Print PageMarine microbes dominate the ocean in abundance, diversity, and metabolic activity. Heterotrophic microbes are ten times more abundant in the surface ocean than autotrophic microbes, and control key ocean biogeochemical processes such as nutrient mineralization and carbon sequestration. Changes in the elemental composition, or stoichiometry, of heterotrophic marine microbes directly affect local biogeochemistry and ocean ecosystem processes. Since studies on the cellular stoichiometry of marine microbes have largely focused on phytoplankton, the range and controls on the stoichiometry of marine heterotrophic microbes is not well understood. Therefore, the aim of this study was to investigate the actual range of elemental ratios among abundant, ecologically relevant heterotrophic marine bacteria. The Roseobacter lineage represents one of the most abundant groups of marine bacteria. Cultivated Roseobacter strains are relatively closely related to cloned environmental sequences, and although they are very physiologically diverse, the group is viewed as phylogenetically coherent. Several Roseobacter strains representing each of the major clades were grown in a common garden design to isolate any phylogenetic effect on cellular stoichiometry. Growth conditions were standardized using a light/dark cycling incubator and seawater media amended with carbon and nutrients. Cells were then harvested for analysis of carbon, nitrogen, and phosphorus content.
Results/Conclusions
On average, 109 cells were harvested and analyzed for each of the 9 Roseobacter strains tested, with some variation due to differences in growth rate among the strains. Our results suggest that the average carbon (C) to nitrogen (N) ratio of the strains (4.07± 0.62) is less than the expected “Redfield ratio” (6.625) for marine plankton. This discrepancy between the observed and expected cellular stoichiometry of the abundant Roseobacter group carries implications for predicting interactions among major biogeochemical cycles. Much of our current understanding of ocean biogeochemistry assumes that carbon and nutrients cycle according to the Redfield ratio, but our results suggest a lower ratio for some marine heterotrophs.
