Award: OCE-1046297

Marine phytoplankton, which are single-celled, photosynthetic organisms living in the sunlit surface of the ocean, link the Earth's atmosphere to the chemistry of the oceans. Phytoplankton grow by absorbing light and carbon dioxide, extracting carbon and combining it with nitrogen, phosphorus, iron, and other nutrients. When phytoplankton die, they sink from the ocean's surface, transporting carbon to the deep sea. This transport reduces the level of carbon dioxide in the atmosphere, cooling the climate. Phytoplankton grow until they deplete available nutrients, and the nutrient requirements of phytoplankton are thus key variables that determine how nutrient supply affects atmosphere and climate. We performed research to better understand the elemental composition of phytoplankton. This was inspired by field data (some of which we collected) showing that phytoplankton have different nutrient requirements in different ecosystems. In warm water, nutrient depleted ecosystems, phytoplankton are rich in carbon and nitrogen but poor in phosphorus, with the opposite pattern holding in nutrient rich, cold water ecosystems. We developed mathematical models of phytoplankton cells that linked their ability to survive in the ocean environment to their nutrient content, enabling us to predict their nutrient requirements in different ocean regions. We validated these mathematical models using both laboratory experiments and field measurements. We identified two key drivers of the observed patterns: phytoplankton that are growing quickly or living in the cold need lots of phosphorus to synthesize proteins rapidly, and phytoplankton can be frugal with phosphorus when it is scarce, using less of it in their cells. We then used these mathematical models to ask questions about how phytoplankton productivity is controlled and how phytoplankton interact with climate, by embedding them in a model ocean. With this work, we found that future atmospheric CO2 levels are sensitive to how phytoplankton regulate their nutrient requirements. Finally, we developed and shared multiple data products including a global database on particulate organic matter concentrations, the data collected from a weekly time-series in Southern California, data collected during three cruises, and our models. Our research and data demonstrate that phytoplankton physiology and diversity can have important implication for present and future global biogeochemical cycles. Last Modified: 12/15/2016 Submitted by: Adam C Martiny