Contribution of Photosynthesis-Driven Oxic Methane Production to the Methane Cycling of a Tropical River Network Latika Patel, Rashmi Singh, and Shoji D. Thottathil* Cite This: ACS EST Water 2024, 4, 2836−2847 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Oxygenated surface layers of aquatic systems are ubiquitously oversaturated with methane (CH 4 ). A growing number of studies suggest that CH 4 oversaturation in surface waters can be sustained, at least partly, by methanogenesis occurring under oxic conditions. Although we are gaining a better understanding of the extent and drivers of oxic CH 4 production (OMP) in oceanic and lake environments, the existence and variability of OMP in rivers and streams remain unknown. Here, we present experimental evidence for the occurrence and a large variability of OMP rates in a tropical river network. The positive correlation between chlorophyll a concentration and OMP rates and reduction of OMP during the experimental inhibition of photosynthesis establishes a clear link between OMP and photosynthesis. At the same time, a general decrease of the OMP rates with increasing total phosphorus (TP) concentration and the correlation between stable carbon isotopic (δ 13 C−CH 4 ) values of the OMP-derived CH 4 and TP suggest the likely involvement of P-availability as well in regulating the OMP rates. While our estimation suggested a minor contribution of the OMP in the CH 4 cycling of the studied tropical system, we show that the OMP in the fluvial environment may be highly sensitive to the current and future changes in algal and nutrient dynamics. KEYWORDS: methane, oxic methane production, rivers and streams, tropical ■ INTRODUCTION 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”, 1,2 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. 3−9 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. 5,6,8,10,11 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. 6,11 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. 12 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 methano- genic archaea in the production of CH 4 in oxygenated waters, 13−15 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, 9,16−20 bacterial conversion of other methylated compounds (e.g., methylamine, trimethyl- amine, glycine betaine, or dimethyl sulfide) to CH 4 , 7,9,21,22 and Received: December 18, 2023 Revised: May 23, 2024 Accepted: May 28, 2024 Published: June 7, 2024 Article pubs.acs.org/estwater © 2024 American Chemical Society 2836 https://doi.org/10.1021/acsestwater.3c00812 ACS EST Water 2024, 4, 2836−2847 Downloaded via SRM UNIVERSITY AP on July 21, 2024 at 15:56:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.