Influence of amino acids on bacterioplankton production, biomass and community composition at Ocean Station Papa in the subarctic Pacific

Bacterioplankton play a central role in carbon cycling, yet their relative contributions to carbon production and removal can be difficult to constrain. As part of the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) program, this study identifies potential influences of bacterioplankton community and dissolved organic matter (DOM) composition on carbon cycling at Ocean Station Papa in August 2018. Surface (5-35 m) bacterioplankton production rates and stocks spanned a 2- to 3-fold range over the 3-week cruise and correlated positively with the DOM degradation state, estimated using the mole proportion of total dissolved amino acids. When the DOM was more degraded, 16S rRNA gene amplicon data revealed a less diverse bacterioplankton community with a significant contribution from members of the Flavobacteriaceae family. Over the course of 7-10 d, as the DOM quality improved (became less degraded) and bacterioplankton productivity increased, the responding bacterioplankton community became more diverse, with increased relative contributions from members of the SAR86, SAR11 and AEGEAN-169 clades. The cruise mean for mixed layer, depth-integrated bacterioplankton carbon demand (gross bacterioplankton production) was 5.2 mmol C m-2 d-1, representing 60% of net primary production, where the difference between net primary production and bacterioplankton carbon demand was less than sinking flux at 50 m. The concentrations of dissolved organic carbon (cruise average of 58.5 mM C) did not exhibit a systematic change over the cruise period. Therefore, we hypothesize that carbon supplied from gross carbon production, values that were 2- to 3-fold greater than net primary production, provided the carbon necessary to account for the sinking flux and bacterioplankton carbon demand that were in excess of net primary production. These findings highlight the central contributions of bacterioplankton to carbon cycling at Ocean Station Papa, a site of high carbon recycling.