Methanotrophy under Versatile Conditions in the Water Column of the Ferruginous Meromictic Lake La Cruz (Spain)

Lakes represent a considerable natural source of methane to the atmosphere compared to their small global surface area. Methanotrophs in sediments and in the water column largely control methane fluxes from these systems, yet the diversity, electron accepting capacity, and nutrient requirements of these microorganisms have only been partially identified. Here, we investigated the role of electron acceptors alternative to oxygen and sulfate in microbial methane oxidation at the oxycline and in anoxic waters of the ferruginous meromictic Lake La Cruz, Spain. Active methane turnover in a zone extending well below the oxycline was evidenced by stable carbon isotope-based rate measurements. We observed a strong methane oxidation potential throughout the anoxic water column, which did not vary substantially from that at the oxic/anoxic interface. Both in the redox-transition and anoxic zones, only aerobic methane-oxidizing bacteria (MOB) were detected by fluorescence in situ hybridization and sequencing techniques, suggesting a close coupling of cryptic photosynthetic oxygen production and aerobic methane turnover. Additions of nitrate, nitrite and to a lesser degree iron and manganese oxides also stimulated bacterial methane consumption. We could not confirm a direct link between the reduction of these compounds and methane oxidation and we cannot exclude the contribution of unknown anaerobic methanotrophs. Nevertheless, our findings from Lake La Cruz support recent laboratory evidence that aerobic methanotrophs may be able to utilize alternative terminal electron acceptors under oxygen limitation

Methane oxidation pathways in Lake La Cruz

Even though lakes only occupy a small percentage of Earth's surface, they represent a natural source of methane, contributing substantially to methane emissions and ultimately to global warming. There is extensive evidence that methane emissions from lakes are mitigated by its oxidation. However, many uncertainties concerning biotic controls on methane release from lakes remain, especially in zones where oxygen is depleted and other electron acceptors might be important. In this study, biological methane oxidation was investigated in the water column of Lake La Cruz, a small karstic lake located in Central Eastern Spain near the city of Cuenca. Permanent stratification and unusually high concentrations of dissolved iron(II) make this lake ideal for studying methane oxidation in the absence of oxygen, particularly the possibility of this process coupled to iron reduction. The physical and chemical properties of Lake La Cruz were investigated, including the presence of methane and possible electron acceptors. Incubation experiments with 13C-methane were carried out to measure methane oxidation rates, test possible electron acceptors and other coupled processes. Highest methane oxidation rates were measured at the oxic-anoxic boundary, where aerobic oxidation dominated. Moreover, the aerobic pathway also seemed relevant below the oxycline, likely triggered by in-situ production of oxygen via photosynthesis. Evidence suggests that nitrate, nitrite and iron also function as electron acceptors for methane oxidation, whereas manganese and sulfate are unlikely oxidants. Methane oxidation coupled to denitrification seemed to proceed in close vicinity to aerobic methane oxidation and was likely limited by nitrate/nitrite availability and competition with denitrifiers. Iron mediated methane oxidation only appeared relevant in lower parts of the lake where both oxygen and nitrate were depleted. Though known aerobic methanotrophs were present and appeared to mediate methane oxidation to some degree, the involvement of unknown groups of aerobic and also anaerobic methane-oxidizers is probable. Methane emissions to the atmosphere are effectively mitigated by a seemingly complex interplay of aerobic and anaerobic processes in Lake La Cruz

Methane oxidation in the waters of a humic-rich boreal lake stimulated by photosynthesis, nitrite, Fe(III) and humics

Small boreal lakes are known to contribute significantly to global CH4 emissions. Lake Lovojarvi is a eutrophic lake in southern Finland with bottom water CH4 concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emission potential, was low (< 0.5 mu M). We studied the biogeochemical processes involved in CH4 removal by chemical profiling and through incubation experiments. delta C-13-CH4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH4 oxidation. We observed a distinct peak in CH4 concentration at the chlorophyll-a maximum, caused by either in situ CH4 production or other CH4 inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in CH4 oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH4 oxidation. Surprisingly, nitrite seemed to inhibit CH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH4 oxidation can be a key pro-cess in CH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH4 emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen

Microbial and abiotic nitrous oxide cycling in the water column of meromictic, iron-rich Lake La Cruz, Spain, 2015-2017

We investigated the microbial and abiotic N2O cycle in the water column of iron-rich, meromictic Lake La Cruz, Spain, during two sampling campaigns in March 2015 and March 2017. At the deepest point of the lake, we used a profiling in situ analyzer equipped with several probes and optodes to detect physicochemical parameters. In addition, we collected water column samples via an in situ pump in order to analyze concentrations of N, S, and Fe species as well as isotope characteristics of several N species. In 2017, we used a Niskin bottle to take water samples from 8.0 and 14.5 m depth for two types of incubation experiments. In the first set of experiments, we added 15N-labeled substrates, and in some incubations Fe2+, to filtered and unfiltered lake water, and analyzed the produced N2O, N2, and NH4+. In the other experiment, we determined the N and O isotope effects of NO2- and N2O during chemodenitrification (reaction of NO2- and Fe2+) in anoxic and sterile lake water from 14.5 m depth

Isotopic signatures of biotic and abiotic N2O production and consumption in the water column of meromictic, ferruginous Lake La Cruz (Spain)

Lakes can be important sources of the potent greenhouse gas nitrous oxide (N2O) to the atmosphere, but to what extent abiotic processes may contribute to lacustrine N2O production remains uncertain. We assessed pathways of N2O production and reduction in the water column of meromictic and iron-rich Lake La Cruz, Spain, including chemodenitrification-induced N2O formation via the reaction of reactive nitrogen (N) (e.g., NO2-) with ferrous iron (Fe[II]). In the oxic waters (similar to 8-10 m), N2O concentrations above atmospheric equilibrium were associated with comparatively low delta N-15-N2O, high delta N-15-NH4+, and high N2O N-15-site-preference (SP) values (up to similar to 29 parts per thousand), suggesting N2O production by nitrification. N2O concentrations were highest (23-33 nM) near the depth of oxygen depletion (similar to 11-14.5 m), likely due to production by nitrifier denitrification and/or denitrification, as indicated by decreasing SP values (as low as 12 parts per thousand). Further below (similar to 14.5-17 m), N2O consumption was indicated by increasing SP values and a delta O-18-vs.-delta N-15 relationship (1.8-2.9) typical for stand-alone N2O reduction. The coupled N-vs.-O isotope signatures thus highlight the spatial, redox-dependent separation of incomplete and complete denitrification. In incubations with sterile-filtered lake water and N-15-labeled or unlabeled substrate, NO2- was reduced by Fe(2+ )to N2O, even at low nitrite concentrations (5 mu M NO2-). In the water column, the spatial separation of NO2- and Fe(II) during our samplings appears to preclude elevated rates of chemodenitrification, but during periods of overlapping NO2- and Fe(II) in Lake La Cruz, and potentially in other lakes, its distinct N2O delta O-18 vs.-delta N-15 relationship of similar to 1 : 1, as experimentally determined, could help to detect it.ISSN:0024-3590ISSN:1939-559