Recent research has shown that freshwater methane emissions make a surprisingly large contribution to global carbon fluxes. However, controls on methanogenesis and methane flux in fluvial environments are still not well understood. Rivers with high discharge might be expected to be unfavorable for methanogenesis because of high aeration, sediment turnover which could disrupt microbial syntrophy, and a continuous input of alternative electron acceptors for anaerobic respiration. The Kanawha-New River watershed encompasses 12,000 square miles of Appalachian Mountains. The watershed is primarily forested but includes large sections of surface mining and considerable riverside industrial development, particularly in the Charleston (WV) area. Yearly river discharge varies widely from 5000-100,000 cu ft per second in the Lower Kanawha region. Our long-term research seeks to understand the impact of land-use practices on riverine microbial biogeochemical cycles at a range of scales. For this study, we sampled biogas produced in river sediment in conjunction with metagenome sampling of microbial functional diversity in order to understand microbial contributions to organic carbon turnover. We sampled the sediment microbiome from five widely-spaced locations along an 80 km section of the Kanawha River. Replicate sediment samples were collected from 0-5 and 5-10 cm layers of sediment using an 8 cm diameter corer. Water chemistry was monitored with a deployed, automated sonde (Eureka). Total DNA was extracted and 72 gigabases of sequence were obtained with Illumina HiSeq. Taxonomic analysis of the metagenomes was done with Kaiju which uses protein-coding genes for classification. In addition, total organic carbon was measured with an Aurora Analyzer. Biogas ebullition was captured underwater and measured with an Agilent Gas Chromatograph.
Results/Conclusions
During two three month periods, hourly measurements of water chemistry showed pH values of 7.89±0.1 (winter) and 7.49±0.5 (summer); and dissolved oxygen values of 13.18±0.83 mg L-1 (winter) and 8.42±1.47 mg L-1 (summer). Methane percentage in biogas varied among locations. The highest concentration of methane (58-60%) was found in areas where high river discharge recently deposited new sediment following heavy rain. Hydrogenotrophic methanogens (Methanomicrobia and Methanobacteria) were found in all eight samples and had a relative abundance ranging from 0.81 to 2.75%. In conclusion, we present evidence for a highly active anaerobic carbon cycle, yielding significant methane production, even in the dynamic upper sediment layer of a high-flow Appalachian river. Acknowledgements: Funding was provided by NSF Appalachian Freshwater Initiative grant #OIA-1458952.