We describe stromatolites forming at an altitude of 3570 m at the shore of a volcanic lake Socompa, Argentinean Andes. The water at the site of stromatolites formation is alkaline, hypersaline, rich in inorganic nutrients, very rich in arsenic, and warm (20–24°C) due to a hydrothermal input. The stromatolites do not lithify, but form broad, rounded and low-domed bioherms dominated by diatom frustules and aragonite micro-crystals agglutinated by extracellular substances. In comparison to other modern stromatolites, they harbour an atypical microbial community characterized by highly abundant representatives of Deinococcus-Thermus, Rhodobacteraceae, Desulfobacterales and Spirochaetes. Additionally, a high proportion of the sequences that could not be classified at phylum level showed less than 80% identity to the best hit in the NCBI database, suggesting the presence of novel distant lineages. The primary production in the stromatolites is generally high and likely dominated by Microcoleus sp. Through negative phototaxis, the location of these cyanobacteria in the stromatolites is controlled by UV light, which greatly influences their photosynthetic activity. Diatoms, dominated by Amphora sp., are abundant in the anoxic, sulfidic and essentially dark parts of the stromatolites. Although their origin in the stromatolites is unclear, they are possibly an important source of anaerobically degraded organic matter that induces in situ aragonite precipitation. To the best of our knowledge, this is so far the highest altitude with documented actively forming stromatolites. Their generally rich, diverse and to a large extent novel microbial community likely harbours valuable genetic and proteomic reserves, and thus deserves active protection. Furthermore, since the stromatolites flourish in an environment characterized by a multitude of extremes, including high exposure to UV radiation, they can be an excellent model system for studying microbial adaptations under conditions that, at least in part, resemble those during the early phase of life evolution on Earth.

Summary: A dataset of 87 020 nifH reads and 16 782 unique nifH protein sequences obtained over 2 years from four locations across a gradient of agricultural soil types in A rgentina were analysed to provide a detailed and comprehensive picture of the diversity, abundance and responses of the N ₂ ‐fixing community in relation to differences in soil chemistry and agricultural practices. Phylogenetic analysis revealed an expected high proportion of A lphaproteobacteria , B etaproteobacteria and D eltaproteobacteria , mainly relatives to B radyrhizobium and M ethylosinus / M ethylocystis , but a surprising paucity of G ammaproteobacteria . Analysis of variance and stepwise regression modelling suggested location and treatment‐specific influences of soil type on diazotrophic community composition and organic carbon concentrations on nifH diversity. nifH gene abundance, determined by quantitative real‐time polymerase chain reaction, was higher in agricultural soils than in non‐agricultural soils, and was influenced by soil chemistry under intensive crop rotation but not under monoculture. At some locations, sustainable increased crop yields might be possible through the management of soil chemistry to improve the abundance and diversity of N ₂ ‐fixing bacteria.

In this survey, a total of 80 787 reads and 28 171 unique NifH protein sequences were retrieved from soil RNA. This dataset extends our knowledge about the structure and diversity of the functional diazotrophic communities in agricultural soils from Argentinean Pampas. Operational taxonomic unit (OTU)‐based analyses showed that nifH phylotypes related to Geobacter and Anaeromyxobacter (44.8%), Rhizobiales (29%), Cyanobacteria (16.7%), and Verrucomicrobiales (8%) are key microbial components of N₂ fixation in soils associated with no‐till management and soil depth. In addition, quantification of nifH gene copies related to Geobacter and Cyanobacteria revealed that these groups are abundant in soils under maize–soybean rotation and soybean monoculture, respectively. The correlation of physicochemical soil parameters with the diazotrophic diversity and composition showed that soil stability and organic carbon might contribute to the functional signatures of particular nifH phylotypes in fields under no‐till management. Because crop production relies on soil‐borne microorganism's activities, such as free N₂ fixation, the information provided by our study on the diazotrophic population dynamics, associated with the edaphic properties and land‐use practices, represents a major contribution to gain insight into soil biology, in which functionally active components are identified.