Freshwater wetlands are the largest single source of methane to the atmosphere. Methane is a potent greenhouse gas (approximately 24 times more potent than CO2), such that the pathways that control the production of methane in wetlands have global implications. Most wetlands, including the Florida Everglades, are affected by anthropogenic nutrient additions. The goal of this research is to determine the enhancement of methane production caused by nutrient pollution. Excess phosphate increases the amount of plant derived-organic matter. This material is then fermented to an array of small molecules in anoxic wetland soils, which are eventually converted to methane. There are two main pathways by which microbes form methane. Approximately 70% of global freshwater wetland-produced methane arises from the acetate fermentation pathway and 30% comes from H2 reduction of CO2. Preliminary data indicate that pathways for methanogenesis are affected by phosphorus runoff from adjacent agricultural lands. Guiding this work is the hypothesis that in addition to increasing organic matter production by plants, excess phosphate also results in higher rates of methane production due to two additional factors affecting these pathways: provision of additional supply of hydrogen substrate from nitrogen fixation that is unrelated to organic production; and shunting of methane production from acetate-derived methane to fermentation of acetate by an unusual group of bacteria, the syntrophic acetate oxidizers. This shunting benefits the CO2 reduction pathway which has higher rates of methane production, is less sensitive to competitive electron acceptors (such as sulfate), is less sensitive to phosphate and ammonium inhibition, and is affected differently by increased temperature. Thus predictions of methane emissions based on temperate wetlands may not accurately forecast methane emissions from warmer climates. A combination of microbiological, isotopic and biogeochemical approaches will be employed to determine how these pathways are affected by nutrient pollution. Wetlands in South Florida and Panama will be examined.

This project is funded by the NSF and is being conducted with Jeff Chanton (FSU), Ramesh Reddy (UF), and Mary Christman (UF Statistics). We will employ a range of approaches, including microarray analysis of fermentation pathways, analysis of natural abundances of stable isotopes to trace the flow of carbon and hydrogen in these systems, and structural equation modeling to pull it all together.