Influence of fresh particulate organic matter input on Hanford 300 Area riverbed sediment microbial metabolism

Recent studies (Stern et al., 2017, Appl. Environ. Microbiol, 83:e00260-17) have suggested that deposition of fresh particulate organic matter (POM) can lead to elevated rates of aerobic microbial metabolism, increased activity of anaerobic heterotrophic organisms, and release of labile dissolved organic carbon (DOC) in riverbed sediments from the Hanford 300 Area of the Columbia River in Washington State. In the current study we investigated carbon and nitrogen metabolism in aerobic vs. anaerobic Hanford 300 Area riverbed sediments with or without the addition of fresh periphyton-derived POM from the Hanford 300 Area. Results revealed that reactors amended with POM (8.7 mmol C/L addition, equivalent to ca. 15% of the endogenous POM pool) increased microbial activity ca. 2-fold compared to unamended controls under both aerobic and anaerobic conditions. Approximately twice as much of the added POM was metabolized under aerobic vs. anaerobic conditions. Fe(III) reduction was the dominant metabolic pathway in all anaerobic reactors. DOC was rapidly scavenged in aerobic reactors, but accumulated to mM levels in anoxic reactors with added fresh POM, demonstrating the potential for DOC release during anaerobic metabolism. Similarly, ammonium decreased to near detection levels in aerobic reactors, but accumulated to several hundred mM under anaerobic conditions. Both the loss of initial ammonium and metabolism of N released during POM decay resulted in the accumulation of 0.5-1.5 mM nitrate in the aerobic reactors. The results of metagenomic sequence analyses were consistent with expected phylogenetic and functional gene differences between microbial communities in the aerobic versus anaerobic reactors. These results have straightforward, broad-ranging implications for the role of near-surface riverbed sediments in the exchange of DOC and nutrients within riverine corridor environments. They also provide a starting point for development of kinetic models of POM degradation and associated biogeochemical processes in Hanford 300 Area riverbed sediments.