Dissertation Research: The effects of precipitation change on microbial drivers of salt marsh greenhouse gas emissions

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It is predicted that, in the next several decades, Eastern North America will experience an overall increase in annual precipitation, as well as an increase in the intensity of precipitation events. Rainfall will be concentrated into fewer, larger storms punctuating longer periods of drought. The effects of this intensification may be especially important in ecosystems like salt marshes, that are already under water stress. Salt marshes provide critical ecosystem services including nursery habitat for fish and birds, and nutrient filtration. They also store vast amounts of organic carbon. Changes in precipitation may alter their ecology, and disrupt their nutrient filtration and carbon storage activities, much of which is mediated by salt marsh microorganisms. The objective of this research is to quantify the effects of precipitation intensification on on microbial processes in a New England salt marsh. This doctoral dissertation improvement grant research will also examine mechanisms behind patterns of greenhouse gas (CH4 and N2O) emissions and uptake by salt marsh microbes by studying microbial gene expression patterns in the environment. Precipitation changes, especially the intensification of precipitation events (e.g. longer droughtsand stronger storms) are an important aspect of anthropogenic global climate change. The impact of precipitation intensification may be especially great in ecosystems that are chronically water stressed, either from extremely dry conditions, or from extremely wet conditions that lead to anoxia in the sediment. As tidal wetlands, salt marshes are subject to a variety of stressors including high salinity and anoxia, making them potentially very sensitive to changes in rain and other freshwater inputs. Over the past year an experiment has been performed to test the effects of precipitation change onsalt marsh biogeochemistry. Rainout shelters have been used to intercept incoming rain, and deliver it to one of four precipitation treatments: ambient rainfall, doubled ambient rainfall, prolonged drought, or simulated intense drought with strong storms. Monthly measurements were made of greenhouse gases, the overall productivity and shoot:shoot allocation of marsh grasses, and the cycling of silica and other nutrients through plants, sediment, and porewater. Funding from this award will support the addition of a molecular component. The primary objectives will be to determine 1) whether climate-induced changes in precipitation (prolonged drought, doubled growing season totals, or alteration between the two) will alter salt marsh microbial community structure and species richness, and 2) to what degree changes in the expression of key functional genes by the microbial community can be linked to measured net rates of trace greenhouse gas flux. This research will test two hypotheses: (1) Prolonged drought and doubled precipitation rates will both lead to decreases in microbial species richness, but when drought and increased precipitation are combined this intensified precipitation regime will lead to an increase in microbial species richness, and (2) trace greenhouse gas fluxes (CH4 and N2O) are positively correlated with functional gene transcript copies for production and negatively correlated with those for consumption of each gas. Hypothesis 1 will be tested by determining microbial community composition and species richness with next-generation sequencing of genomic DNA in sediments at an ongoing field experiment. Hypothesis 2 will be tested by pairing high-sensitivity gas flux measurements with quantification of transcript copies of 6 functional genes (mcrA, pmoA, mxaF, nosZ, norB, and norZ) governing microbial production and consumption of CH4 and N2O. The proposed research will be integrated with new outreach activities. A Ph.D. student will work with the BU Upward Bound Math and Science program, which prepares low-income and first generation college bound students for a successful higher education experience. She will lead a one-day workshop with Upward Bound high school students on the importance of salt marshes, and the linkages between microbial communities and greenhouse gases. She will also host a summer Upward Bound high school student intern to work on this project. Second, the Ph.D. student will coordinate in a Citizen Science Outreach Program that leverages the experiment's high-visibility location in a frequently-visited area of a National Park to involve interested parties in the monitoring of the experimental treatments.