This project is an ongoing team effort.
It started in collaboration with colleagues at the Center of Oceanology of Marseille in France.
We developed a protocol to measure taxon-specific phosphate uptake in live and unfiltered samples using flow cytometry sorting:
Talarmin A., Van Wambeke F., Duhamel S., Catala P., Moutin T., and P. Lebaron, 2011. Improved methodology to measure taxon-specific phosphate uptake in live and unfiltered samples. Limnology and Oceanography: Methods 9, 443-453. Open access
We then succesfully applied this method to study (1) microbial group specific uptake kinetics of P, (2) light dependance of microbial P uptake and (3) microbial response to enhanced P cycling in the North Pacific subtropical gyre:
Björkman K.M., Duhamel S., and D.M. Karl. 2012. Microbial group specific uptake kinetics of inorganic phosphate and adenosine-5’-triphosphate (ATP) in the North Pacific Subtropical Gyre. Frontiers in Microbiology 3(189):1-17. Open access
Duhamel S., Björkman K.M., and D.M. Karl. 2012. Light dependence of phosphorus uptake by microorganisms in the North and South Pacific subtropical gyres. Aquatic Microbial Ecology, doi: 10.3354/ame01593. Open access
Duhamel S., Björkman K.M., Doggett J.K., and D.M. Karl. 2014. Microbial response to enhanced phosphorus cycling in the North Pacific Subtropical Gyre. Marine Ecology Progress Series, 504: 43–58, doi: 10.3354/meps10757. Open Access
The Duhamel Lab was then funded by NSF to determine the rates of cellular phosphorus and biomass turnover in the dominant picoplankton groups (Synechococcus, Prochlorococcus and heterotrophic bacteria) inhabiting the oligotrophic, phosphorus-depleted, surface seawaters: NSF award #1434914: “Role of variable picoplankton cellular phosphorus turnover and allocation in marine phosphorus cycling” (2014-2016). This project was led by Dr. Kim Popendorf, a postdoc in the Duhamel Lab (2013-2015) who is now an Assistant Professor at RSMAS.
Overview: Phosphorus is a key element for life, present in low amounts in the vast oligotrophic ocean, 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, microbes’ 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. The proposed research will use field samples to determine the rates of cellular phosphorus and biomass turnover in the dominant picoplankton groups (Synechococcus, Prochlorococcus and heterotrophic bacteria) inhabiting the oligotrophic, phosphorus-depleted, surface waters of the Sargasso Sea. To build a mechanistic understanding of the processes controlling these rates, the proposed research will also determine the variation in picoplankton allocation of phosphorus into intracellular biochemicals. These 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. This work 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.
Intellectual Merit: 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. 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, 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, this project will answer crucial questions about the relative impact of different microbial groups on surface ocean phosphorus fluxes and the cellular dynamics that drive these fluxes.
Here are our findings:
Popendorf K., and S. Duhamel 2015. Variable phosphorus uptake rates and allocation across microbial groups in the oligotrophic Gulf of Mexico. Environmental Microbiology 17(10), 3992–4006. doi: 10.1111/1462-2920.12932. Open Access
Ocean Sciences Meeting 2016 posters by Ana Gomez and Sarah Raney