Project: Role of variable picoplankton cellular phosphorus turnover and allocation in marine phosphorus cycling

NSF award abstract:
Phosphorus is a key element for life, often present in low amounts in the vast central regions of the ocean and sometimes reaching limiting concentrations for biological productivity. Microbial uptake of dissolved phosphorus is an important lever in controlling both microbial production and the fate and cycling of marine phosphorus. This project will investigate the hypothesis that in oligotrophic environments, microbial cellular turnover of phosphorus occurs more rapidly than cellular biomass turnover, leading to a significant return of phosphorus to the dissolved pool. This rapid return of phosphorus could resupply the pool of bioavailable phosphorus, impacting microbial dynamics and fueling significant recycling of phosphorus in the surface ocean. In particular, this research will use field samples to determine the rates of cellular phosphorus and biomass turnover in the dominant groups of very small algae and bacteria inhabiting the phosphorus-depleted, surface waters of the Sargasso Sea (the region of the central North Atlantic Ocean near Bermuda). The project will be co-directed by two early-career women scientists. Their educational efforts will include training of an undergraduate student assistant as well as public educational outreach to K-12 students.

A primary objective in the study of biogeochemical cycles is linking chemical fluxes to the activity of organisms. This work will make fundamental contributions to the understanding of the biogeochemical cycling of phosphorus by linking phosphorus fluxes in the surface ocean to the activity of specific groups of microbes, and providing a mechanistic framework for the factors that control these fluxes. A significant, novel product of this research will be the determination of cellular phosphorus turnover rates relative to biomass turnover rates for individual picoplankton groups in the open ocean. To build a mechanistic understanding of the processes controlling these rates, this project will also determine the variation in picoplankton allocation of phosphorus into intracellular biochemicals. Field measurements will be augmented with experiments on axenic cultures, representative of the cell-sorted groups from the Sargasso Sea, to conduct more detailed biochemical analyses. The research team will utilize a unique suite of tools to make novel measurements from environmental samples: flow cytometry and fluorescence activated cell sorting will be combined with radioisotope labeling and biochemical analyses to quantify cell-specific phosphorus fluxes and characterize the chemical speciation of these fluxes. The significance of a high ratio of cellular phosphorus turnover rate to biomass turnover rate would be two-fold: (1) the amount of phosphorus in microbial biomass would underestimate the total phosphorus demand necessary to support microbial growth, and (2) significant recycling of dissolved phosphorus may occur in the surface ocean through microbial uptake and rapid return to the dissolved pool. By measuring these rates in field samples, the team expects to answer crucial questions about the relative impact of different microbial groups on surface ocean phosphorus fluxes and the cellular dynamics that drive these fluxes.