Pelagic primary production is the outcome of complex interplay among biochemical, physiological and ecological processes that include photosynthesis and the large-scale dynamics of various forms of carbon. Photosynthesis is the photochemical reduction of carbon dioxide to carbohydrate, drawing upon radiant energy to synthesise a store of potential chemical energy, pending its discharge when the carbohydrate (or its derivatives) is oxidised (respiration). As in other pho-toautotrophs, algae and photosynthetic bacteria employ two sequenced, chlorophyll-based photosystems. In the first, electrons are stripped from water and transported to a reductant pool. In the second, photon power re-elevates the electrochemical potential sufficiently to transfer electrons to carbon dioxide, through the reduction of nicotinamide adenine dinuceotide phosphate (NADP to NADPH). The carbon reduction process is built around the cyclical regeneration of ribu-lose 1,5-biphosphate (RuBP). RuBP is first combined with (carboxylated) carbon dioxide and water to form sugar precursors, under the control of the enzyme RUBISCO, and from which hexose is generated and RuBP is liberated (the Calvin cycle). The hexose may be polymerised (e.g. to starch or glycogen) or stored.

The theoretical photosynthetic quantum yield is 1 mol carbon for 8 mol photon, or 0.125 mol C (mol photon captured)-1. Actual efficiency is closer to 0.08 mol C (mol photon)-1, equivalent to 2.821 kJ (mol C fixed)-1, or ~470kJ (gC)-1. The maximum rates of photosynthesis are related to the rate of electron clearance from the reductant pool (and which responds to the photon flux), as well as to an adequate supply of CO2 to the RUBISCO reaction (if a concentration of >0.01 mM is not maintained, the enzyme acts as an oxygenase).

Physiologically, photosynthetic rate is sensitive to temperature, to light and carbon dioxide availability. Even at 30 °C, given saturating light and an adequate carbon supply, photosynthesis achieves <20mgC (mg chla)-1 h-1. Maximum photosynthetic rates are generally halved for each 10 °C drop in temperature. Below saturation (usually <150 mol photons m-2 s-1), photosynthetic rates fall in a light-dependent manner, in the proportion 6-18 mg C (mg chla)-1 (mol photon)-1 m2. Carbon dioxide concentrations below air saturation may also limit photosynthetic rates. Some algae are extremely efficient in adapting to photon harvesting under very low light fluxes or in the fluctuating light experienced by phy-toplankton entrained in mixed water columns. Some algae are restricted to carbon dioxide as a carbon source and are sensitive to the very low concentrations experienced at pH >8. Others can use bicarbonate or employ energy-consuming carbon-concentrating mechanisms to focus the limited fluxes at the sites of synthesis. In this way, low light and low carbon availability select strongly for well-adapted species.

On a local basis, it is possible to calculate the carrying capacity of the environment and the rates of biomass assembly that might be sustainable. Down-mixing and light dilution place important limits on both. The carbon flux from the atmosphere is potentially - and, at times, is - a constraint on area-specific photosynthesis but is avoided in most lakes and at most times by inflowing CO2-saturated and internal recycles. Indeed, most smaller lakes probably release more CO2 to the atmosphere than they take from it. They are considered to be net het-erotrophic. Only in very large, oligotrophic systems does the sedimentary export of carbon balance the atmospheric inorganic uptake flux (at some 50-90gCm-2 a-1).

Globally, pelagic photosynthesis accounts for around 45% of the planetary carbon fixation. In some circumstances, when photosynthesis is constrained (especially by light dilution) the carbon is invested in the growth of the pho-toautotroph. These organisms become potential food to pelagic grazers. In many other cases, light saturation or nutrient depletion result in carbon fixation in excess of contemporaneous growth requirements and photosynthate is either reoxidised or excreted as dissolved organic carbon (DOC). This augments an already relatively large pool of dissolved humic matter (DHM) but presents a much more amenable substrate for pelagic bacteria. Like those of photoautotrophs, concentrations of heterotrophic bacteria reflect the availability of inorganic nutrients and there is mutual competition. However, bacterial growth is often more carbon limited while the main producers are usually nutrient limited. Besides the mutualism that this situation engenders, the acquisition by bacteria of organic carbon products of the phytoplankton and the consumption of bacteria by microzooplankton represents the main route of pelagic photosynthate to the pelagic food web. This 'microbial loop' commonly dominates the first steps in the food chain, is certainly of great antiquity, and should no longer be regarded as a special exception to alga-herbivore-fish linkages. It is the latter that are the exception, being sustainable only in relatively resource-rich conditions.

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