Photosynthesis and respiration in marine phytoplankton: Relationship with cell size, taxonomic affiliation, and growth phase

We determined the rates of photosynthesis and respiration in batch cultures of 22 marine phytoplankton species from five phyla covering a range of 7 orders of magnitude in cell size. Rates were determined during the exponential growth phase and also during the stationary phase, when cell growth was limited by nitrogen availability. We observed, in all growth phases, a curvature in the size scaling of carbon fixation, such that the relationship between carbon-specific photosynthesis and cell size was unimodal, with the highest rates being measured in intermediate-size species. The log–log relationship between individual metabolic rates and cell size showed an overall lineal pattern with a slope equal or near 1, irrespective of whether volume or carbon is used as a metric for cell size. Thus, our results demonstrate that when small species (