Role of phytoplankton in marine silicon cycling

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Marine phytoplankton are key components of the Earth system. They contribute half of the world's annual photosynthetic fixation of carbon and play a fundamental role in nutrient cycling in the ocean. Of all phytoplankton groups, diatoms account for 20% of Earth's photosynthesis and are the main biological drivers of processes involved in the cycling of silicon, carbon, nitrogen and other nutrients. Production of silica cell walls by diatoms in surface waters, dissolution of this biogenic silica through the water column, and export of biogenic silica and associated carbon from surface to deep waters are some of the most important fluxes in the oceanic silicon and carbon cycles. This proposal describes an integrated field and laboratory program that will advance our understanding of these critical, but poorly known, processes of the silicon cycle. During the next 5 years, my objectives are to quantify:***(A) the linkage between diatom silicon use in surface waters and the natural silicon isotopic composition of suspended and sinking particles in Subarctic waters and in Pacific source waters into the Arctic,***(B) the impact of the structure of Subarctic and Arctic diatom assemblages on the silicon cycle, evaluated under natural conditions and under simulated greenhouse scenarios,***(C) the coupling between silicon, nitrogen, carbon and phosphorus in marine particulate and dissolved pools, from pelagic to benthic environments.***These short-term objectives will contribute to the long-term goal of my research program, which is to better understand the links between phytoplankton eco-physiology and biogeochemical cycling in the ocean. ***Field work will be conducted in high-latitude oceans where phytoplankton and nutrient dynamics are currently poorly described. Climate variability is disproportionally affecting these regions, which are experiencing dramatic changes in sea-ice cover, water mass circulation, temperature and seawater chemistry. It is therefore critical to experimentally evaluate phytoplankton responses and biogeochemical processes in Subarctic and Arctic seas of the NE Pacific and western Arctic.***Experimental work will employ leading-edge methodologies, such as high-resolution inductively coupled plasma mass spectrometry for measuring silicon isotopic abundance, and low-level beta counting of radioactive silicon and fluorescence imaging techniques for measuring silica production and dissolution. The projects described in this proposal will provide excellent training opportunities for graduate and undergraduate students who will be exposed to interdisciplinary science.***This research will fill critical gaps in our current knowledge of biologically mediated processes of the silicon cycle, and will advance our understanding of variations in marine paleoproductivity, controls on biogeochemical cycles, and the magnitude of carbon export in high-latitude oceans.**