The potential for biogeochemically-critical marine organisms, such as the N2-fixing cyanobacterium Trichodesmium, to adapt to a rapidly changing environment is a poorly understood but key determinant of future ocean food webs and elemental cycles. Despite an exponential increase in the sophistication of molecular tools available to the ocean science community, no study has yet applied these methods to relevant marine organisms in conjunction with microbial evolution experiments such as those pioneered by R.E. Lenski and colleagues for the model enteric bacterium Escherichia coli.
Intellectual Merit: In this EAGER project, the PIs will conduct an exploratory approach that uses tiled microarray methods to evaluate changes in the expression of both coding and non-coding regions of the genome in Trichodesmium cultures that have been maintained in long-term (>3 years) high CO2 adaptation experiments. The objective of this work is to demonstrate that a novel combination of evolutionary experimental techniques and state-of-the-art gene expression methods can be used to yield unique insights into adaptive changes in keystone marine micro-organisms such as Trichodesmium in response to selection by environmental change variables. The over-arching goal is to increase our mechanistic understanding of the ways that evolution could shape the responses of marine biota to future changes in ocean chemistry and climate.
Broader Impacts: This project will help support and train a new USC Ph.D. student, and research activities will include USC undergraduate biology majors. Public education and communication efforts for this project will be greatly enhanced by an annual series of public outreach and professional colloquia sponsored by a USC-funded "2020" initiative to integrate scientific and societal responses to climate change in the Southern California region. Any evolution-driven shifts in the growth and N2 fixation patterns of Trichodesmium, a keystone oceanic functional group, have large consequences for marine ecology, carbon cycling, and the food chains that support important living resources. The biggest scientific impact of this EAGER project could be to offer verification of a pioneering new approach to help determine how the microbes that are fundamental to today's ocean biogeochemical cycles will adapt to anticipated and unprecedented rates of change in marine ecosystems.
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.
