Warming the phycosphere: Differential effect of temperature on the use of diatom‐derived carbon by two copiotrophic bacterial taxa
Néstor Arandia‐Gorostidi, Laura Alonso‐Sáez, Hryhoriy Stryhanyuk, Hans H. Richnow, Xosé Anxelu G. Morán, Niculina Musat
Abstract
Summary 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.