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The Khor Al-Adaid sabkha in Qatar is among the rare extreme environments on Earth where it is possible to study the formation of dolomite—a carbonate mineral whose origin remains unclear and has been hypothetically linked to microbial activity. By combining geochemical measurements with microbiological analysis, we have investigated the microbial mats colonizing the intertidal areas of sabhka. The main aim of this study was to identify communities and conditions that are favorable for dolomite formation. We inspected and sampled two locations. The first site was colonized by microbial mats that graded vertically from photo-oxic to anoxic conditions and were dominated by cyanobacteria. The second site, with higher salinity, had mats with an uppermost photo-oxic layer dominated by filamentous anoxygenic photosynthetic bacteria (FAPB), which potentially act as a protective layer against salinity for cyanobacterial species within the deeper layers. Porewater in the uppermost layers of the both investigated microbial mats was supersaturated with respect to dolomite. Corresponding to the variation of the microbial community’s vertical structure, a difference in crystallinity and morphology of dolomitic phases was observed: dumbbell-shaped proto-dolomite in the mats dominated by cyanobacteria and rhombohedral ordered-dolomite in the mat dominated by FAPB.

Benthic cyanobacterial mats are increasingly reported to cover major coral reef areas. Although suggestions have been made that cyanobacterial mats impair coral reef health in multiple ways, information is lacking regarding the distribution, morphotypical variation and bacterial species composition of these microbial mats. As such, this study aimed to (1) Reveal the bacterial community diversity and composition of different mat morphotypes, (2) Identify the most abundant community members and closely related organisms, (3) Assess to what extent, morphotype, colonized substrate (coral or abiotic substrate), depth, and site were significant predictors of bacterial composition. Data were collected on reefs surrounding the island of Koh Tao (Gulf of Thailand). A total of 201 mats from 16 different locations around the island were classified into eight distinct morphotypes. Of these, the bacterial communities of 44 mats, representing colour groups from multiple sites, were characterized using 16S rRNA gene high-throughput sequencing. Our data revealed that Proteobacteria, Cyanobacteria, Bacteriodetes and Planctomycetes were the four most abundant phyla and occurred in all samples. Abundant cyanobacterial zero-radius operational taxonomic units (ZOTUs) were closely related to prokaryotic sequences found in previous studies of coastal mats (98–100%) and were assigned to genera in the order Oscillatoriales, e.g. Hormoscilla, Okeania, and Oscillatoria. Abundant proteobacterial ZOTUs were assigned to orders in the classes Alpha- and Gammaproteobacteria, e.g. Rhodobacterales, Rhizobiales and Alteromonadales. Abundant Bacteriodetes ZOTUs were mainly assigned to the class Bacteroidia and order Cytophagales. Our results showed that mats consist of a diverse and variable bacterial consortium, with mat colour (morphotype), substrate type and geographic location only explaining a small part of the total variation in composition.

There is growing evidence that life has been on land for billions of years. Microbial mats fuelled by oxygenic photosynthesis were probably present in terrestrial habitats from c. 3.0 billion yr ago (Ga) onwards, creating localized ‘oxygen oases’ under a reducing atmosphere, which left a characteristic oxidative weathering signal. After the Great Oxidation c. 2.4 Ga, the now oxidizing atmosphere masked that redox signal, but ancient soils record the mobilization of phosphorus and other elements by organic acids in weathering profiles. Evidence for Neoproterozoic ‘greening of the land’ and intensification of weathering c. 0.85–0.54 Ga is currently equivocal. However, the mid-Palaeozoic c. 0.45–0.4 Ga shows global atmospheric changes consistent with increased terrestrial productivity and intensified weathering by the first land plants.

This review compiles principal data on Antarctic cynobacteria published in recent decades and focuses on the diversity, environmental adaptations, and ecotypes of these microorganisms. Multidisciplinary investigation of Antarctica is important especially in times of global climate change and anthropogenic threats to unaltered ecosystems. Antarctica, which has been disconnected from Gondwana for the last 65 Myr, harbors relict biota mainly including microorganisms adapted to multiextreme environments. Cyanobacteria represent the most well studied Antarctic microorganisms related to the cell biology, abundance, distribution, and symbioses. However, substantial drawbacks remain, e.g., (i) the diversity of Antarctic cyanobacteria has been evaluated without a consensus between bacteriologists and phycologists due to the intricate interplay of morphological records, genomic data and culturing restrictions as well as taxonomic obstacles; (ii) although the main strategy of Antarctic cyanobacteria is biofilms (in particular, microbial mats), detailed knowledge on these symbiotic systems is still fragmentary; (iii) the specificity of the ‘Antarctic cyanobacteria biosphere’ should be better understood in light of unresolved questions related to endemism in prokaryotes, and (iv) the strains of Antarctic cyanobacteria maintained in culture are not numerous and not very diverse. These issues are at the forefront of cyanobacteriology, which is supported by related biosciences and earth sciences.

Arsenic (As) is a metalloid present in the earth’s crust and widely distributed in the environment. Due to its high concentrations in the Andean valleys and its chemical similarity with phosphorus (P), its biological role in Andean Microbial Ecosystems (AMEs) has begun to be studied. The AMEs are home to extremophilic microbial communities that form microbial mats, evaporites, and microbialites inhabiting Andean lakes, puquios, or salt flats. In this work, we characterize the biological role of As and the effect of phosphate in AMEs from the Laguna Tebenquiche (Atacama Desert, Chile). Using micro X-ray fluorescence, the distribution of As in microbial mat samples was mapped. Taxonomic and inferred functional profiles were obtained from enriched cultures of microbial mats incubated under As stress and different phosphate conditions. Additionally, representative microorganisms highly resistant to As and able to grow under low phosphate concentration were isolated and studied physiologically. Finally, the genomes of the isolated Salicola sp. and Halorubrum sp. were sequenced to analyze genes related to both phosphate metabolism and As resistance. The results revealed As as a key component of the microbial mat ecosystem: (i) As was distributed across all sections of the microbial mat and represented a significant weight percentage of the mat (0.17 %) in comparison with P (0.40%); (ii) Low phosphate concentration drastically changed the microbial community in microbial mat samples incubated under high salinity and high As concentrations; (iii) Archaea and Bacteria isolated from the microbial mat were highly resistant to arsenate (up to 500 mM), even under low phosphate concentration; (iv) The genomes of the two isolates were predicted to contain key genes in As metabolism (aioAB and arsC/acr3) and the genes predicted to encode the phosphate-specific transport operon (pstSCAB-phoU) are next to the arsC gene, suggesting a functional relationship between these two elements.

We acquired and analyzed metagenome and 16S/18S rRNA gene amplicon data of green-colored microbial mats from two hot springs within the Onikobe geothermal region (Miyagi Prefecture, Japan). The two collection sites—Tamago and Warabi—were in proximity and had the same temperature (40 °C), but the Tamago site was connected to a nearby stream, whereas the Warabi site was isolated. Both the amplicon and metagenome data suggest the bacterial, especially cyanobacterial, dominance of the mats; other abundant groups include Chloroflexota, Pseudomonadota, Bacteroidota/Chlorobiota, and Deinococcota. At finer resolution, however, the taxonomic composition entirely differed between the mats. A total of 5 and 21 abundant bacterial 16S rRNA gene OTUs were identified for Tamago and Warabi, respectively; of these, 12 are putative chlorophyll- or rhodopsin-based phototrophs. The presence of phylogenetically diverse microbial eukaryotes was noted, with ciliates and amoebozoans being the most abundant eukaryote groups for Tamago and Warabi, respectively. Fifteen metagenome-assembled genomes (MAGs) were obtained, represented by 13 bacteria, one ciliate (mitochondrion), and one giant virus. A total of 15 novel taxa, including a new deeply branching Chlorobiota species, is noted from the amplicon and MAG data, highlighting the importance of environmental sequencing in uncovering hidden microorganisms.

Main conclusion Haematococcus lacustris inhabits supralittoral rock ponds and forms, under natural conditions, biofilms including layered cyanobacterial and fermentative microbial mats. Dry mats, formed under extremely stressful conditions, contained only haematocysts. Under favorable growth conditions, modeled for dry biofilms in vitro, microalgal free-living stages were detected. Haematococcus lacustris is the microalga known for its high potential to survive under a wide range of unfavorable conditions, particularly in the supralittoral temporal rock ponds of the White Sea. Previously, we described microbial communities containing H. lacustris in this region. In many cases, they were organized into systems exhibiting complex three-dimensional structure similar to that of natural biofilms. In this study, for the first time, we clarify structural description and provide microscopic evidence that these communities of H. lacustris and bacteria are assembled into the true biofilms. There are (1) simple single layer biofilms on the surface of rocks and macrophytic algae, (2) floccules (or flocs) not attached to a surface, (3) as well as stratified (layered) biofilms, wet, and dehydrated in nature. Being involved into primary organic production, H. lacustris and cyanobacteria are located exclusively in the upper layers of stratified biofilms, where they are capable to absorb sufficient for photosynthesis amount of light. The presence of acidic polysaccharides in the extracellular matrix revealed by specific staining with ruthenium red in the H. lacustris -containing microbial communities is a biochemical evidence of biofilm formation. Meanwhile, the presence of bacterial L-form is an ultrastructural confirmation of that fact. Under favorable conditions, modeled in vitro, H. lacustris from the dry microbial mats moves to the free-living states represented by vegetative palmelloid cells and motile zoospores. Owing to the fact that inside biofilms cells of microorganisms exist under stable conditions, we consider the biofilm formation as an additional mechanism that contributes to the survival of H. lacustris in the supralittoral zone of the White Sea.

Photosynthetic microbial mats are macroscopic microbial ecosystems consisting of a wide array of functional groups and microenvironments arranged along variable redox gradients. Light energy ultimately drives primary production and a cascade of daisy-chained metabolisms. Heterotrophic members of these communities remineralise organic material, decreasing net primary production, and returning nutrients to the aqueous phase. However, reduced inorganic and one-carbon substrates such as trace gases and those released as metabolic byproducts in deeper anoxic regions of the mat, could theoretically also fuel carbon fixation, mitigating carbon loss from heterotrophy and enhancing net primary production. Here, we investigated the intricate metabolic synergies that sustain community nutrient webs in a biomineralising microbial mat from a hypersaline lake. We recovered 331 genomes spanning 40 bacterial and archaeal phyla that influence the biogeochemistry of these ecosystems. Phototrophy is a major metabolism found in 17% of the genomes, but over 50% encode enzymes to harness energy from inorganic substrates and 12% co-encode chemosynthetic carbon fixation pathways that use sulfide and hydrogen as electron donors. We experimentally demonstrated that the microbial community oxidises ferrous iron, ammonia, sulfide, and reduced trace gas substrates aerobically and anaerobically. Furthermore, carbon isotope assays revealed that diverse chemosynthetic pathways contribute significantly to carbon fixation and organic matter production alongside photosynthesis. Chemosynthesis in microbial mats results from a complex suite of spatially organised metabolic interactions and continuous nutrient cycling, which decouples carbon fixation from the diurnal cycle, and enhances the net primary production of these highly efficient ecosystems.

ABSTRACT Ecological communities are regulated by the flow of energy through environments. Energy flow is typically limited by access to photosynthetically active radiation (PAR) and oxygen concentration (O2). The microbial mats growing on the bottom of Lake Fryxell, Antarctica, have well-defined environmental gradients in PAR and (O2). We analyzed the metagenomes of layers from these microbial mats to test the extent to which access to oxygen and light controls community structure. We found variation in the diversity and relative abundances of Archaea, Bacteria and Eukaryotes across three (O2) and PAR conditions: high (O2) and maximum PAR, variable (O2) with lower maximum PAR, and low (O2) and maximum PAR. We found distinct communities structured by the optimization of energy use on a millimeter-scale across these conditions. In mat layers where (O2) was saturated, PAR structured the community. In contrast, (O2) positively correlated with diversity and affected the distribution of dominant populations across the three habitats, suggesting that meter-scale diversity is structured by energy availability. Microbial communities changed across covarying gradients of PAR and (O2). The comprehensive metagenomic analysis suggests that the benthic microbial communities in Lake Fryxell are structured by energy flow across both meter- and millimeter-scales. Energy input affects the composition and diversity of microbial mats growing at the bottom of an Antarctic lake.

Cyanobacterial mat communities are the main drivers of primary productivity in the McMurdo Dry Valleys, Antarctica. These microbial communities form laminar mats on desert pavement surfaces adjacent to glacial meltwater streams, ponds, and lakes. The low-density nature of these communities and their patchy distribution make assessments of distribution, biomass, and productivity challenging. We used satellite imagery coupled with in situ surveying, imaging, and sampling to systematically estimate microbial mat biomass in selected wetland regions in Taylor Valley, Antarctica. On January 19th, 2018, the WorldView-2 multispectral satellite acquired an image of our study areas, where we surveyed and sampled seven 100 m2 plots of microbial mats for percent ground cover, ash-free dry mass (AFDM), and pigment content (chlorophyll-a, carotenoids, and scytonemin). Multispectral analyses revealed spectral signatures consistent with photosynthetic activity (relatively strong reflection at near-infrared wavelengths and relatively strong absorption at visible wavelengths), with average normalized difference vegetation index (NDVI) values of 0.09 to 0.28. Strong correlations of microbial mat ground cover (R2 = 0.84), biomass (R2 = 0.74), chlorophyll-a content (R2 = 0.65), and scytonemin content (R2 = 0.98) with logit transformed NDVI values demonstrate that satellite imagery can detect both the presence of microbial mats and their key biological properties. Using the NDVI—biomass correlation we developed, we estimate carbon (C) stocks of 21,715 kg (14.7 g C m−2) in the Canada Glacier Antarctic Specially Protected Area, with an upper and lower limit of 74,871 and 6312 kg of C, respectively.