The first aim of this chapter is to summarise the biochemical basis of photosynthesis in planktic algae and to review the physiological sensitivities of carbon fixation and assimilation under the environmental conditions experienced by natural populations of phytoplankton. These fundamental aspects of autotrophy are plainly relevant to the dynamics and population ecology of individual algal species, functioning within the constraints set by temperature and by the natural fluxes of light energy and inorganic carbon. They are also relevant to the function of entire pelagic systems as, frequently, they furnish the major source of energy, in the form of reduced carbon, to heterotrophic consumers. The yields of fish, birds and mammals in aquatic systems are ultimately related to the harvestable and assimilable sources of carbon bonds. In turn, the energy and resource fluxes through the entire biosphere are greatly influenced by pelagic primary producers, impinging on the gaseous composition of the atmosphere and the heat balance of the whole planet.
Here, we shall be concerned with events at the population, community and ecosystem levels. However, it is necessary to emphasise at the start of the chapter that recent advances in understanding of planetary carbon stores and fluxes assist our appreciation of the relative global importance of aquatic photosynthesis. To those biologists of my generation brought up with the exclusive axiom that animals derive their energy by respiring (oxidising) the carbohydrates and proteins manufactured (reduced) by photosynthesising plants, the presently perceived realities of aquatic-reductant fluxes may seem quite counter-intuitive. The original postulate is not in error: it succeeds in describing how a section of the trophic relationships of the pelagic is conducted. It is just that it is far from being the whole story. For instance, the photosynthetic reduction of carbon is antedated by several hundreds of millions of years (>0.4 Ga: Falkowski, 2002) by the chemosynthe-sis by Archaeans of reduced carbon. This continues to be maintained in deep-ocean hydrothermal vents, where there is no sunlight and only minimal supplies of organic nutrients (Karl et al., 1980; Jannasch and Mottl, 1985). Even in the upper, illuminated waters of lakes and seas, most (perhaps 60-95%) of the organic carbon present is not organismic but in solution (Sugimura and Suzuki, 1988; Wetzel, 1995; Thomas, 1997). A large proportion of this is humic in character and, thus, supposed to be derived from terrestrial soils and ecosystems. True, much of this carbon would have been reduced orginally through terrestrial photosynthesis but the extent of its contribution to the assembly of marine biomass is still not fully clear. Setting this aside, the direct phagotrophic transfer of photosynthetic primary products from phytoplankton to zoo-planktic consumers is not universally achieved in the pelagic but is, in fact, commonly mediated by the activities of free-living microbes. Thus, the dynamic relationships among phytoplankton and their potential phagotrophic consumers acquire a new interpretative significance, which is to be addressed in this and later chapters.
The present chapter prepares some of the ground necessary to understanding the relation of planktic photoautotrophy to the dynamics of phytoplankton populations. After considering the biochemical and physiological basis of photo-synthetic production, the chapter compares the various limitations on the assembly of photoau-totrophic biomass in natural lakes and seas, and it considers the implications for species selection and assemblage composition.
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