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Aerobic methane oxidation (MOX) significantly reduces methane (CH 4 ) emissions from inland water bodies and is, therefore, an important determinant of global CH 4 budget. Yet, the magnitude and controls of MOX rates in rivers - a quantitatively significant natural source of atmospheric CH 4 - are poorly constrained. Here, we conducted a series of incubation experiments to understand the magnitude and environmental controls of MOX rates in tropical fluvial systems. We observed a large variability in MOX rate (0.03 - 3.45 μmol l -1 d -1 ) shaped by a suit of environmental variables. Accordingly, we developed an empirical model for MOX that incorporate key environmental drivers, including temperature, CH 4 , total phosphorus, and dissolved oxygen (O 2 ) concentrations, based on the results of our incubation experiments. We show that temperature dependency of MOX (activation energy: 0.66 ± 0.18 eV) is lower than that of sediment methanogenesis (0.71 ± 0.21 eV) in the studied tropical fluvial network. Furthermore, we observed a non-linear relationship between O 2 concentration and MOX, with the highest MOX rate occuring ∼135 μmol O 2 l -1 , above or below this "optimal O 2 " concentration, MOX rate shows a gradual decline. Together, our results suggest that the relatively lower temperature response of MOX compared to methanogenesis along with the projected decrease of O 2 concentration due to organic pollution may cause elevated CH 4 emission from tropical southeast Asian rivers. Since estimation of CH 4 oxidation is often neglected in routine CH 4 monitoring programs, the model developed here may help to integrate MOX rate into process-based models for fluvial CH 4 budget., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Rivers are globally significant natural sources of atmospheric methane (CH 4 ). However, the effect of land use changes on riverine CH 4 dynamics, particularly in tropical zones, remain ambiguous, yet important to predict and anticipate the present and future contribution of rivers to the global CH 4 budget. The present study examines the magnitude and drivers of riverine CH 4 concentration and emission in the tropical Krishna River (KR) basin, India. The large spatial variability of CH 4 concentration (0.03 to 185.34 μmol L -1 ) and emissions (0.04 mmol m -2 d -1 to 1666.24 mmol m -2 d -1 ) in the KR basin was linked to the site-specific features of the catchments through which rivers are draining. Several fold higher CH 4 concentration and emission was observed for the urban river sites (64.63 ± 53.17 µmol L -1 and 294.15 ± 371.52 mmol m 2 d -1 , respectively) than the agricultural (1.05 ± 2.22 µmol L -1 and 3.45 ± 9.72 mmol m 2 d -1 , respectively) and forested (0.49 ± 0.23 µmol L -1 and 1.26 ± 0.73 mmol m 2 d -1 , respectively) sites. The concentrations of dissolved oxygen, total phosphorus, and Chlorophyll-a were significant hydrochemical variables strongly coupled with the dissolved CH 4 concentrations. On the other hand, percentage of built-up area emerged as the most important landscape-level driver indicating that urbanization has an overriding effect on riverine CH 4 concentration in the agriculture dominated KR basin. Our study supports the growing notion that tropical urban rivers are hotspot of CH 4 emission. Furthermore, we show that the pattern of increasing in riverine CH 4 concentration with built-up area (%) is a general feature of Asian river basins. As the urban land cover and population following an exponential increase, Asian rivers might contribute substantially to the regional and global CH 4 budget., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper, (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH 4 ) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH 4 production (OMP) have remained ambiguous. Based on the summer CH 4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km 2 ), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L -1 day -1 ) is linked to the concentrations of chlorophyll- a , total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH 4 (δ 13 C-CH 4 ) indicates direct photoautotrophic release as the most likely source of oxic CH 4 . Furthermore, we show that the oxic CH 4 contribution to the SML CH 4 saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km 2 . Given that global lake CH 4 emissions are dominated by small lakes (SA of <1 km 2 ), the large contribution of oxic CH 4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH 4 emissions.

Previous stable isotope and biomarker evidence has indicated that methanotrophy is an important pathway in the microbial loop of freshwater ecosystems, despite the low cell abundance of methane-oxidizing bacteria (MOB) and the low methane concentrations relative to the more abundant dissolved organic carbon (DOC). However, quantitative estimations of the relative contribution of methanotrophy to the microbial carbon metabolism of lakes are scarce, and the mechanism allowing methanotrophy to be of comparable importance to DOC-consuming heterotrophy remained elusive. Using incubation experiments, microscopy, and multiple water column profiles in six temperate lakes, we show that MOB play a much larger role than their abundances alone suggest because of their larger cell size and higher specific activity. MOB activity is tightly constrained by the local methane:oxygen ratio, with DOC-rich lakes with large hypolimnetic volume fraction showing a higher carbon consumption through methanotrophy than heterotrophy at the whole water column level. Our findings suggest that methanotrophy could be a critical microbial carbon consumption pathway in many temperate lakes, challenging the prevailing view of a DOC-centric microbial metabolism in these ecosystems., (© 2022. The Author(s).)

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH 4 ) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH 4 emission and isotopic signature. Here, we examine the magnitude of CH 4 ebullition (bubbling) and stable carbon isotopic signature (δ 13 C-CH 4 ) of bubble CH 4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH 4 flux and bubble δ 13 C-CH 4 varied from 0.01 to 37.0 mmol m -2  d -1 and between -71.0‰ and -50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ 13 C-CH 4 signature. Subsequently, in addition to the temperature dependence (1.4 ± 0.1 eV), the rates of ebullition strongly correlated with the variability of δ 13 C-CH 4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ 13 C-CH 4 values (-64.6 ± 0.6‰ to -60.1 ± 3.2‰), yet the signature of the total CH 4 emission (ebullition + diffusion) was relatively enriched (-60.7‰ to -52.6‰) due to high methanotrophic activity in the water column. We show that δ 13 C-CH 4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH 4 source signature to northern lakes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Lake methane (CH 4 ) emissions are largely controlled by aerobic methane-oxidizing bacteria (MOB) which mostly belong to the classes Alpha- and Gammaproteobacteria (Alpha- and Gamma-MOB). Despite the known metabolic and ecological differences between the two MOB groups, their main environmental drivers and their relative contribution to CH 4 oxidation rates across lakes remain unknown. Here, we quantified the two MOB groups through CARD-FISH along the water column of six temperate lakes and during incubations in which we measured ambient CH 4 oxidation rates. We found a clear niche separation of Alpha- and Gamma-MOB across lake water columns, which is mostly driven by oxygen concentration. Gamma-MOB appears to dominate methanotrophy throughout the water column, but Alpha-MOB may also be an important player particularly in well-oxygenated bottom waters. The inclusion of Gamma-MOB cell abundance improved environmental models of CH 4 oxidation rate, explaining part of the variation that could not be explained by environmental factors alone. Altogether, our results show that MOB composition is linked to CH 4 oxidation rates in lakes and that information on the MOB community can help predict CH 4 oxidation rates and thus emissions from lakes., (© 2019 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Changes in the phytoplankton biomass (chlorophyll a), production rate, and species composition were studied over two seasons using the time series measurements in the northern limb of the Cochin estuary in relation to the prevailing hydrological conditions. The present study showed the significant seasonal variation in water temperature (F=69.4, P<0.01), salinity (F=341.93, P<0.01), dissolved inorganic phosphorous (F=17.71, P<0.01), and silica (F=898.1, P<0.01) compared to nitrogen (F=1.646, P>0.05). The uneven input of ammonia (3.4-224.8 μM) from upstream (Periyar River) leads to the inconsistency in the N/P ratio (range 6.8-262). A distinct seasonality was observed in Si/N (F=382.9, P<0.01) and Si/P (F=290.3, P<0.01) ratios compared to the N/P ratio (F=1.646, P>0.05). The substantial increase in chlorophyll a (average, 34.8±10 mg m(-3)) and primary production (average, 1,304±694 mg C m(-3) day(-1)) indicated the mesotrophic condition of the study area during the premonsoon (PRM) and it was attributed to the large increase in the population of nanoplankton (size<20 μ) such as Skeletonema costatum, Thalassiosira subtilis, Nitzschia closterium, and Navicula directa. In contrast, during the post monsoon (PM), low chlorophyll a concentration (average, 9.3±9.2 mg m(-3)) and primary production (average, 124±219 mg C m(-3) day(-1)) showed heterotrophic condition. It can be stated that favorable environmental conditions (optimum nutrients and light intensity) prevailing during the PRM have enhanced the abundance of the nanoplankton community in the estuary, whereas during the PM, the light limitation due to high turbidity can reduce the nanoplankton growth and abundance, even though high nutrient level exists.