Award: OCE-1436865

Roughly half of photosynthesis on Earth each day happens in the oceans. Most of the organic carbon formed is rapidly recycled to CO2 by heterotrophic bacteria. This project examined this process by focusing on the ecology and metabolism of the most abundant heterotrophic bacterioplankton in the oceans, an evolutionarily related set of small bacteria known as SAR11. One of the pressing questions about SAR11 biology is, what organic compounds do they oxidize? Major scientific outcomes of this research were: 1) from measurements SAR11 cell mass and elemental composition we estimated that 6% to 37% of ocean photosynthesis is converted to CO2 by SAR11 cells; 2) in culture SAR11 cells oxidized a broad range of organic compounds, including osmolyte, polyamine, and volatile organic carbon compounds produced by algae, showing that these molecules are important components of the ocean carbon cycle; 3) a SAR11 transport protein was shown to be multifunctional and to have superior kinetic properties, demonstrating that SAR11 can successfully compete with other cells for dissolved organic compounds in nutrient depleted regions of the oceans; 3) experiments showed that as much as 20% of the carbon fixed by diatoms could be transferred to SAR11 cells in the form of volatile compounds, indicating that molecules that can easily diffuse across cell membranes can transfer large amounts of carbon from algae to heterotrophic cells; 4) ocean cruises supported by this project allowed a large group of scientists to gather data about SAR11 and other cells types from the North Atlantic ocean; 5) a large database of SAR11 genomes was assembled to predict the carbon oxidation metabolic pathways of these cells. Collectively, these findings expanded our knowledge of the intricate inner workings of the ocean carbon cycle, particularly helping us understand the biochemistry and cell biology of rapidly recycled organic carbon. These findings will lead to better mathematical models that simulate the movement of ocean carbon and help predict the future behavior of ocean ecosystems. This project also provided training experiences for five students and postdoctoral scholars. In bi-annual events, an outreach component of this project brought scientific experiences and training to teachers and students from rural Oregon high schools. One product of this activity, a board game called "Oligotroph" is being used to help students understand how life adapts to the extreme nutrient limitation found in the central gyres of warm oceans. Last Modified: 12/30/2019 Submitted by: Stephen J Giovannoni