Award: OCE-1260490

Important living ocean resources like fisheries depend entirely on energy and materials supplied by micro-organisms at the base of the marine food chain. One of the most important groups of microbes are the cyanobacteria (blue-green algae) that can use nitrogen gas from the atmosphere to build their cells, and who ultimately provide the essential element nitrogen to nearly all living things in the open ocean. Understanding how these key micro-organisms will respond to a changing ocean containing higher levels of carbon dixoide is therefore a priority, in order to accurately predict how the ocean's food chains will function in the future. This project brought together methods and ideas from marine science and evolutionary biology to test how these 'nitrogen-fixing' cyanobacteria will react to long-term changes in the acidity and carbon dioxide content of the ocean. Previous work had showed that these microbes might be one of the few groups of marine organisms that will actually benefit from future higher carbon dioxide levels, since their growth and nitrogen fixation were stimulated under these conditions in short-term experiments. This project however was specifically designed to carry out these experiments over much longer periods (almost a decade), timescales that are much more relevant to real acidification processes in the ocean. These long-term experiments had the unique advantage of allowing the cyanobacteria enough time to adapt or evolve in response to their changing environment. The results showed that the short term responses showing that brief exposures to higher carbon dioxide stimulated nitrogen fixation were retained, even after years of adaptation to high carbon dioxide, A very unexpected finding was that these stimulatory responses became permanent and irreversible. Even when the high carbon dioxide adapted cells were moved back to lower current carbon dioxide levels, they still grew and fixed nitrogen just as if they were under high carbon dioxide conditions. This is an evolutionary response that seems to be unique among all the similar experiments that have been done with microbes. It also suggests that these important microbial links in the ocean food chain may adapt to rising carbon dioxide in the ocean in surprising ways, and that these adaptations could even prove to be irreversible. In addition to these main findings, the results of this project were leveraged to provide additional information on the cell-level controlling mechanisms and basic ecology of marine cyaobacteria. These value-added experiments showed how adaptation to high carbon dioxide can affect the requirements of marine cyanobacteria for other nutrient elements like phosphorus and iron, and explored how the community of many living bacteria that typically grows in association with these nitrogen fixers is involved in their acitivity and growth. The project had major impacts on education, as it supported three different graduate students, including two who have now completed their doctoral degrees. It also contributed to active undergraduate engagement in scientific research. The biggest contribution this project made to science was to highlight the need to more realistically consider evolutionary processes in order to make good projections of how important groups of marine organisms will respond to a rapidly changing ocean. Last Modified: 05/25/2018 Submitted by: David A Hutchins