Regulation of primary productivity in the Southern Ocean: Phytoplankton photosynthetic characteristics from individual cell measurements

Robert J. Olson and Heidi M. Sosik

Central to the mission of the US Joint Global Ocean Flux Study in the Southern Ocean is an understanding of the composition of the planktonic food web, and the efficiency with which it transports carbon to the deep ocean. One of the specific questions being posed is the mechanism of regulation of phytoplankton growth and productivity in this area; light limitation, micronutrient availability, and grazing pressure have all been proposed to explain the persistence of high macronutrient concentrations and the relatively low productivity often observed.

To distinguish among these mechanisms, we are examining photosynthetic characteristics of phytoplankton in natural populations under a range of environmental conditions. Specifically, we estimate, from chlorophyll fluorescence induction measurements of individual cells, photochemical efficiency, and absorption cross-sections of photosystem 2 (PS2) as well as relative pigment content per cell for different groups of phytoplankton, that can be distinguished either microscopically or flow cytometrically.

These measurements, carried out on samples from depth profiles and experimentally manipulated assemblages of phytoplankton, should allow us to distinguish among limiting factors for phytoplankton growth. For example, limitation by micronutrients should result in low photochemical efficiency (reflecting loss of functioning PS2 reaction centers), whereas low-light limitation (due to deep mixing) would be indicated by high fluorescence (i.e., pigment content) in near-surface cells. Grazing control would be implied if photosynthetic characteristics indicate no limitation of intrinsic growth rates.

The individual cell nature of our measurements allow us to evaluate the condition of different groups of phytoplankton (or different cells) in a given water sample, and to examine unambiguously the effects of environmental gradients or experimental treatments on given species. In addition, in the course of our measurements the composition of the phytoplankton community can be characterized by microscopic and flow cytometric analysis.

Observations of the physiological state of cells in natural assemblages and experiments, combined with information on community structure, allow us to evaluate the relative importance of light, iron, and grazing as limiting factors for productivity in the Southern Ocean, and to better interpret bulk measurements of phytoplankton properties, such as can be obtained in a high resolution survey with FRR (fast repetition rate) fluorometry.

This research was supported by the National Science Foundation.

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