What is the limit for photoautotrophic plankton growth rates?

Knowing the potential maximum photoautotrophic growth rate for planktonic primary producers is fundamental to our understanding of trophic and biogeochemical processes, and of importance in applied phycology. When day-integrated C-specific growth is considered over natural light:dark cycles, plausible RuBisCO activity (Kcat coupled with cellular RuBisCO content) caps growth to less than a few doubling per day. Prolonged periods of C-specific growth rates above ca. 1.3 d−1 thus appear increasingly implausible. Discrepancies between RuBisCO-capped rates and reported microalgal-specific growth rates, including temperature–growth rate relationships, may be explained by transformational errors in growth rate determinations made by reference to cell counts or most often chlorophyll, or by extrapolations from short-term measurements. Coupled studies of enzyme activity and day-on-day C-specific growth rates are required to provide definitive evidence of high growth rates. It seems likely, however, that selective pressure to evolve a RuBisCO with a high Kcat (with a likely concomitant increase in Km for CO2) would be low, as other factors such as light limitation (developing during biomass growth due to self-shading), nutrient limitations, CO2 depletion and pH elevation, would all rapidly depress realized specific growth rates.