Assessing community metabolism and flexibility: metabolomics and microbial diversity across the North Pacific Transition Zone and in response to nutrient amendments

Nutrient limitation and co-limitation play a key role in determining marine microbial community structure and activity. Metabolites, the small organic molecules resulting from cellular activity, can be used as tracers of cell taxonomy and metabolism. Here we measure intracellular metabolite pools across a gradient of macro- and micro-nutrients between the North Pacific Subtropical Gyre and the North Pacific Subpolar Gyre as well as in nutrient amendment experiments. These measurements reveal how the genetic potential of a microbial community is manifested in situ and in response to changes in nutrient ratios. We found that metabolite pools reflected community composition and nutrient availability across the transect. In the incubations, while chlorophyll concentrations increased in all nitrate addition experiments, metabolite pools and community composition show that microbial communities in the subtropical gyre and the transition zone have different responses to nutrient amendments. For example, the ratio of glutamine to glutamate, a marker for nitrogen uptake and assimilation strategy, changes in opposite directions in response to the addition of nitrate to the different communities. These data provide a unique opportunity to evaluate the importance of community composition and metabolic flexibility in determining community metabolism. Compounds that change across this transect and in response to nutrient amendments are key targets for future investigations as potentially important players in microbial physiological adaptation and biogeochemical cycling.