Stromatolites on the rise in peat-bound karstic wetlands

Salinity-driven ecology and diversity changes of heterocytous cyanobacteria in Australian freshwater and coastal-marine microbial mats

N₂ -fixing heterocytous cyanobacteria are considered to play a minor role in sustaining coastal microbial mat communities developing under normal marine to hypersaline conditions. Here, we investigated microbial mats growing under different salinities from freshwater mats of Giblin River (Tasmania) to metahaline and hypersaline mats of Shark Bay (Western Australia). Analyses of genetic (rRNA and mRNA) and biological markers (heterocyte glycolipids) revealed an unexpectedly large diversity of heterocytous cyanobacteria in all the studied microbial mat communities. It was observed that the taxonomic distribution as well as abundance of cyanobacteria is strongly affected by salinity. Low salinity favoured the presence of heterocytous cyanobacteria in freshwater mats, while mats thriving in higher salinities mainly supported the growth unicellular and filamentous non-heterocytous genera. However, even though mRNA transcripts derived from heterocytous cyanobacteria were lower in Shark Bay (<6%) microbial mats, functional analyses revealed that these diazotrophs were transcribing a substantial proportion of the genes involved in biofilm formation and nitrogen fixation. Overall, our data reveal an unexpectedly high diversity of heterocytous cyanobacteria (e.g. Calothrix, Scytonema, Nodularia, Gloeotrichia, Stigonema, Fischerella and Chlorogloeopsis) that had yet to be described in metahaline and hypersaline microbial mats from Shark Bay and that they play a vital role in sustaining the ecosystem functioning of coastal-marine microbial mat systems.

Genome-resolved metagenomics provides insights into the functional complexity of microbial mats in Blue Holes, Shark Bay

The present study describes for the first time the community composition and functional potential of the microbial mats found in the supratidal, gypsum-rich, and hypersaline region of Blue Holes, Shark Bay. This was achieved via high throughput metagenomic sequencing of total mat community DNA and complementary analyses using hyperspectral confocal microscopy. Mat communities were dominated by Proteobacteria (29%), followed by Bacteroidetes/Chlorobi Group (11%), and Planctomycetes (10%). These mats were found to also harbor a diverse community of potentially novel microorganisms including members from the DPANN, Asgard archaea, and Candidate Phyla Radiation, with highest diversity found in the lower regions (∼14-20 mm depth) of the mat. In addition to pathways for major metabolic cycles, a range of putative rhodopsins with previously uncharacterized motifs and functions were identified along with heliorhodopsins and putative schizorhodopsins. Critical microbial interactions were also inferred, and from 117 medium-to-high quality metagenome-assembled genomes (MAGs), viral defense mechanisms (CRISPR, BREX, and DISARM), elemental transport, osmoprotection, heavy metal and UV resistance were also detected. These analyses have provided a greater understanding of these distinct mat systems in Shark Bay, including key insights into adaptive responses and proposing that photoheterotrophy may be an important lifestyle in Blue Holes.

Novel Gloeobacterales spp. from Diverse Environments across the Globe

Photosynthetic Cyanobacteria and their descendants are the only known organisms capable of oxygenic photosynthesis. Their metabolism permanently changed the Earth’s surface and the evolutionary trajectory of life, but little is known about their evolutionary history. Genomes of the Gloeobacterales, an order of deeply divergent photosynthetic Cyanobacteria, may hold clues about the evolutionary process. However, there are only three published genomes within this order, and it is difficult to make broad inferences based on such little data. Here, I describe five species within the Gloeobacterales retrieved from publicly available databases and examine their photosynthetic gene content and the environments in which Gloeobacterales genomes and 16S rRNA gene sequences are found. The Gloeobacterales contain reduced photosystems and inhabit cold, wet-rock, and low-light environments. They are likely present in low abundances due to their low growth rate. Future searches for Gloeobacterales should target these environments, and samples should be deeply sequenced to capture the low-abundance taxa. Publicly available databases contain undescribed taxa within the Gloeobacterales. However, searching through all available data with current methods is computationally expensive. Therefore, new methods must be developed to search for these and other evolutionarily important taxa. Once identified, these novel photosynthetic Cyanobacteria will help illuminate the origin and evolution of oxygenic photosynthesis.

IMPORTANCE Early branching photosynthetic Cyanobacteria such as the Gloeobacterales may provide clues into the evolutionary history of oxygenic photosynthesis, but there are few genomes or cultured taxa from this order. Five new metagenome-assembled genomes suggest that members of the Gloeobacterales all contain reduced photosystems and lack genes associated with thylakoids and circadian rhythms. Their distribution suggests that they may thrive in environments that are marginal for other species, including wet-rock and cold environments. These traits may aid in the discovery and cultivation of novel species in this clade.

Functional Gene Expression in Shark Bay Hypersaline Microbial Mats: Adaptive Responses

Microbial mat communities possess extensive taxonomic and functional diversity, which drive high metabolic rates and rapid cycling of major elements. Modern microbial mats occurring in hypersaline environments are considered as analogs to extinct geobiological formations dating back to ∼ 3.5 Gyr ago. Despite efforts to understand the diversity and metabolic potential of hypersaline microbial mats in Shark Bay, Western Australia, there has yet to be molecular analyses at the transcriptional level in these microbial communities. In this study, we generated metatranscriptomes for the first time from actively growing mats comparing the type of mat, as well as the influence of diel and seasonal cycles. We observed that the overall gene transcription is strongly influenced by microbial community structure and seasonality. The most transcribed genes were associated with tackling the low nutrient conditions by the uptake of fatty acids, phosphorus, iron, and nickel from the environment as well as with protective mechanisms against elevated salinity conditions and to prevent build-up of ammonium produced by nitrate reducing microorganisms. A range of pathways involved in carbon, nitrogen, and sulfur cycles were identified in mat metatranscriptomes, with anoxygenic photosynthesis and chemoautotrophy using the Arnon–Buchanan cycle inferred as major pathways involved in the carbon cycle. Furthermore, enrichment of active anaerobic pathways (e.g., sulfate reduction, methanogenesis, Wood–Ljungdahl) in smooth mats corroborates previous metagenomic studies and further advocates the potential of these communities as modern analogs of ancient microbialites.

Symbiolite formation: a powerful in vitro model to untangle the role of bacterial communities in the photosynthesis-induced formation of microbialites

Microbially induced calcification is an ancient, community-driven mineralisation process that produces different types of microbialites. Symbiolites are photosynthesis-induced microbialites, formed by calcifying co-cultures of dinoflagellates from the family Symbiodiniaceae and bacteria. Symbiolites encase the calcifying community as endolithic cells, pointing at an autoendolithic niche of symbiotic dinoflagellates, and provide a rare opportunity to study the role of bacteria in bacterial-algal calcification, as symbiodiniacean cultures display either distinct symbiolite-producing (SP) or non-symbiolite-producing (NP) phenotypes. Using Illumina sequencing, we found that the bacterial communities of SP and NP cultures differed significantly in the relative abundance of 23 genera, 14 families, and 2 phyla. SP cultures were rich in biofilm digesters from the phylum Planctomycetes and their predicted metagenomes were enriched in orthologs related to biofilm formation. In contrast, NP cultures were dominated by biofilm digesters from the Bacteroidetes, and were inferred as enriched in proteases and nucleases. Functional assays confirmed the potential of co-cultures and bacterial isolates to produce biofilms and point at acidic polysaccharides as key stimulators for mineral precipitation. Hence, bacteria appear to influence symbiolite formation primarily through their biofilm-producing and modifying activity and we anticipate that symbiolite formation, as a low-complexity in vitro model, will significantly advance our understanding of photosynthesis-induced microbial calcification processes.

A phylogenetically novel cyanobacterium most closely related to Gloeobacter

Clues to the evolutionary steps producing innovations in oxygenic photosynthesis may be preserved in the genomes of organisms phylogenetically placed between non-photosynthetic Vampirovibrionia (formerly Melainabacteria) and the thylakoid-containing Cyanobacteria. However, only two species with published genomes are known to occupy this phylogenetic space, both within the genus Gloeobacter. Here, we describe nearly complete, metagenome-assembled genomes (MAGs) of an uncultured organism phylogenetically placed near Gloeobacter, for which we propose the name Candidatus Aurora vandensis {Au’ro.ra. L. fem. n. aurora, the goddess of the dawn in Roman mythology; van.de’nsis. N.L. fem. adj. vandensis of Lake Vanda, Antarctica}. The MAG of A. vandensis contains homologs of most genes necessary for oxygenic photosynthesis including key reaction center proteins. Many accessory subunits associated with the photosystems in other species either are missing from the MAG or are poorly conserved. The MAG also lacks homologs of genes associated with the pigments phycocyanoerethrin, phycoeretherin and several structural parts of the phycobilisome. Additional characterization of this organism is expected to inform models of the evolution of oxygenic photosynthesis.

Distinct responses of planktonic and sedimentary bacterial communities to anthropogenic activities: Case study of a tributary of the Three Gorges Reservoir, China

Anthropogenic activities can cause serious negative effects on ecosystems. Despite the ecological significance of bacterial communities, the integrated biogeography of planktonic and sedimentary bacterial communities in response to anthropogenic activities is not adequately understood. Here, we examined environmental parameters and the composition of planktonic and sedimentary bacteria in the Yulin River, a tributary of Three Gorges Reservoir, in response to changes in land use and dam construction. The results revealed that human-induced land use changes enhanced the nutrient concentrations in surface water and dam construction reduced the content of carbon and nitrogen in immediately downstream sediments. Intensified human-dominated land use showed a slight impact on sedimentary bacterial communities but largely reduced the diversity of planktonic bacterial communities. Moreover, human-induced land use changes increased the abundance of genes associated with denitrification, nitrification, and anammox in planktonic bacterial communities by 19.04%, 32.40% and 30.45%, respectively. In dam construction regions, the diversity and nutrient-related metabolic activity of sedimentary bacterial communities immediately downstream of the dam were decreased, whereas these changes were not observed in planktonic bacterial communities. Additionally, bacterial community composition was significantly related to nutrient concentrations variability and followed a distance-decay pattern. Furthermore, environmental effects explained more of the variation in planktonic bacterial community composition as compared with spatial effects did, whereas, sedimentary bacterial communities were more closely related to spatial effects. Our results demonstrated the distinct responses of planktonic and sedimentary bacterial communities to anthropogenic activities, and offered new insight for understanding their potential ecological influence on rivers.

Isolation of novel quorum-sensing active bacteria from microbial mats in Shark Bay Australia

Quorum sensing is a potent system of genetic control allowing phenotypes to be coordinated across localized communities. In this study, quorum sensing systems in Shark Bay microbial mats were delineated using a targeted approach analyzing whole mat extractions as well as the creation of an isolate library. A library of 165 isolates from different mat types were screened using the AHL biosensor E. coli MT102. Based on sequence identity 30 unique isolates belonging to Proteobacteria, Actinobacteria and Firmicutes were found to activate the AHL biosensor, suggesting AHLs or analogous compounds were potentially present. Several of the isolates have not been shown previously to produce signal molecules, particularly the members of the Actinobacteria and Firmicutes phyla including Virgibacillus, Halobacillius, Microbacterium and Brevibacterium. These active isolates were further screened using thin-layer chromatography (TLC) providing putative identities of AHL molecules present within the mat communities. Nine isolates were capable of producing several spots of varying sizes after TLC separation, suggesting the presence of multiple signalling molecules. This study is the first to delineate AHL-based signalling in the microbial mats of Shark Bay, and suggests quorum sensing may play a role in the ecosphysiological coordination of complex phenotypes across microbial mat communities.

Comparative Metagenomics Provides Insight Into the Ecosystem Functioning of the Shark Bay Stromatolites, Western Australia

Stromatolites are organosedimentary build-ups that have formed as a result of the sediment trapping, binding and precipitating activities of microbes. Today, extant systems provide an ideal platform for understanding the structure, composition, and interactions between stromatolite-forming microbial communities and their respective environments. In this study, we compared the metagenomes of three prevalent stromatolite-forming microbial mat types in the Spaven Province of Hamelin Pool, Shark Bay located in Western Australia. These stromatolite-forming mat types included an intertidal pustular mat as well as a smooth and colloform mat types located in the subtidal zone. Additionally, the metagenomes of an adjacent, non-lithifying mat located in the upper intertidal zone were also sequenced for comparative purposes. Taxonomic and functional gene analyses revealed distinctive differences between the lithifying and non-lithifying mat types, which strongly correlated with water depth. Three distinct populations emerged including the upper intertidal non-lithifying mats, the intertidal pustular mats associated with unlaminated carbonate build-ups, and the subtidal colloform and smooth mat types associated with laminated structures. Functional analysis of metagenomes revealed that amongst stromatolite-forming mats there was an enrichment of photosynthesis pathways in the pustular stromatolite-forming mats. In the colloform and smooth stromatolite-forming mats, however, there was an increase in the abundance of genes associated with those heterotrophic metabolisms typically associated with carbonate mineralization, such as sulfate reduction. The comparative metagenomic analyses suggest that stromatolites of Hamelin Pool may form by two distinctive processes that are highly dependent on water depth. These results provide key insight into the potential adaptive strategies and synergistic interactions between microbes and their environments that may lead to stromatolite formation and accretion.