Phytoplankton utilization of nitrogen in the surface ocean.
Malcolm Woodward, who heads the PML Nutrient Facility, has been invited to join Professor Bess Ward and her research group from Princeton University, USA, on a 25 day research cruise in the subarctic North Atlantic as part of her United States National Science Foundation funded project “Dimensions of Biodiversity: Functional Diversity of Marine Eukaryotic Phytoplankton”.
The goal of the project is to investigate the taxonomic, genetic and functional diversity of eukaryotic phytoplankton (i.e. single cell algae, which are related to the higher plants) and link diversity and assemblage composition to the carbon (C) and nitrogen (N) biogeochemistry of the surface ocean. The PML nutrient results will provide some of the basic information for these experiments and will be able to report very low level concentration results for ammonium and nitrate, two of the essential chemical ‘building blocks’ in the ocean that influence the phytoplankton. The eukaryotic phytoplankton tend to be larger and favour utilizing nitrate, rather than ammonium, which is in contrast to the smaller but more abundant prokaryotic phytoplankton (i.e. single cell microbes related to bacteria), and because of this the eukaryotic plankton are much more important in sequestering carbon away from the surface ocean. Thus the response of the ocean food webs to ocean warming caused by climate change depends on the composition and activity of these phytoplankton assemblages. Eukaryotic phytoplankton contribute disproportionately to fish production higher up the food chain, and their abundance is predicted to decrease as the surface ocean warms.
The cruise is on board the R/V Endeavor and departed from Morehead City, NC, on 22 August on a sampling transect towards the study sites south of the Rockall Plateau in the North Atlantic.
The genetic diversity of the water will be explored at several levels, including direct sequencing of clone libraries of key functional genes and metatranscriptomic sequencing, and the microarray analysis of phytoplankton assemblages using size fractionation and flow cytometric sorting. Using natural abundance and tracer stable isotope methods, the team will link genetic and taxonomic diversity to functional diversity in C and N assimilation in size- fractionated and taxon-sorted populations. Taxonomic diversity will be investigated by identifying the components of the phytoplankton assemblages using molecular, chemical and microscope methods. The combination of cutting edge methods makes these investigations unique in their ability to determine the contributions made by the different phytoplankton groups to Carbon and Nitrogen cycling.
Previous work from Ward’s lab (the work of then graduate student Sarah Fawcett with colleague Daniel Sigman) using similar methods in the Sargasso Sea, found that small eukaryotic phytoplankton and prokaryotic picoplankton could be distinguished by their nitrogen stable isotope signatures, implying that the two groups rely on different sources of nitrogen. The eukaryotes were disproportionately important in the utilization of nitrate, sometimes relying almost entirely on nitrate for their nitrogen needs, even when nitrate was present in the surface water at vanishingly low concentrations, and this is where the PML expertise is so important for this project.
The team start very early, with a pre-dawn sampling station providing the water column samples for nutrients, isotopic analysis and primary production measurements. At two process stations around 50North 20West, differential Nitrogen utilization will be investigated using natural abundance and tracer Nitrogen isotope methods. The PML nutrient team’s real time measurements of low level (nanomolar) dissolved inorganic nutrient concentrations will provide the crucial information necessary for the incubations, which are the primary experimental basis of the project. Natural seawater samples are incubated with tracer Nitrogen compounds (ammonium and nitrate) in order to determine the rates of NitrogRunning seawater incubators on the Endeavor fantail, with Nicolas van Oostende (post doc at Princeton), Jessica Lueders-Dumont (graduate student at Princeton) and Erich Gruebel (Marine Technician on R/V Endeavor). en assimilation by the different phytoplankton groups, hence the crucial component in understanding the processes involved in the ocean.
The incubated samples will by analyzed by flow cytometry and mass spectrometry in the lab back at Princeton University. PML carries out all the analysis for the nutrients on board the RV Endeavor and this project follows on from other international programmes.
The cruise has an international flavour with scientists from the host laboratory in America, South Africa, France, Italy, Belgium and also with participation from the Craig Ventor Institute (all part of Ward’s project).
Malcolm is supported on the cruise by Amandine Sabadel from Otago University, New Zealand, who is co-supervised for her PhD at PML. The overall project involves four cruises, two in the Sargasso Sea and two in the subarctic North Atlantic; two environments where the contributions of eukaryotic and prokaryotic phytoplankton are expected to be quite different.
After the process stations, the cruise ends in the Azores. A repeat cruise in May of 2014 will carry out the same experiments at the time of the spring bloom. The team expect to find major ecological contrasts in rates of C and N cycling and in phytoplankton community composition between the August and May seasons, likely reflecting differences in the major N utilization pathways and fates of primary production.