Essential biochemistry of photosynthesis

It has been stated or implied several times already that the paramount requirement of photoau-totrophic plankton to prolong residence in, or gain frequent access to, the upper, illuminated layers of the pelagic is consequential upon the requirement for light. The need to capture solar energy in order to drive photosynthetic carbon fixation and anabolic growth is no different from that experienced by any other chlorophyll-containing photoautotroph inhabiting the surface of the Earth. Indeed, the mechanisms and ultrastructural provisions for bringing this about constitutes one of the most universally conserved processes amongst all photoautotrophic organisms. On the other hand, to achieve, within the bounds of an effectively opaque and fluid environment, a net excess of energy harvested over the energy consumed in metabolism requires certain features of photosynthetic production that are peculiar to the plankton. Thus, our approach should be to rehearse the fundamental requirements and sensitivities of photosynthetic production and then seek to review the aspects of the pelagic lifestyle that constrain their adaptation and govern their yields.

Enormous strides in photosynthetic chemistry have been made, especially over the last 30 years or so, especially at the molecular and submolecular levels (Barber and Anderson, 2002). This progress is not likely to stop so that, undoubtedly, whatever is written here will have soon been overtaken by new information. At the same time, it is possible to predict that future progress will concern the biochemical and biophysical intricacies of control and regulation more than the broad principles of process and order-of-magnitude yields, which are generally accepted by physiological ecologists. Thus, the contemporary biochemical basis for assessing phytoplankton production will continue to be valid for some time to come.

Photosynthesis comprises a series of reactions that involve the absorption of light quanta (photons); the deployment of power to the reduction of water molecules and the release of oxygen; and the capture of the liberated electrons in the synthesis of energy-conserving compounds, which are used ultimately in the Calvin cycle of carbon-dioxide carboxylation to form hexose (Falkowski and Raven, 1997; Geider and MacIntyre, 2002). The aggregate of these reactions may be summarised:

As with most summaries, Eq. (3.1) omits not merely detail but several important intermediate feedback switches, involving carbon, oxygen and reductant, all of which have a bearing upon the output products and their physiological allocation in active phytoplankters. These are best appreciated against the background of the supposed 'normal pathway' of photosyn-thetic electron transport. The latter was famously proposed by Hill and Bendall (1960). Their z-model of two, linked redox gradients (photosystems) has been well substantiated, biochemically and ultrastructurally. In the first of these (perversely, still referred to as photosystem II, or PSII), electrons are stripped, ultimately from water, and transported to a reductant pool. In the second (photosystem I, or PSI), photon energy is used to re-elevate the electrochemical potential sufficiently to transfer electrons to carbon dioxide, through the reduction of nicoti-namide adenine dinuceotide phosphate (NADP to NADPH).

The (Calvin cycle) carbon reduction is based on the carboxylation reaction. Catalysed by ribu-lose 1,5-biphosphate carboxylase (RUBISCO), one molecule each of carbon dioxide, water and ribulose 1,5-biphosphate (RuBP) react to yield two molecules of the initial fixation product, glycer-ate 3-phosphate (G3P). This latter reacts with ATP and NADPH to form the sugar precursor, glycer-aldehyde 3-phosphate (GA3P), which now incorporates the high energy phosphate bond. In the remaining steps of the Calvin cycle, GA3P is further metabolised, first to triose, then to hexose, and RuBP is regenerated.

At the molecular level, photosynthetic reactivity is plainly sensitive to the supply of carbon and water, the photon harvesting and, like all other biochemical processes, to the ambient temperature. Measurement of photosynthesis may invoke a yield of fixed carbon, the quantum efficiency of its synthesis (yield per photon), or the amount of oxygen liberated. None of these is any longer difficult to quantify but the difficulty is still the correct interpretation of the bulk results. It is still necessary to consider carefully the regulatory role of the ultrastructural and biochemical components that govern the photosynthesis of phytoplankton. Special attention is directed to the issues of photon harvesting, the internal electron transfer, carbon uptake, RUBISCO activity and the behaviour of the regulatory safeguards that phytoplankters invoke in order to function in highly variable environments.

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