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Life in hypersaline environments is typically limited by bioenergetic constraints. Microbial activity at the thermodynamic edge, such as the anaerobic oxidation of methane ( AOM) coupled to sulphate reduction ( SR), is thus unlikely to thrive in these environments. In this study, carbon and sulphur cycling was investigated in the extremely hypersaline cold seep sediments of Mercator mud volcano. AOM activity was partially inhibited but still present at salinity levels of 292 g L−1 ( c. eightfold sea water concentration) with rates of 2.3 nmol cm−3 day−1 and was even detectable under saturated conditions. Methane and evaporite-derived sulphate comigrated in the ascending geofluids, which, in combination with a partial activity inhibition, resulted in AOM activity being spread over unusually wide depth intervals. Up to 79% of total cells in the AOM zone were identified by fluorescence in situ hybridization ( FISH) as anaerobic methanotrophs of the ANME-1. Most ANME-1 cells formed monospecific chains without any attached partner. At all sites, AOM activity co-occurred with SR activity and sometimes significantly exceeded it. Possible causes of these unexpected results are discussed. This study demonstrates that in spite of a very low energy yield of AOM, microorganisms carrying this reaction can thrive in salinity up to halite saturation. [ABSTRACT FROM AUTHOR]

Summary The anaerobic oxidation of methane (AOM) by methanotrophic archaea and sulfate-reducing bacteria is the major sink of methane formed in marine sediments. The study of AOM as well as of methanogenesis in different habitats is essentially connected with the in situ analysis of stable isotope (13C/12C, D/H) signatures ( δ-values). For their kinetic interpretation, experimental (cultivation-based) isotope fractionation factors ( α-values) are richly available in the case of methanogenesis, but are scarce in the case of AOM. Here we used batch enrichment cultures with high AOM activity and without background methanogenesis from detrital remnants to determine 13C/12C and D/H fractionation factors. The enrichment cultures which originated from three marine habitats (Hydrate Ridge, NE Pacific; Amon Mud Volcano, Mediterranean Sea; NW shelf, Black Sea) were dominated by archaeal phylotypes of anaerobic methanotrophs (ANME-2 clade). Isotope fractionation factors calculated from the isotope signatures as a function of the residual proportion of methane were 1.012-1.039 for 13CH4/12CH4 and 1.109-1.315 for CDH3/CH4. The present values from in vitro experiments were significantly higher than values previously estimated from isotope signature distributions in marine sediment porewater, in agreement with the overlap of other processes with AOM in the natural habitat. [ABSTRACT FROM AUTHOR]

The deep anoxic shelf of the northwestern Black Sea has numerous gas seeps, which are populated by methanotrophic microbial mats in and above the seafloor. Above the seafloor, the mats can form tall reef-like structures composed of porous carbonate and microbial biomass. Here, we investigated the spatial patterns of CH4 and CO2 assimilation in relation to the distribution of ANME groups and their associated bacteria in mat samples obtained from the surface of a large reef structure. A combination of different methods, including radiotracer incubation, beta microimaging, secondary ion mass spectrometry, and catalyzed reporter deposi- tion fluorescence in situ hybridization, was applied to sections of mat obtained from the large reef structure to locate hot spots of methanotrophy and to identify the responsible microbial consortia. In addition, CO2 reduction to methane was investigated in the presence or absence of methane, sulfate, and hydrogen. The mat had an average δ13C carbon isotopic signature of -67.1%v, indicating that methane was the main carbon source. Regions dominated by ANME-1 had isotope signatures that were significantly heavier (-66.4%v - 3.9 %0 [mean - standard deviation; n = 7]) than those of the more central regions dominated by ANME-2 (-72.9‰ ± 2.2 %v; n = 7). Incorporation of 14C from radiolabeled CH4 or CO2 revealed one hot spot for methanotrophy and CO2 fixation close to the surface of the mat and a low assimilation efficiency (1 to 2% of methane oxidized). Replicate incubations of the mat with 14CH4 or 14CO2 revealed that there was intercon- version of CH4 and CO2. The level of CO2 reduction was about 10% of the level of anaerobic oxidation of methane. However, since considerable methane formation was observed only in the presence of methane and sulfate, the process appeared to be a rereaction of anaerobic oxidation of methane rather than net methanogenesis. [ABSTRACT FROM AUTHOR]

Microbial communities in marine sediments represent some of the densest and most diverse biological communities known, with up to a billion cells and thousands of species per milliliter. Among this taxonomic diversity, the class Acidimicrobiia, within the phylum Actinomycetota, stands out for its consistent presence, yet its limited taxonomic understanding obscures its ecological role. We used metagenome-assembled genomes from a 5-year Arctic fjord sampling campaign and compared them to publicly available Acidimicrobiia genomes using 16S rRNA gene and whole-genome phylogenies, alongside gene prediction and annotation to study their taxonomy and genomic potential. Overall, we provide a taxonomic overview of the class Acidimicrobiia and show its significant prevalence in Isfjorden and Helgoland coastal sediments, representing over 90% of Actinomycetota 16S rRNA gene sequences, and 3-7% of Bacteria. We propose Benthobacter isfjordensis gen. nov., sp. nov., Hadalibacter litoralis gen. nov., sp. nov., and two new species from Ilumatobacter, following SeqCode guidelines. In addition, we report the first in situ quantification of the family Ilumatobacteraceae, revealing its substantial presence (1-6%) in coastal sediments. This work highlights the need of refining the taxonomy of Acidimicrobiia to better understand their ecological contributions., (Copyright © 2024. Published by Elsevier GmbH.)

The Kryos Basin is a deep-sea hypersaline anoxic basin (DHAB) located in the Eastern Mediterranean Sea (34.98°N 22.04°E). It is filled with brine of re-dissolved Messinian evaporites and is nearly saturated with MgCl 2 -equivalents, which makes this habitat extremely challenging for life. The strong density difference between the anoxic brine and the overlying oxic Mediterranean seawater impedes mixing, giving rise to a narrow chemocline. Here, we investigate the microbial community structure and activities across the seawater-brine interface using a combined biogeochemical, next-generation sequencing, and lipid biomarker approach. Within the interface, we detected fatty acids that were distinctly 13 C-enriched when compared to other fatty acids. These likely originated from sulfide-oxidizing bacteria that fix carbon via the reverse tricarboxylic acid cycle. In the lower part of the interface, we also measured elevated rates of methane oxidation, probably mediated by aerobic methanotrophs under micro-oxic conditions. Sulfate reduction rates increased across the interface and were highest within the brine, providing first evidence that sulfate reducers (likely Desulfovermiculus and Desulfobacula) thrive in the Kryos Basin at a water activity of only ~0.4 A w . Our results demonstrate that a highly specialized microbial community in the Kryos Basin has adapted to the poly-extreme conditions of a DHAB with nearly saturated MgCl 2 brine, extending the known environmental range where microbial life can persist.

The anaerobic oxidation of methane (AOM) by methanotrophic archaea and sulfate-reducing bacteria is the major sink of methane formed in marine sediments. The study of AOM as well as of methanogenesis in different habitats is essentially connected with the in situ analysis of stable isotope ((13) C/(12) C, D/H) signatures (δ-values). For their kinetic interpretation, experimental (cultivation-based) isotope fractionation factors (α-values) are richly available in the case of methanogenesis, but are scarce in the case of AOM. Here we used batch enrichment cultures with high AOM activity and without background methanogenesis from detrital remnants to determine (13) C/(12) C and D/H fractionation factors. The enrichment cultures which originated from three marine habitats (Hydrate Ridge, NE Pacific; Amon Mud Volcano, Mediterranean Sea; NW shelf, Black Sea) were dominated by archaeal phylotypes of anaerobic methanotrophs (ANME-2 clade). Isotope fractionation factors calculated from the isotope signatures as a function of the residual proportion of methane were 1.012-1.039 for (13) CH4 /(12) CH4 and 1.109-1.315 for CDH3 /CH4 . The present values from in vitro experiments were significantly higher than values previously estimated from isotope signature distributions in marine sediment porewater, in agreement with the overlap of other processes with AOM in the natural habitat., (© 2009 Society for Applied Microbiology and Blackwell Publishing Ltd.)