Award: OCE-1303287

The global ocean absorbs roughly 25% of global carbon dioxide emissions through the processes of photosynthesis, termed primary production, and sinking of newly formed particulate organic material, termed carbon export. This combination of processes is complex and is not well enough understood as too allow confident predictions of the future abilities of ocean biology to continue absorbing carbon dioxide release. There are many reasons for this uncertainty in our understanding ranging from limitations in experimental and observational methodologies, to undersampling in space and time, to variability in the physiology and behavior of plankton that is not currently measured or understood in the ocean. This project focused on the latter, understanding the variability in phytoplankton physiology to changes in environment over a wide range of oceanic environments. Specifically, we studied how the elemental ratios of the key nutrients carbon, nitrogen and phosphorus changed between different phytoplankton and in response to changes in environment within the same phytoplankton group. The particulate elemental ratio of carbon to nitrogen to phosphorus is termed the Redfield Ratio and is a central oceanographic tenant that directly bears on the ability of ocean biology to take up carbon dioxide. We studied the ratios of carbon, nitrogen and phosphorus in three primary phytoplankton groups, the photosynthetic bacteria (cyanobacteria) Prochlorococcus and Synechoccocus, and the small flagellated true (eukaryotic) phytoplankton. Collectively these three groups comprise the majority of the phytoplankton biomass over roughly 60% of the global ocean surface area. We observed that on average, the cyanobacteria had significantly greater carbon, nitrogen to phosphorus ratios than the eukaryotic phytoplankton. This pattern held across a wide range of environmental nutrient conditions. When the data were examined more deeply, it is clear that there are regions where both cyanobacteria and phytoplankton have cellular elemental ratios that match the Redfield Ratio, but this is uncommon. The cyanobacteria showed a particularly strong ability to change their physiological condition such that they would reach a minimum cellular nitrogen and carbon content, but their cellular phosphorus content would still decrease by an order of magnitude. While eukaryotic phytoplankton sometimes were observed to show a similar pattern, their overall ability, their cellular plasticity, was much less than that of the cyanobacteria. The implications of this work are that the cyanobacteria, which are of major importance over much of the oceanÆs surface area, more efficient at fixing carbon per unit phosphorus than eukaryotes and thus, as those organisms are grazed upon or sink in the open ocean they remove relatively more carbon than other phytoplankton. Thus in a future ocean, if these cyanobacteria were to expand their geographical extent or increase their cell abundance, the oceanÆs biological sequestration of carbon would continue and perhaps may even increase. Last Modified: 04/28/2016 Submitted by: Michael W Lomas