Abstract
A large annual carbon flux occurs through the surface ocean's labile dissolved organic carbon (DOC) pool, with influx dominated by phytoplankton-derived metabolites and outflux by heterotrophic bacterioplankton uptake. We addressed the dynamics of this carbon flow between microbial primary and secondary producers through analysis of the Thalassiosira pseudonana CCMP1335 endometabolome, a proxy for the labile DOC released upon phytoplankton lysis, as temperature and bacterial presence were altered. Diatom strains acclimated at one of three different temperatures (14°C, 20°C, or 28°C) were cultured either axenically or with the bacterium Ruegeria pomeroyi DSS-3, and their endometabolites analyzed by NMR. Median concentration variation between conditions was ~1.5-fold across all identified endometabolites. Those with roles as osmolytes varied most, exhibiting concentration differences up to 170-fold across conditions with the largest variations triggered by the presence/absence of the heterotrophic bacterium. Differential expression observed for diatom metabolite synthesis pathways suggested changes in synthesis rates as a mechanism for endometabolome remodeling. Consistent with expectations of high turnover by heterotrophic bacteria, endometabolite mean lifetimes in a DOC pool were <2 h to 12 h. IMPORTANCE: The role of labile DOC in the transfer of marine carbon between phytoplankton and heterotrophic bacteria was first recognized 40 years ago, yet the identity and dynamics of phytoplankton metabolites entering the labile DOC pool are still poorly known. Using metabolome and transcriptome profiling, we found highly variable composition and concentration of diatom endometabolites, depending on growth conditions and arising over time frames as short as a single growth cycle. This strong response to external conditions, both biotic and abiotic, suggests that the chemical composition of phytoplankton intracellular pools released during lysis shift with ocean conditions. As phytoplankton cell lysis is one of the largest sources of labile dissolved compounds in the ocean, dynamic compositional changes in the metabolites released to heterotrophic bacteria have implications for the fate of surface ocean carbon.