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Oceans have a fundamental role in the global climate system on account of their capacity to store and transport heat and absorb and emit tract gases, which directly or indirectly affect the earth's radiation budget. The ocean biota, in particular the phytoplankton, contribute significantly to these processes: photosynthetic carbon fixation and the downward transport of organic carbon increase the storage capacity of oceans for the major greenhouse gas, carbon dioxide. Changes in the oceanic cycling of carbon are believed to have been a major factor in the control of atmospheric carbon dioxide concentrations during the Pleistocene ice age cycles, although biological mechanisms that contribute to these changes are still not well understood.

Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (ΣPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1τPSII), the chlorophyll-specific maximum rate of carbon fixation (P*max), and the light-saturation intensity (Ek), all decreased from the surface to the subsurface chlorophyll maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. [ABSTRACT FROM AUTHOR]