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

Michael R Landry,Claire H Davies,Raleigh R Hood,Lynnath E Beckley,Karen E Selph

doi:10.1002/lol2.10224

Abstract

AbstractWe 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.

Highlights

We investigated temperature sensitivities of picophytoplankton growth along a natural 10C (18–28C) temperature gradient in the eastern Indian Ocean characterized by deep mixing and consistently low dissolved nitrogen
We investigated microbial food-web interactions along a transect in the eastern Indian Ocean (IO) in 2019 and here report biomass (B), production (P), and growth rate estimates for photosynthetic bacteria and small EUK along an oligotrophic thermal gradient of 10C
The present study considers 17 stations (35–11.5S) located north of the subtropical front, where the euphotic zones were oligotrophic and well mixed to at least the penetration depth of 7.6% incident light (% Io measured as PAR, photosynthetically active radiation)

Summary

Introduction

We investigated temperature sensitivities of picophytoplankton growth along a natural 10C (18–28C) 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. 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 below prediction, and picoeukaryotes showed high variability, including negative rates suggesting lytic cycles, at high. Author Contribution Statement: MRL and LEB conceived the study. MRL analyzed results and drafted the manuscript. All authors contributed to comments and edits of the manuscript

Methods

Results

Conclusion