Diatom derived dissolved organic matter as a driver of bacterial productivity: The role of nutrient limitation

Variable inorganic nutrient concentrations were used to generate silicate (Si) or nitrogen (N) limited conditions in cultures of the marine diatom Skeletonema costatum. Dissolved organic matter (DOM) harvested in the nutrient limited phase of these cultures was added to a natural bacterial community. Enhanced bacterial abundance and bacterial production were observed, in comparison to un-supplemented controls, when Si-limited (Si-DOM) rather than N-limited (N-DOM) diatom derived DOM or inorganic nutrients was added. This indicates that the bacterial population was limited by organic rather than inorganic resources but only Si-DOM had sufficient lability to alleviate this. Within the bacterial assemblage, a notable increase in the proportion of γ-Proteobacteria was evident on receipt of only Si-DOM. Assessment of the composition of the added DOM suggested that the observed dynamics were related to organic matter composition rather than molecular size as, within the Si- and N-DOM, the proportions of low and high molecular weight compounds were similar, but the polysaccharide and protein signatures were different.