Aerobic methane synthesis and dynamics in a river water environment

Reports of aerobic biogenic methane (${\text{CH}}_4$) have generated new views about ${\text{CH}}_4$ sources in nature. We examine this phenomenon in the free-flowing Yellowstone river wherein ${\text{CH}}_4$ concentrations were tracked as a function of environmental conditions, phototrophic microorganisms (using chlorophyll $a$, Chl $a$, as proxy), as well as targeted methylated amines known to be associated with this process. ${\text{CH}}_4$ was positively correlated with temperature and Chl $a$, although diurnal measurements showed ${\text{CH}}_4$ concentrations were greatest during the night and lowest during maximal solar irradiation. ${\text{CH}}_4$ efflux from the river surface was greater in quiescent edge waters (71-94 μmol m^-2 d) than from open flowing current (~ 57 μmol m^-2 d). Attempts to increase flux by disturbing the benthic environment in the quiescent water directly below (~ 1.0 m deep) or at varying distances (0-5 m) upstream of the flux chamber failed to increase surface flux. Glycine betaine (GB), dimethylamine and methylamine (MMA) were observed throughout the summer-long study, increasing during a period coinciding with a marked decline in Chl $a$, suggesting a lytic event led to their release; however, this did not correspond to increased ${\text{CH}}_4$ concentrations. Spiking river water with GB or MMA yielded significantly greater ${\text{CH}}_4$ than nonspiked controls, illustrating the metabolic potential of the river microbiome. In summary, this study provides evidence that: (1) phototrophic microorganisms are involved in ${\text{CH}}_4$ synthesis in a river environment; (2) the river microbiome possesses the metabolic potential to convert methylated amines to ${\text{CH}}_4$; and (3) river ${\text{CH}}_4$ concentrations are dynamic diurnally as well as during the summer active months.