Seasonal and stable heterotrophic guilds drive Arctic benthic microbiome functioning across polar day and night

The remineralization of organic matter by benthic bacteria is an essential process in the marine carbon cycle. In polar regions, strong variation in daylength causes pronounced seasonality in primary productivity, but the responses of sedimentary bacteria to these fluctuations are not well understood. We investigated the seasonal dynamics of benthic bacterial communities from an Arctic fjord and found a partitioning of the communities into seasonally responsive and stable guilds. We separately analyzed the fractions of cells in the porewater and those loosely and firmly attached to sand grains through 16S ribosomal RNA gene sequencing, cell counting, rate measurements, and geochemical analyses. The porewater and loosely attached bacterial communities showed a dynamic response in composition and activity, suggesting that they play a central role in benthic-pelagic coupling by responding rapidly to seasonal fluctuations in organic matter availability. In contrast, the majority of the firmly attached cells showed a more buffered response, as reflected, e.g. in the consistently high cell numbers of <i>Woeseiaceae</i>. This fraction is potentially key to maintaining baseline remineralization processes throughout the year, independent of fresh organic matter input. These findings provide a new mechanistic understanding of carbon cycling in Arctic surface sediments that may also apply beyond polar regions.