorption cross-section spectra of randomly oriented blue-green algal colonies of various shapes and sizes (calculated by Kirk (1976a) for idealized colonies containing 2%, dry mass, chlorophyll a). In every case the data apply to 100 000 p,m~3 of algal volume. This corresponds

Fig. 9.2 Effect of size and shape on light absorption properties of phyto-plankton. (a) The absorption cross-section spectra of randomly oriented blue-green algal colonies of various shapes and sizes (calculated by Kirk (1976a) for idealized colonies containing 2%, dry mass, chlorophyll a). In every case the data apply to 100 000 p,m~3 of algal volume. This corresponds

That the package effect really does have a major influence on the light-harvesting capability of algal cells has now been demonstrated in numerous experiments comparing different phytoplankton species varying in cellular size and pigment content, or comparing cells of a given species having a range of pigment contents due to variation in growth irradi-ance.101,284,379,516,614,947,954,982,1173,1268 A striking example in the field has been described by Robarts and Zohary (1984): in Hartbeespoort Dam (South Africa), as the colony size of the dominant blue-green alga Micro-cystis aeruginosa increased, there was a corresponding increase in eupho-tic depth resulting from the less efficient light interception of biomass distributed in larger packages. Phytoplankton populations in the coastal waters of the Antarctic Peninsula appeared, on the basis of chlorophyll-specific vertical attenuation coefficients measured at wavelengths through the photosynthetic spectrum, to have a markedly greater package effect than populations from temperate oceans: Mitchell and Holm-Hansen (1991a) attributed this to the presence of larger cells with high cellular pigment concentration resulting from chronic low-light adaptation in nutrient-rich waters. In another Antarctic oceanic region, the southwestern Ross Sea, Arrigo et al. (1998) found that a bloom dominated by large-celled cryptophytes had substantially lower chlorophyll-specific absorption values, both in the red and the blue spectral regions, than blooms of the much smaller diatoms or of the prymnesiophyte, Phaeocystis antarctica. In the Huon River estuary, Tasmania, Clementson et al. (2004) found that the specific absorption coefficients (per mg chlorophyll) of blooms dominated by diatoms were substantially greater than those of blooms dominated by the much larger cells of the dinofla-gellate, Gymnodinium catenatum, the difference being —2^-fold at the 675 nm chlorophyll a peak.

to one particle in the cases of the 57.6 mm diameter spheres (•), the 230.4 x 28.8 mm prolate spheroids (A) and the 3537 x 6 mm cylinders (A); and to 884 particles in the case of the 6 mm diameter spheres (o). (b) Specific absorption coefficient of phytoplankton chlorophyll at the red maximum (670-680 nm) as a function of cell or colony size (Kirk, unpublished). The values were obtained from the particle absorptance values calculated using the equation of Duysens (1956), assuming that the cells/colonies contained 2% dry mass (~4 gl—1cell volume) of chlorophyll a and that the (natural logarithm) specific absorption coefficient of chlorophyll a in solution at its red peak is about 0.0233m2mg—1, as it is, for example, in diethyl ether.410 Similar calculations have been carried out by Morel and Bricaud (1981).

Geider and Osborne (1987) showed that for the relatively small diatom Thalassiosira (~5 mm diameter) grown in culture, the package effect reduced light-absorption efficiency by 50% at the blue absorption maximum (435 nm), and by 30% at the chlorophyll a red maximum (670 nm), but had no significant effect at the absorption minimum (600 nm): larger diatoms would show an even greater package effect. In the picoplankton (<2 mm diameter), such as the unicellular cyanophytes and prochlorophytes of the ocean, however, the package effect should be of no significance.708

It is now recognized that the influence of the package effect on the light-absorption efficiency of oceanic phytoplankton varies in a systematic manner with the nutrient status of the water. In the western North Atlantic, for a large number of stations with chlorophyll a concentrations ranging from 0.1 to 8.0 mgm-3 (Sargasso Sea to Gulf of Maine), Yentsch and Phinney (1989) found that the absorption coefficient due to particles (predominantly phytoplankton) at 670 nm increased with chlorophyll concentration, not linearly but in accordance with [chl]0758, indicating that there was a progressive decrease in the specific absorption coefficient as total phytoplankton chlorophyll increased. They attributed this to an increased package effect associated with the greater proportion of larger cells in the more eutrophic waters. Bricaud et al. (1995), using a more wide-ranging oceanic database (815 stations), and correcting the spectra of the particulate matter for the contribution of non-algal material, arrived at an equation of the form a$*(X) = A(A)[Ch1]-B(1)

for wavelengths from 400 to 698 nm, where [Chl] is the concentration of phytoplankton chlorophyll a in mgm-3, af*(A) is the specific absorption coefficient of the phytoplankton (m2 mg chlorophyll a-1), and A (A) and B (A) are coefficients applicable at each wavelength. A (A) and B(A) were 0.403 and 0.332, respectively at 440 nm, and 0.0201 and 0.158 at 675 nm. They found that, accompanying an increase in [Chl] from 0.02 to 25 mg m-3 across this range of oceanic environments, the package effect brought about a three-fold reduction in the height of the red absorption band. In the blue region of the spectrum the reduction was around ten-fold: this, however, was due, not only to the package effect, but also to variations in the proportion of accessory pigments, including an increase in the cellular concentration of photoprotective carotenoids in oligotrophic waters.

For a data set of 465 samples from a range of estuarine, coastal and oceanic stations, Staehr and Markager (2004) calculated aph*, the numerical mean of the chlorophyll-specific phytoplankton absorption coefficient (m2 mg chlorophyll a-1), over the 400 to 700 nm range (not to be confused with ap(z), the effective absorption coefficient of the phyto-plankton for PAR at depth z, see eqn 9.9, below), and found that it decreased with increasing phytoplankton concentration (mgm-3), in accordance with

which may be regarded as a measure of the variation in package effect with phytoplankton concentration, for absorption across the whole photosynthetic waveband.

In the southern Beaufort Sea, Canadian Arctic, Matsuoka et al. (2009) found that as ambient light levels declined from autumn when open water still existed, to early winter when sea ice cover formed, there was an increase in a^*(X) across the spectrum, resulting from a biological selection of smaller-sized phytoplankton, more efficient at absorbing light.

In aquatic macrophytes also, the efficiency of light absorption is influenced by the package effect, but because of the complexities imposed by the three-dimensional structure, does not lend itself to mathematical analysis. In two seagrass species sampled at different times of the year, Cummings and Zimmerman (2003) found that a five-fold increase in leaf chlorophyll content (in seasonally low light) was accompanied by only an 18.5% increase in photosynthetic light-harvesting efficiency, a disparity which they attributed to the package effect. In brown, red and green macroalgae, Grzymski et al. (1997), by comparing absorption spectra of intact tissue with those of suspensions of sonically disrupted thylakoids, concluded that the package effect was responsible for a significant flattening of the absorption peaks in these species.

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