Colin G Finlay
Published: 2023
Total Pages: 0
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Wetlands provide valuable ecosystem functions including nutrient recycling, carbon storage, flood mitigation, and habitat in support of biodiversity. However, land use change and climate change stressors continue to threaten wetland ecosystems. Specifically, climate change is predicted to increase rates of sea-level rise and increase frequency of storm surges. Therefore, we need to better understand how the combined saltwater intrusion and flooding environmental stressors influence coastal wetland structure and function. Environmental stressors modify soil redox potential which directly influences microbial community structure and function in ways that alter transformations of carbon, nitrogen, sulfur, and iron, at the ecosystem level. Abiotic and biotic factors, including hydrology and plant presence, can affect terminal electron acceptor availability and dictate rates and types of metabolic microbial functions to different degrees. In a previous experiment, a soil mesocosm approach was used to examine how hydrology (wet, dry, interim) and plant presence (with or without plants) influenced wetland soils sampled from varying hydrologic histories (wet, dry, interim) in a restored, coastal wetland. After eight weeks of hydrologic manipulation, 16S rRNA amplicon sequencing and shotgun metagenomic sequencing were performed to characterize the microbial communities and greenhouse gas concentrations were measured to assess microbial function. Soil redox potential and soil physicochemical properties were also measured. Previous results showed that plant presence decreased greenhouse gas concentrations even in flooded conditions, and hydrology (history and contemporary treatment) altered wetland soil microbial community structure and the composition of carbohydrate metabolic genes. Functional genes involved in methanogenesis, and aerobic respiration, also differed in composition across hydrologic histories. In this study, we address the questions (1) how do hydrologic and plant related redox shifts relate to the composition of metabolic genes involved in sulfur/iron cycling and (2) how do patterns of iron-sulfur metabolic composition relate to carbon and nitrogen metabolic composition and greenhouse gas production? We hypothesized that the most reducing conditions (i.e., prolonged flooded, no plants) modify anaerobic metabolisms in similar ways. We predict that (i) in oxidizing conditions (dry and/or plant presence), functional gene composition of sulfate reduction will not correlate to the gene composition of iron reduction, and (ii) in reducing conditions (i.e., wet and/or plant absence), functional gene composition of sulfate reduction will correlate to patterns in iron reduction metabolic genes. In addition, iron and sulfur metabolic gene composition will contribute to carbon dioxide production while competing with methanogenesis. Results revealed that hydrologic treatment impacted assimilatory sulfate reduction gene composition, while hydrologic history impacted dissimilatory sulfate reduction composition. Hydrologic history significantly affected total iron active gene composition and iron reduction gene composition. We also identified correlations between sulfate reduction and iron reduction, especially in flooded conditions, while sulfate reduction and iron reduction compositions explained variation in biogenic greenhouse gas concentrations (carbon dioxide and methane). These results demonstrate the role of historical hydrology, saltwater exposure, and soil iron in shaping microbial community responses to future changes in hydrology and plant cover. Salinization events (e.g., saltwater intrusion) and changing precipitation patterns impact soil redox dynamics by altering sulfate and oxygen availability, and challenge estimates of biogenic greenhouse gas emissions. Therefore, a better understanding of microbial community responses to hydrologic manipulations, plant presence/absence, and soil physicochemistry will inform wetland greenhouse gas emissions predictions and management strategies (e.g., plant presence and hydrologic flows).