Salt marshes are extraordinarily productive ecosystems found in estuaries worldwide. Located between the coastal ocean and coastal watersheds, salt marshes are often heavily influenced by human activities. Many receive high nitrate input from land, degrading water quality and leading in some cases to harmful algal blooms and low oxygen zones harmful to fish. Previous research has shown that salt marshes can act as cleansing sites where pollutant nitrate can be transformed to harmless nitrogen gas and released to the atmosphere. This occurs through heterotrophic denitrification, a microbe-mediated process that transforms plant-available nitrate to nitrogen gas (N2) using organic carbon. However, this transformation to harmless nitrogen gas is not always the fate of salt marsh nitrate. More recent research suggests that the forms of sulfur and carbon compounds in the marsh sediment directly affect the types of microbes and their activities determining nitrate?s fate, i.e. sulfur, nitrogen, and carbon transformations are all linked via microbial activities. For example, instead of being converted to nitrogen gas, nitrate can be converted to ammonium via the microbially controlled process dissimilatory nitrate reduction (DNRA). This different fate of nitrate is environmentally important in several ways. If nitrate is converted to nitrogen gas, it is lost from the ecosystem to the atmosphere, whereas if DNRA dominates nitrate reduction, pollutant nitrogen remains in the system as ammonium. Also, depending on the type of microbial process governing nitrate?s fate, if organisms are using nitrate to help degrade organic matter, less carbon is stored in the ecosystem, potentially influencing the ability of marshes to keep up with sea level rise. To investigate the environmental and microbial controls affecting the fate of nitrate in salt marshes, lab and field experiments will be carried out at Plum Island Estuary. The project focuses on sulfur-oxidizing bacteria, a group of particularly important chemosynthetic microbes that use energy trapped in sulfur compounds in sediment to for energy production, and thus contribute to carbon storage. The proposed studies aim (1) to identify sulfur-oxidizers present in sediment densely populated with the salt marsh grass, Spartina alterniflora, and to examine their gene expression linked to sulfur and nitrate processing under shifting environmental conditions; and (2) to combine this molecular information with measurements of rates and characteristics of biogeochemical reactions occurring in the sediment to detect whether sulfur-oxidizer-linked DNRA (thus retention of nitrogen in the ecosystem) or denitrification (thus loss of nitrogen gas from the ecosystem) dominates under specific environmental conditions.
Broader Impacts. This multidisciplinary research integrates biogeochemical process measurements with molecular analyses, and will be synergistic with ongoing studies at the Plum Island Ecosystem Long-Term Ecological Research (PIE-LTER) site. Salt marshes provide a variety of ecosystem services to humanity, including nutrient removal and storm protection, but they are under pressure from increasing coastal development and rising sea level. A detailed understanding of marsh microbial function will likely contribute to restoration efforts, particularly if the form of sulfur present in marshes allows prediction of whether pollutant nitrogen will most likely be lost (as nitrogen gas) or retained over time. Project personnel will participate as research supervisors or teachers in MBL?s annual fall Semester in Environmental Science, which each year draws mostly women from undergraduate liberal arts colleges into rigorous classroom learning and individual ecosystems research. One post-doc will also be trained in this interdisciplinary atmosphere, and collaboration between PIE-LTER and Massachusetts Audubon Society provides a conduit for the investigators to teach middle and high school students about nitrogen loading from human activities on land, and its effects in local estuaries.
