Award: OCE-1143760

This project was a pilot study to pioneer the use of a group of organisms called marine cyanobacteria (or blue-green agae) in experimental studies of their evolutionary responses to increased carbon dioxide levels. Cyanobacteria play a key role in ocean ecosystems since they can "fix" or obtain nitrogen gas from the atmosphere, which very few other organisms can do. Therefore, they are very important in providing the nitrogen that is needed to support all ocean food chains. It is well known that concentrations of carbon dioxide are increasing in the surface ocean, and past studies have suggested that marine cyanobacteria may benefit from this ongoing change by increasing their nitrogen fixation rates. However, all of this previous research has been done after the cyanobacteria were exposed to increasing carbon dioxide only briefly (typically weeks). How they may respond after lengthy conditioning at high carbon dioxide levels (years or decades), and whether they may adapt in an evolutionary sense to this seawater chemical change, is completely unknown. To address this question, we used cultures of the marine cyanobacterium Trichodesmium that our lab group had maintained under high carbon dioxide levels for 4-5 years, or about 400-500 generations. This long conditioning period was intended to allow the cultures a chance to evolve in response to selection by carbon dioxide changes. Measurements of nitrogen fixation rates at the end of this period showed surprisingly that the cultures adapted in an irreversible way: the high nitrogen fixation rates under high carbon dioxide became "constitutive", or in other words were maintained even when they were switched back to low carbon dioxide conditions. We also obtained samples of DNA from the cultures to evaluate if and how the cultures had mutated to produce this effect, as well as RNA samples to measure how their gene expression changed. These two sets of samples are currently being analyzed and the data worked up to be published in the scientific literature. In summary, our exploratory project showed that this important group of marine organisms may adapt to future increasing carbon dioxide levels in unexpected ways, with possibly large implications for the productivity of marine food chains. We also demonstrated the feasibility of using marine cyanobacteria in extended laboratory experimental evolution studies, offering marine scientists a potentially important new tool to understand human impacts on ocean ecosystems. Last Modified: 06/09/2013 Submitted by: David A Hutchins