Osmolyte cycling mid-Atlantic

Each year, half of all marine primary production (25 Pg C) is remineralized by microorganisms. Both identity and abundances of the molecules and microbes involved regulate this flux. Organic osmolytes likely comprise a significant, but unconstrained, component of this carbon flux. Osmolytes regulate osmotic pressure and thus accumulate at millimolar concentrations inside the cell. Given their high intracellular concentrations, upon their release to the microbial loop osmolytes are valuable carbon substrates as well as sources of nitrogen and sulfur. In fact, a few osmolytes have been shown to fulfill up to 4-38% of bacterial carbon demand, which could translate to a significant carbon flux. The goal of our proposed work is to produce a quantitative budget of osmolyte fate in the context of the labile carbon cycle by developing a greater mechanistic understanding of the biological and environmental controls of osmolyte composition and concentrations. We will do this through culture and field experiments where production, drawdown, and assimilation of organic osmolytes will be measured and linked to transcriptional activity of the microbial producers and consumers. Our field component aims to collect osmolyte standing stock concentrations, rates, and associated microbial molecular information along a coastal to open ocean gradient extending from the Mid-Atlantic Bight out past the Gulf Stream to oligotrophic waters. This region is typified by heightened primary productivity in the in-shore region, with coastal seasonal blooms that occur annually. Based on a preliminary analysis of surface biogeochemical conditions satellite, we propose a cruise track to take place in mid-summer (July) that will transit a strong productivity and nitrogen gradient from productive coastal waters, and eastward through the Gulf Stream and into stratified open ocean waters of the North Atlantic. The intersection of biogeochemical regimes will allow us to assess the impact of changes in community composition and nitrogen availability on in situ osmolyte cycling. We will pair quantitative metabolomic and stable isotope methods with quantitative metatranscriptomics to produce a budget of standing stock concentrations of osmolytes, bounded by the activity of microbial transcription.