Award: OCE-1559002

The Indian Ocean accounts for nearly a fifth of global ocean photosynthesis and is likely a key component in global ocean nutrient and carbon cycles. However, the central Indian Ocean may be the least studied major marine body on the planet. Our limited understanding suggests extensive variations in physical and chemical environmental conditions, but how this variation influences biodiversity, nutrient stress, and more broadly regional differences in the functioning of phytoplankton has been unknown. To help address these gaps, we participated in two major US-led cruises (IO9N and IO7N) traversing large sections of the Indian Ocean. The project has led to a direct understanding of the interconnectedness of environmental conditions, phytoplankton, and nutrient biogeochemistry in the central Indian Ocean. By combining genomics analyses of phytoplankton populations, bioassays, and biogeochemical observations, we showed that a large section of the Indian Ocean was N-stressed. However, we also found that Fe-availability played an important role in regulating N-availability but did not reach growth-limiting concentrations. This growth limitation pattern was supported by presence of specific phytoplankton ecotypes associated with macronutrient limitation. Thus, the project has fundamentally advanced our understanding of how phytoplankton growth in the Indian Ocean is regulated. The project also led to a broader understanding of phytoplankton diversity and their biogeochemical role. We demonstrated how the microdiversity of phytoplankton is both important for ocean ecosystem functioning but also as a tracer of biogeochemical variation. Furthermore, we used our observations to develop a general theory for how the elemental composition of phytoplankton is regulated in the ocean. This has led to a reformulation of global earth system models. As such, the observations from the Indian Ocean have been instrumental to our understanding of the global ocean carbon and nutrient cycles. The research had broader impacts on many levels. First, it increased the public awareness of the role of phytoplankton on ocean functioning, climate, and people's lives through a partnership with the Long Beach Aquarium of the Pacific (AOP). Secondly, we trained postdoctoral scholars, graduate students and undergraduate students. Thirdly, we have shared the collected data enabling other researchers to further study the Indian Ocean. Fourthly, we gave many presentations on the biogeochemical functioning of the Indian Ocean and its important role in the Earth System. Last Modified: 02/12/2019 Submitted by: Adam C Martiny