Geology defines microbiome structure and composition in nunataks and valleys of the Sør Rondane Mountains, East Antarctica.

Understanding the relation between terrestrial microorganisms and edaphic factors in the Antarctic can provide insights into their potential response to environmental changes. Here we examined the composition of bacterial and micro-eukaryotic communities using amplicon sequencing of rRNA genes in 105 soil samples from the Sør Rondane Mountains (East Antarctica), differing in bedrock or substrate type and associated physicochemical conditions. Although the two most widespread taxa (Acidobacteriota and Chlorophyta) were relatively abundant in each sample, multivariate analysis and co-occurrence networks revealed pronounced differences in community structure depending on substrate type. In moraine substrates, Actinomycetota and Cercozoa were the most abundant bacterial and eukaryotic phyla, whereas on gneiss, granite and marble substrates, Cyanobacteriota and Metazoa were the dominant bacterial and eukaryotic taxa. However, at lower taxonomic level, a distinct differentiation was observed within the Cyanobacteriota phylum depending on substrate type, with granite being dominated by the Nostocaceae family and marble by the Chroococcidiopsaceae family. Surprisingly, metazoans were relatively abundant according to the 18S rRNA dataset, even in samples from the most arid sites, such as moraines in Austkampane and Widerøefjellet ("Dry Valley"). Overall, our study shows that different substrate types support distinct microbial communities, and that mineral soil diversity is a major determinant of terrestrial microbial diversity in inland Antarctic nunataks and valleys.

Ecophysiological and genomic approaches to cyanobacterial hardening for restoration.

Cyanobacteria inhabit extreme environments, including drylands, providing multiple benefits to the ecosystem. Soil degradation in warm drylands is increasing due to land use intensification. Restoration methods adapted to the extreme stress in drylands are being developed, such as cyanobacteria inoculation to recover biocrusts. For this type of restoration method to be a success, it is crucial to optimize the survival of inoculated cyanobacteria in the field. One strategy is to harden them to be acclimated to stressful conditions after laboratory culturing. Here, we analyzed the genome and ecophysiological response to osmotic desiccation and UVR stresses of an Antarctic cyanobacterium, Stenomitos frigidus ULC029, which is closely related to other cyanobacteria from warm and cold dryland soils. Chlorophyll a concentrations showed that preculturing ULC029 under moderate osmotic stress improved its survival during an assay of desiccation plus rehydration under UVR. Additionally, its sequential exposure to these stress factors increased the production of exopolysaccharides, carotenoids, and scytonemin. Desiccation, but not osmotic stress, increased the concentrations of the osmoprotectants trehalose and sucrose. However, osmotic stress might induce the production of other osmoprotectants, for which the complete pathways were observed in the ULC029 genome. In total, 140 genes known to be involved in stress resistance were annotated. Here, we confirm that the sequential application of moderate osmotic stress and dehydration could improve cyanobacterial hardening for soil restoration by inducing several resistance mechanisms. We provide a high-quality genome of ULC029 and a description of the main resistance mechanisms (i.e., production of exopolysaccharides, osmoprotectants, chlorophyll, and carotenoids; DNA repair; and oxidative stress protection).

Novel diversity of polar Cyanobacteria revealed by genome-resolved metagenomics.

Benthic microbial mats dominated by Cyanobacteria are important features of polar lakes. Although culture-independent studies have provided important insights into the diversity of polar Cyanobacteria, only a handful of genomes have been sequenced to date. Here, we applied a genome-resolved metagenomics approach to data obtained from Arctic, sub-Antarctic and Antarctic microbial mats. We recovered 37 metagenome-assembled genomes (MAGs) of Cyanobacteria representing 17 distinct species, most of which are only distantly related to genomes that have been sequenced so far. These include (i) lineages that are common in polar microbial mats such as the filamentous taxa Pseudanabaena, Leptolyngbya, Microcoleus/Tychonema and Phormidium; (ii) the less common taxa Crinalium and Chamaesiphon; (iii) an enigmatic Chroococcales lineage only distantly related to Microcystis; and (iv) an early branching lineage in the order Gloeobacterales that is distributed across the cold biosphere, for which we propose the name Candidatus Sivonenia alaskensis. Our results show that genome-resolved metagenomics is a powerful tool for expanding our understanding of the diversity of Cyanobacteria, especially in understudied remote and extreme environments.