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Terrestrial hydrothermal springs and aquifers are excellent sites to study microbial biogeography because of their high physicochemical heterogeneity across relatively limited geographic regions. In this study, we performed 16S rRNA gene sequencing and metagenomic analyses of the microbial diversity of 11 different geothermal aquifers and springs across the tectonically active Biga Peninsula (Turkey). Across geothermal settings ranging in temperature from 43 to 79°C, one of the most highly represented groups in both 16S rRNA gene and metagenomic datasets was affiliated with the uncultivated phylum " Candidatus Bipolaricaulota" (former " Ca. Acetothermia" and OP1 division). The highest relative abundance of " Ca. Bipolaricaulota" was observed in a 68°C geothermal brine sediment, where it dominated the microbial community, representing 91% of all detectable 16S rRNA genes. Correlation analysis of " Ca. Bipolaricaulota" operational taxonomic units (OTUs) with physicochemical parameters indicated that salinity was the strongest environmental factor measured associated with the distribution of this novel group in geothermal fluids. Correspondingly, analysis of 23 metagenome-assembled genomes (MAGs) revealed two distinct groups of " Ca. Bipolaricaulota" MAGs based on the differences in carbon metabolism: one group encoding the bacterial Wood-Ljungdahl pathway (WLP) for H 2 dependent CO 2 fixation is selected for at lower salinities, and a second heterotrophic clade that lacks the WLP that was selected for under hypersaline conditions in the geothermal brine sediment. In conclusion, our results highlight that the biogeography of " Ca. Bipolaricaulota" taxa is strongly correlated with salinity in hydrothermal ecosystems, which coincides with key differences in carbon acquisition strategies. The exceptionally high relative abundance of apparently heterotrophic representatives of this novel candidate Phylum in geothermal brine sediment observed here may help to guide future enrichment experiments to obtain representatives in pure culture., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Coskun, Gomez-Saez, Beren, Ozcan, Hosgormez, Einsiedl and Orsi.)

Bioremediation of polluted groundwater is one of the most difficult actions in environmental science. Nonetheless, the clean-up of nitrate polluted groundwater may become increasingly important as nitrate concentrations frequently exceed the EU drinking water limit of 50 mg L -1 , largely due to intensification of agriculture and food production. Denitrifiers are natural catalysts that can reduce increasing nitrogen loading of aquatic ecosystems. Porous aquifers with high nitrate loading are largely electron donor limited and additionally, high dissolved oxygen concentrations are known to reduce the efficiency of denitrification. Therefore, denitrification lag times (time prior to commencement of microbial nitrate reduction) up to decades were determined for such groundwater systems. The stimulation of autotrophic denitrifiers by the injection of hydrogen into nitrate polluted regional groundwater systems may represent a promising remediation strategy for such environments. However, besides high costs other drawbacks, such as the transient or lasting accumulation of the cytotoxic intermediate nitrite or the formation of the potent greenhouse gas nitrous oxide, have been described. In this article, we detect causes of incomplete denitrification, which include environmental factors and physiological characteristics of the underlying bacteria and provide possible mitigation approaches., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Duffner, Wunderlich, Schloter, Schulz and Einsiedl.)