Microbial mat
A microbial mat is a cohesive, vertically stratified, self-sustaining community of microorganisms—primarily bacteria and archaea, along with some eukaryotes—that develops at the interface between liquids and solids, such as sediment-water boundaries, and is embedded within an extracellular polymeric substance (EPS) matrix. These mats form layered biofilms ranging from millimeters to over a meter in thickness, where microbial populations organize into distinct zones influenced by steep physicochemical gradients, including light penetration, oxygen levels, pH, and nutrient availability.[1][2][3] Microbial mats exhibit high functional and taxonomic diversity, often comprising dozens of phyla and hundreds of species that interact through complex metabolic dependencies and trophic relationships. They thrive in diverse and extreme environments worldwide, including hypersaline lagoons like those in Guerrero Negro, Mexico; thermophilic hot springs in Yellowstone National Park; oligotrophic coastal sediments; and even psychrophilic settings in Antarctic ice shelves. This adaptability stems from their ability to cycle key elements internally, such as carbon, nitrogen, sulfur, and hydrogen, making them resilient to environmental fluctuations like temperature, salinity, and hydrodynamic forces.[2][1][3] In terms of structure, the upper oxic layers of microbial mats are typically dominated by photosynthetic cyanobacteria and diatoms that fix carbon and produce oxygen during daylight, while deeper anoxic zones host anaerobic bacteria performing sulfate reduction, fermentation, and methanogenesis, often synchronized by diel cycles. These communities trap sediments and stabilize substrates, contributing to the formation of microbialites, and play pivotal roles in global biogeochemical processes, including oxygen production, nutrient recycling, and greenhouse gas regulation in aquatic ecosystems.[1][2][3] As ancient ecosystems, microbial mats represent some of the earliest evidence of life on Earth, with fossilized forms known as stromatolites dating back at least 3.5 billion years to the Archean eon, preserved in sedimentary rocks from sites like the Pilbara Craton in Australia. Modern mats serve as analogs for studying early Earth conditions and potential extraterrestrial life, such as on Mars, due to their self-contained nature and preservation potential. Additionally, they hold biotechnological promise, yielding extremozymes like Taq polymerase from thermophilic mats and applications in bioremediation for pollutant degradation.[1][3]Description and Structure
Physical Structure
Microbial mats exhibit a distinctive layered architecture, characterized by vertical stratification that arises from environmental gradients. The surficial layer, typically 0.5–3 mm thick, is a photosynthetic zone where light penetration supports oxygenic activity, often appearing green or brown due to pigment concentrations.[4] Beneath this lies the middle anoxic layer, where oxygen levels drop sharply, fostering anaerobic conditions conducive to sulfate reduction and resulting in darker, sulfide-rich zones up to several millimeters deep.[4] The basal layer interfaces with underlying sediments, facilitating nutrient exchange and sediment incorporation, which anchors the mat to the substrate.[4] These layers form cohesive biofilms primarily through the production of extracellular polymeric substances (EPS), a mucilaginous matrix secreted by microorganisms that binds cells, detritus, and minerals together, enhancing structural integrity.[4] Mats typically range in thickness from a few millimeters to several centimeters, though some can reach decimeters in stable, low-energy environments.[5] The EPS matrix imparts resistance to erosion by increasing sediment cohesion and stabilizing surfaces against hydrodynamic forces, allowing mats to persist in flowing or wave-exposed settings.[6] Stratification develops through vertical gradients in light, oxygen, and nutrients, which drive microbial zonation and ecological succession starting with photosynthetic colonizers at the surface.[4] This results in diverse morphologies, such as flat, laminated sheets in calm, silty substrates or domed, stromatolitic forms in marine settings like Shark Bay, where upward growth and mineral precipitation create conical or columnar structures.[4]Microbial Composition
Microbial mats are composed of highly diverse microbial communities, primarily dominated by prokaryotes with minor contributions from eukaryotes, forming layered consortia that exploit distinct metabolic niches. Cyanobacteria, such as Microcoleus chthonoplastes and Lyngbya spp., often form the foundational phototrophic layer, binding the mat through extracellular polymeric substances and driving primary production.[1] Sulfate-reducing bacteria, including genera like Desulfovibrio, prevail in deeper anoxic zones, utilizing organic compounds produced by upper layers for dissimilatory sulfate reduction.[1] Archaea, particularly methanogenic Euryarchaeota such as those in the Methanobacteriales order, occupy anaerobic subsurface regions, contributing to methane production from simple substrates.[7] Eukaryotes play a subordinate role, with diatoms (e.g., Navicula and Amphora spp.) and green algae (Chlorophyta) integrating into surface layers to enhance oxygenic photosynthesis and silicification.[1] Biodiversity within microbial mats exhibits pronounced vertical zonation, shaped by gradients in oxygen availability, light penetration, and sulfide concentrations, which align microbial distribution with their physiological tolerances. Aerobic and microaerobic taxa like cyanobacteria and proteobacteria dominate the upper oxygenated zones (0-2 mm), while obligate anaerobes such as sulfate-reducers and methanogens thrive below 3 mm in sulfidic, anoxic strata.[1] This stratification results in exceptionally high cell densities, estimated at 10^8 to 10^9 cells per cm³, reflecting the compact, three-dimensional architecture of the mat.[8] Overall diversity can encompass dozens of phyla, with hypersaline mats hosting up to 42 bacterial phyla and over 750 operational taxonomic units, underscoring the ecological complexity within millimeters-scale gradients.[1] Symbiotic interactions among mat microbes foster resilience and efficiency, particularly through mutualistic partnerships between oxygenic photosynthesizers like cyanobacteria and anaerobic heterotrophs. For instance, cyanobacteria supply organic carbon via exudates, which sulfate-reducers and methanogens metabolize, in turn recycling nutrients like sulfide and ammonium back to upper layers to mitigate toxicity and support growth.[1] These consortia enable closed-loop nutrient cycling, where bacterial and archaeal groups form stable syntrophic networks, enhancing overall mat productivity and stability against environmental fluctuations.[7] Metagenomic studies have revealed extensive genetic diversity in microbial mats, highlighting numerous uncultured lineages that evade traditional isolation methods. 16S rRNA gene profiling, often via high-throughput sequencing of variable regions like V6, identifies rare phyla such as candidate divisions TM7, WS3, and OD1, comprising up to 10-20% of the community in coastal mats.[7] Whole-genome metagenomics from sites like Guerrero Negro further uncovers functional genes in uncultured cyanobacteria and proteobacteria, indicating specialized adaptations that contribute to the mat's metabolic versatility.[1] This physical layering provides the spatial framework for such microbial zonation, allowing coexistence of diverse taxa in close proximity.[1]Functional Processes
Microbial mats exhibit stratified functional processes driven by light penetration, oxygen availability, and substrate gradients, enabling efficient biogeochemical cycling within compact communities. In the upper oxygenated layers, cyanobacteria dominate oxygenic photosynthesis, converting CO₂ and H₂O into organic matter and O₂ using light energy, with primary production rates typically ranging from 10 to 100 μmol O₂ cm⁻² h⁻¹ under optimal conditions.[9] This process establishes steep vertical gradients of O₂, which diffuse downward to support aerobic respiration by heterotrophs.[10] In the deeper anoxic layers, anaerobic respiration prevails, including sulfate reduction by bacteria such as Desulfovibrio species, where sulfate (SO₄²⁻) serves as an electron acceptor, producing hydrogen sulfide (H₂S) as a byproduct: SO₄²⁻ + 2CH₂O → H₂S + 2HCO₃⁻.[11] Methanogenesis occurs even deeper in highly reduced zones, primarily via acetoclastic pathways by archaea like Methanosaeta, reducing CO₂ or acetate to CH₄, which can accumulate and influence overlying layers.[12] Nitrogen fixation, facilitated mainly by diazotrophic cyanobacteria in surface layers under microaerobic conditions, incorporates atmospheric N₂ into biomass through the nitrogenase enzyme, supporting mat productivity in nutrient-limited environments.[13] Diurnal cycles profoundly shape these processes, with daytime photosynthesis generating supersaturated O₂ levels (up to several hundred percent above air saturation) that penetrate millimeters into the mat, creating dynamic micro-oxic zones and enabling sulfide oxidation.[10] At night, O₂ depletes rapidly due to aerobic and anaerobic respiration, leading to H₂S accumulation from sulfate reduction, which reverses upon re-illumination.[11] These fluctuations drive internal recycling, where photosynthetic O₂ reoxidizes H₂S produced anaerobically, as represented by the simplified coupled reaction:$2\mathrm{H_2S} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O} + 2\mathrm{S}
This chemosynthetic coupling minimizes external nutrient loss and maintains mat stability.[14] Nutrient and gas fluxes occur primarily via diffusion along concentration gradients, with O₂, H₂S, and CO₂ exhibiting sharp profiles: O₂ decreasing from surface maxima to near-zero within 1-2 mm, while H₂S rises inversely in deeper strata.[15] Flux rates for O₂ into the mat can reach 50-200 μmol cm⁻² h⁻¹ during peak photosynthesis, facilitating efficient exchange with overlying water or sediment.[9] Biogeochemical models of these mats often integrate measured rates of photosynthesis, sulfate reduction (typically 0.1–2 μmol cm⁻³ h⁻¹), and methanogenesis to simulate coupled redox dynamics, highlighting the mats' role as self-sustaining microcosms.[11]