Aerobic methane synthesis and dynamics in a river water environment

Abstract

Reports of aerobic biogenic methane (CH4) have generated new views about CH4 sources in nature. We examine this phenomenon in the free‐flowing Yellowstone river wherein CH4 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. CH4 was positively correlated with temperature and Chl a, although diurnal measurements showed CH4 concentrations were greatest during the night and lowest during maximal solar irradiation. CH4 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 CH4 concentrations. Spiking river water with GB or MMA yielded significantly greater CH4 than nonspiked controls, illustrating the metabolic potential of the river microbiome. In summary, this study provides evidence that: (1) phototrophic microorganisms are involved in CH4 synthesis in a river environment; (2) the river microbiome possesses the metabolic potential to convert methylated amines to CH4; and (3) river CH4 concentrations are dynamic diurnally as well as during the summer active months.

Contribution of Photosynthesis-Driven Oxic Methane Production to the Methane Cycling of a Tropical River Network

“…On the other hand, a lack of correlation between dissolved CH 4 concentration and TP across our study sites suggests that anoxic CH 4 production is not directly linked to the TP unlike that proposed in previous studies. , Together, our results suggest that with reduced P-availability in areas away from external nutrient sources, degradation of methylated compounds could contribute toward the background CH 4 saturation. Therefore, our data suggest that phytoplankton-mediated OMP can contribute toward the CH 4 oversaturation observed in nutrient-replete waters, − while P-limitation and subsequent CH 4 production through the degradation of methylated compounds could contribute to the observed CH 4 oversaturation in oligotrophic streams, corroborating some recent studies. , Together, we show that the OMP can be widespread in rivers and streams irrespective of their trophic status due to the multiple mechanisms at play.…”

“…Therefore, our data suggest that phytoplanktonmediated OMP can contribute toward the CH 4 oversaturation observed in nutrient-replete waters, 55−59 while P-limitation and subsequent CH 4 production through the degradation of methylated compounds could contribute to the observed CH 4 oversaturation in oligotrophic streams, corroborating some recent studies. 9,30 Together, we show that the OMP can be widespread in rivers and streams irrespective of their trophic status due to the multiple mechanisms at play.…”

“…Methane (CH 4 ) production in aquatic environments is no longer considered a strictly anaerobic process. In contrast to the early reports that considered CH 4 oversaturation in oxic marine waters as a “paradox”, , a growing number of experimental and field studies have unveiled the involvement of multiple oxic methanogenesis processes and dispelled the notion of “methane paradox” in oxic surface waters. − Several recent studies have highlighted the significant contribution of oxic CH 4 production (OMP) in sustaining CH 4 oversaturation and up to 90% of emissions in some freshwater lakes. ,,,, The disproportionate contribution of OMP to the lake CH 4 emission is partly attributed to the rapid evasion of CH 4 produced in the oxygenated and turbulent surface mixed layers to the atmosphere as well as the links between OMP and morphology of lakes. , If the OMP occurs in highly turbulent rivers and streams, the contribution of the OMP to global CH 4 emission becomes even more significant given that fluvial CH 4 emission likely accounts for nearly half of the total inland water CH 4 emission . Nonetheless, the limited evidence of the occurrence of OMP in rivers and streams presents a new challenge in comprehending CH 4 cycling in fluvial systems.…”

“…While early studies suggested the involvement of methanogenic archaea in the production of CH 4 in oxygenated waters, − recent evidence indicates a diverse range of organisms and processes contributing to OMP. These include CH 4 synthesis by the bacterial cleavage of methylphosphonate (MPn) in phosphate-limited waters, ,− bacterial conversion of other methylated compounds (e.g., methylamine, trimethylamine, glycine betaine, or dimethyl sulfide) to CH 4 , ,,, and even the direct release of CH 4 by phytoplankton and cyanobacteria. − However, these studies have been conducted in lakes or marine waters or using laboratory-grown single autotrophic species in culture media. Consequently, our understanding of OMP and its role in the CH 4 cycling of rivers and streams remains elusive.…”

“…30 Another recent study in the temperate Yellowstone River showed potential CH 4 production from methylated compounds in oxic waters. 9 However, direct experimental estimation of the level of OMP in rivers and streams is still lacking, and we are yet to determine the contribution, controls, and possible pathways of the OMP in fluvial environments.…”