Warming the phycosphere: Differential effect of temperature on the use of diatom-derived carbon by two copiotrophic bacterial taxa

Abstract

Heterotrophic bacteria associated with microphytoplankton, particularly those colonizing the phycosphere, are major players in the remineralization of algal-derived carbon. Ocean warming might impact dissolved organic carbon (DOC) uptake by microphytoplankton-associated bacteria with unknown biogeochemical implications. Here, by incubating natural seawater samples at three different temperatures, we analysed the effect of experimental warming on the abundance and C and N uptake activity of Rhodobacteraceae and Flavobacteria, two bacterial groups typically associated with microphytoplankton. Using a nano-scale secondary ion mass spectrometry (nanoSIMS) single-cell analysis, we quantified the temperature sensitivity of these two taxonomic groups to the uptake of algal-derived DOC in the microphytoplankton associated fraction with ¹³ C-bicarbonate and ¹⁵ N-leucine as tracers. We found that cell-specific ¹³ C uptake was similar for both groups (~0.42 fg C h^-1 μm^-3 ), but Rhodobacteraceae were more active in ¹⁵ N-leucine uptake. Due to the higher abundance of Flavobacteria associated with microphytoplankton, this group incorporated fourfold more carbon than Rhodobacteraceae. Cell-specific ¹³ C uptake was influenced by temperature, but no significant differences were found for ¹⁵ N-leucine uptake. Our results show that the contribution of Flavobacteria and Rhodobacteraceae to C assimilation increased up to sixfold and twofold, respectively, with an increase of 3°C above ambient temperature, suggesting that warming may differently affect the contribution of distinct copiotrophic bacterial taxa to carbon cycling.