Low temperature sensitivity of picophytoplankton P : B ratios and growth rates across a natural 10°C temperature gradient in the oligotrophic Indian Ocean

Abstract

We investigated temperature sensitivities of picophytoplankton growth along a natural 10°C (18–28°C) temperature gradient in the eastern Indian Ocean characterized by deep mixing and consistently low dissolved nitrogen. Population biomass (B), cell carbon, and chlorophyll were measured by flow cytometry. Instantaneous growth (μ) and production (P) were calculated from dilution incubations at four light levels. Contrary to most empirical and theoretical predictions, Prochlorococcus, the biomass dominant, showed insignificant temperature sensitivity, with nominal Q10 values of 1.06 and 1.18 for P : B and μ, respectively, and activation energies (Ea) of 0.05 and 0.12 eV. Q10 and Ea values for Synechococcus (1.36–1.42 and 0.23–0.27 eV) were also below prediction, and picoeukaryotes showed high variability, including negative rates suggesting lytic cycles, at high temperature. We emphasize the importance of using adapted communities in natural environmental gradients to test climate predictions and hypothesize that mortality defenses are a significant selection criterion in balanced oligotrophic systems.