We measured seasonal variations in the vertical distribution of methane concentration, methane oxidation rates, and lipid biomarkers in the northern basin of Lake Lugano. Methane consumption below the oxic-anoxic interface co-occurred with concentration maxima of 13 C-depleted C 16 fatty acid biomarkers (with d 13 C values as low as 270%) in the anoxic water column, as well as characteristic d 13 C CH 4 profiles. We argue that the conspicuous methane concentration gradients are primarily driven by (micro-)aerobic methane oxidation (MOx) below the chemocline. We measured a strong MOx potential throughout the anoxic water column, while MOx rates at in situ O 2 concentration . 10 nmol L 21 were undetectable. Similarly, we found MOx-related biomarkers and gene sequences encoding the particulate methane monooxygenase in the anoxic, but not the oxic, water. The mechanism of (episodic) oxygen supply sustaining the MOx community in anoxic waters is still uncertain. Our results indicate that a bacterial methanotrophic community is responsible for the methane consumption in Lake Lugano, without detectable contribution from archaeal methanotrophs. Bacterial populations that accumulated both at the suboxic-anoxic interface and in the deeper anoxic hypolimnion, where maximum potential MOx rates were observed throughout the year (1.5-2.5 mmol L 21 d 21 ) were mainly related to Methylobacter sp. Close relatives are found in lacustrine environments throughout the world, and their potential to thrive under micro-and anoxic conditions in Lake Lugano may imply that micro-aerobic methane oxidation is important in methane cycling and competition for methane and oxygen in stratified lakes worldwide.

Anaerobic oxidation of ammonium (anammox) is recognized as an important process in the marine nitrogen cycle yet nothing is known about the distribution, diversity and activity of anammox bacteria in the terrestrial realm. In this study, we report on the detection of anammox sequences of Candidatus 'Brocadia', 'Kuenenia', 'Scalindua' and 'Jettenia' in marshes, lakeshores, a contaminated porous aquifer, permafrost soil, agricultural soil and in samples associated with nitrophilic or nitrogen-fixing plants. This suggests a higher diversity of anammox bacteria in terrestrial than in marine ecosystems and could be a consequence of the larger variety of suitable niches in soils. Anammox bacteria were not ubiquitously present but were only detected in certain soil types and at particular depths, thus reflecting specific ecological requirements. As opposed to marine water column habitats where Candidatus 'Scalindua' dominates anammox guilds, 'Kuenenia' and 'Brocadia' appear to be the most common representatives in terrestrial environments.

Most of the sulfi de produced in surface marine sediments is eventually oxidized back to sulfate via sulfur compounds of intermediate oxidation state in a complexweb of competing chemical and biological reactions. Improved handling, derivatization, and chromatographic techniques allowed us to more closely examine the occurrence and fate of the sulfur intermediates elemental sulfur (S 0 ), thiosulfate (S 2 O 3 2− ), tetrathionate (S 4 O 6 2− ), and sulfi te (SO 3 2− ) in Black Sea and North Sea sediments. Elemental sulfur was the most abundant sulfur intermediate with concentrations ~3 orders of magnitude higher than the dissolved species, which were typically in the low micromolar range or below. Turnover times of the intermediate sulfur compounds were inversely correlated with concentration and followed the order: SO 3 2− ≈ S 4 O 6 2− > S 2 O 3 2− > S 0 .Experiments with anoxic but non-sulfi dic surface sediments from the Black Sea revealed that added sulfi de and sulfi te disappeared most rapidly, followed by thiosulfate. Competing chemical reactions, including the reaction of sulfi te with sedimentary S 0 that led to temporarily increased thiosulfate concentrations, resulted in the rapid disappearance of SO 3 2− . Conversely, low thiosulfate concentrations in the Black Sea sediments (<3µM) were attributed to the activity of thiosulfate-consuming bacteria. Experiments with anoxic but non-sulfi dic sediments revealed that 1 mol of tetrathionate was rapidly converted to 2 moles of thiosulfate. This tetrathionate reduction was bacterially mediated and occurred generally much faster than thiosulfate consumption. The rapid reduction of tetrathionate back to thiosulfate creates a cul-de-sac in the sulfur cycle, with thiosulfate acting as a bottleneck for the oxidation pathways between sulfi de and sulfate.

The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were -7.2‰ and -8.5‰ for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only -0.3‰ for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of -25.2‰ for VC and -18.5‰ for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichment factors were larger than those of aerobic oxidation (-1.8‰ for VC, -1.5‰ for cDCE), they were not as large as typically expected for a primary chlorine isotope effect suggesting that no cleavage of C-Cl bonds takes place during the initial ratelimiting step. The ratio of isotope enrichment factors for chlorine and carbon were substantially different for the two reaction mechanisms suggesting that the reaction mechanisms can be differentiated at the field scale using a dual isotope approach.

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SignificanceReliable prediction of future climate conditions requires a thorough understanding of climate variability throughout Earth’s history. Microbial molecular fossils, such as bacterial membrane-spanning tetraether lipids [branched glycerol dialkyl glycerol tetraethers (brGDGTs)], have proven to be particularly useful for the assessment of past climatic conditions, because they occur ubiquitously in the environment and show compositional changes related to temperature. However, the identity and ecology of brGDGT-producing bacteria is largely unknown, and a mechanistic basis for brGDGT-based paleoclimate reconstruction is still lacking. Here, we present insights into the ecological parameters that affect brGDGT synthesis in lakes, demonstrating that eutrophic lakes with oxygen-deprived bottom waters are the preferred sites for brGDGT-based reconstructions of continental climate.

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