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Replication rates under sub-ideal conditions

The corollary of the previous section is that the achievement of maximal growth rates is not dependent upon maximal photosynthetic rates and nutrient uptake rates being achieved: the resource-gathering provisions have capacity in excess over the heaviest resource demands of growth. This luxury can be enjoyed only under ideal conditions of an abundant supply of resources, which scarcely obtain in the natural environments of phytoplankton: darkness alternates with periods of daylight, depth is equated with (at best) sub-saturating light levels and a balanced, abundant supply of nutrients is extremely rare. The question that arises relates to the levels at which resources start to impose restrictive 'limitations' on the dynamics of growth. The question is not wholly academic, as it impinges on the prevalent theories about competition for light and nutrients. Some, indeed, may need revision, while growth-simulation models founded upon resource harvesting are likely to be erroneous, except at very low nutrient levels.

5.4.1 The effect of truncated photoperiod

Perhaps the most obvious shortcoming that real habitats experience in comparison with idealised cultures is the alternation between light and dark: this is avoidable in nature but only at polar latitudes and, even then, for just a short period of the year. It also seems likely, following the calculations in Section 5.3.3, that growth performance is most vulnerable to interruptions to net photosynthetic output and the supply of reduced-carbon skeletons to cell assembly. It is self-evident that photoperiod truncation by phases of real or effective darkness must eventually detract from the ability to sustain rapid growth. For small algae, even maintaining any adequate reserve of condensed photosynthate (such as starch, glyco-gen, paramylum, etc.) soon becomes problematic during extended periods of darkness, whereas all light-independent anabolism will become rapidly starved of newly fixed carbon.

Of the responses to light/dark alternation that might be anticipated, the most plausible is that the rate of cell replication becomes a direct function of the aggregate of the light periods. If the cell required a 24-h period of continuous light exposure in which to complete one generation, then, other things being equal, a day/night alternation of 12 h should determine that at least two photoperiods must be passed (not less than 36 h real time) before the cell can complete its replication. By extension of this logic, day/night alternations giving 6 h of light to 18 h of darkness, or 3 h to 21 h will each double the real time of replication. We may note that, on the basis of this logic, alternations of 6 h light to 6 h dark (or, for that matter, 1-minute alternations from light to dark) should not extend the generation time beyond the 12-h cycle of alternations.

Surprisingly, there is not a lot of experimental evidence to confirm or dismiss these conjectures. Although there are indications that the logic is neither ill-founded nor especially unrealistic, it does exclude two influential effects. One is the ongoing maintenance requirement of the cell - all those phosphorylations have to be sustained! The power demand, which persists through the hours of darkness and light, is met through the respirational reoxidation of carbohydrate. Unfortunately, it is still difficult to be certain about precisely how great that demand might be, at least partly because of historic difficulties in making good measurements and, in part, because sound testable hypotheses about maintenance have been lacking (see Sections 3.3.1, 3.3.2 and 3.5.1). Most of the available information about the respiration rates of plank-tic algae and Cyanobacteria comes from the 'dark controls' to experimental measurements of pho-tosynthetic rate (of, for instance, Talling, 1957b; Steel, 1972; Jewson, 1976; Robarts and Zohary, 1987). Rates, appropriately expressed as a proportion of chlorophyll-specific Pmax, typically range between 0.04 and 0.10, over a reasonable range of customary temperatures. Translating from the chlorophyll base to one of cell carbon, Reynolds (1990) deduced specific basal metabolic rates for Chlorella, Asterionella and Microcystis at 20 °C of (respectively) 1.3, 1.1 and 0.3 x 10-6 mol C (mol cell C)-1 s-1. These fit sufficiently well to a regression parallel to the one describing maximal replication rate (Eq. 5.4, Fig. 5.2), that it is possible to hypothesise that basal respiration at 20 °C (R20) conforms to something close to:

In this case, R is roughly equivalent to 0.055 r'. Moreover, there is no a priori reason to suppose that basal respiration rate necessarily carries a temperature sensitivity different from other biochemical processes in the cell (that is, Qjo ~ 2). It is suggested, again provisionally, that the proportionality of Eq. (5.6) holds at other temperatures, i.e. Re ~ 0.055r'e.

In the context of growth, however, the value is academic rather than deterministic, for it is by no means proven that the basal respiration applies equally in the light and in the dark. The implication of photosynthetic quotients (PQ) of 1.1-1.15 (Section 3.3.2) is that 10-15% more oxygen is generated in photosynthetic carbon fixation than is predicted by the stoichiometric fixed-carbon yield and that the 'missing' balance represents respirational losses in the light. Ganf (1980) observed that when Microcystis colonies were transferred from zero to saturating-light intensities, their respiration rate accelerated rapidly, from ~25 to ~50 |imol O2 (mg chla)-1 h-1, and did not fall back until after the colonies had been transferred back to the darkness. The time taken to return to base rate was proportional to the time spent at light saturation. Insofar as the same inability to store excess photosyn-thate requires some homeostatic defence reaction, such as accelerated respiration, glycolate excretion, or photorespiration (see Section 3.3.4), the carbon losses from cells must be expected often to exceed basal metabolism. The experiments of Peterson (1978) show how easily the coupling between respiration and growth is broken and why the fractions of photosynthetic production lost to respiration, reported in the literature (reviewed by Tang and Peters, 1995), are so variable.

The second reservation about extrapolating growth over summated photoperiods concerns the photoadaptation of cells. Culturing cells under ideal irradiances actually tends to lead to a reduction in the cell-specific chlorophyll content, often to as little as 4 mg (g ash-free dry mass)-1 ; Reynolds, 1987a; C : Chl ~100). On the other hand, populations exposed to low light intensities are able to adapt by increasing their pigment content (see Section 3.3.4). It is almost as if the cell's photosynthetic potential varies to match the growth requirement, rather than the opposite, as is generally presumed. Turning off the light for a part of the day provokes photoadaptative responses in respect of the shortened pho-toperiod to the extent that the energy available to invest in growth, normalised per light hour, is compensated, as shown by the data of Foy et al. (1976) (see Fig. 3.16).

There are limits to this argument, of course. On the other hand, the abilities of certain diatoms (Talling, 1957b; Reynolds, 1984a) and, especially, of some of the filamentous Cyanobac-teria, such as Planktothrix (formerly Oscillatoria) agardhii (Jones, 1978; Foy and Gibson, 1982; Post et al., 1985) to function on very low light doses has been well authenticated. The curves plotted in Fig. 5.4 represent a selection of experimentally derived fits of specific growth rates of named phytoplankters at 20 °C in cultures fully acclimatised to the daily photon fluxes noted. Far from being a linear function of light dose, except at very low average photon fluxes, the more adaptable species are able to increase biomass-specific photosynthetic efficiency so that the growth demand can continue to be saturated at significantly lowered light intensities.

How far the photosynthetic apparatus can be pushed to turn photons into new biomass is ultimately dependent upon the integrated

Figure 5.4

Light dependence of growth rate at 20 ° C, as a function of intensity, in a selection of freshwater phytoplanktic species. The algae are: Ana, Anabaena flos-aquae; Aphan, Aphanizomenon flos-aquae; Coel, Coelastrum microporum; Diet, Dictyosphaerium pulchellum; Fra b, Fragilaria bidens; Lim red, Limnothrix redekei; Mic, Microcystis aeruginosa; Monor, Monoraphidium sp.; Ped b, Pediastrum boryanum; Pla ag, Planktothrix agardhii; Scen q, Scenedesmus quadricauda. Redrawn with permission from Reynolds (1997a).

Figure 5.4

Light dependence of growth rate at 20 ° C, as a function of intensity, in a selection of freshwater phytoplanktic species. The algae are: Ana, Anabaena flos-aquae; Aphan, Aphanizomenon flos-aquae; Coel, Coelastrum microporum; Diet, Dictyosphaerium pulchellum; Fra b, Fragilaria bidens; Lim red, Limnothrix redekei; Mic, Microcystis aeruginosa; Monor, Monoraphidium sp.; Ped b, Pediastrum boryanum; Pla ag, Planktothrix agardhii; Scen q, Scenedesmus quadricauda. Redrawn with permission from Reynolds (1997a).

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