Award: OCE-1046001

Marine phytoplankton, which are single­celled, 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 (which was gathered by our research partners on this grant) 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 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. We showed that over long periods of time, phytoplankton productivity can be controlled by all three major nutrients, nitrogen, phosphorus, and iron. This work challenged the conventional paradigm that nitrogen supply does not affect phytoplankton when the ocean is in steady- state, and we found that previously unaccounted for differences in the geographic distribution and nutrient composition of certain phytoplankton called nitrogen ­fixers (which are able to get their nitrogen directly from the atmosphere), break negative feedbacks between external nitrogen additions and the population of nitrogen­ fixers and so that nitrogen additions can increase overall phytoplankton productivity. We also studied the factors controlling the amount of nitrogen dissolved in the deep sea, and found that it is regulated by the differences in ocean circulation between cold and warm water regions, the supply of iron or phosphorus, and the elemental composition of warm water phytoplankton. Comparison with data from the modern ocean provided evidence that the iron supply controls primary productivity in the ocean. We were able to use a similar framework to determine how productivity in different ocean regions influences atmospheric carbon dioxide. We found that heterogeneities in elemental composition among different ocean regions caused counterintuitive effect: We predict that increased production in tropical regions might actually increase atmospheric carbon dioxide by cutting off the flow of nutrients to the subtropical ocean regions, where carbon export is more efficient. Our model suggests that temperature could increase the efficiency of phytoplankton in transporting carbon, acting as a negative feedback on climate in a warming planet. Uncertainty remains about the relative roles of temperature and nutrients in controlling the composition of phytoplankton, and more research is required to quantify more effectively the effects of environmental factors. Last Modified: 11/23/2016 Submitted by: Simon A Levin