Benthic mats are complex, layered communities of microorganisms that adhere to sediments or other submerged surfaces in various aquatic environments, including marine and freshwater systems. Formed by diverse microbial life (bacteria, archaea, and microeukaryotes), these assemblages play a role in the health and functioning of many aquatic ecosystems.
Formation and Microscopic Makeup
Benthic mats develop through the accumulation and layering of microbial populations and sediment particles. The uppermost layers are dominated by oxygen-producing phototrophs, such as cyanobacteria and diatoms, which harness sunlight for energy. Below these light-dependent layers, other microbial groups like sulfide-oxidizing bacteria and sulfate-reducing bacteria are found, adapted to lower oxygen or anaerobic conditions. This vertical stratification allows microorganisms to occupy specific niches based on light availability and chemical gradients.
Extracellular polymeric substances (EPS) are key to mat formation and stability. These sticky, gel-like molecules, a complex mixture of biopolymers (polysaccharides and proteins), are secreted by the microbes. EPS acts as a natural glue, binding individual cells and sediment particles together, contributing to the mat’s structural integrity and its ability to adhere to surfaces. This matrix also influences the mat’s three-dimensional architecture and can affect chemical communication and nutrient diffusion within the community.
Ecological Contributions
Benthic mats contribute to various ecosystem processes, particularly where larger plant life may be sparse. They serve as primary producers, converting energy into organic matter through both photosynthesis and chemosynthesis. Photosynthetic microbes, like cyanobacteria, capture sunlight to produce organic carbon and release oxygen. In deep-sea or light-limited environments, chemosynthetic bacteria within the mats use chemical compounds, such as reduced iron or sulfur from hydrothermal vents, as an energy source for organic matter production, supporting diverse food webs.
These microbial communities are involved in global nutrient cycling, cycling elements like carbon, nitrogen, and sulfur. For instance, nitrogen-fixing cyanobacteria convert atmospheric nitrogen into forms usable by other organisms, a process important in low-nutrient settings. Sulfide-oxidizing and sulfate-reducing bacteria mediate sulfur cycles, influencing the availability of sulfur compounds in sediments and water. This metabolic activity facilitates the regeneration of nutrients back into the water column, supporting broader ecosystem productivity.
The extensive EPS networks of benthic mats contribute to sediment stabilization. The sticky exopolymers secreted by diatoms and bacteria enhance the cohesion of sediment particles, increasing resistance to erosion by currents and waves. This physical binding helps prevent the resuspension of sediments, which can improve water clarity and protect underlying habitats. The mats also serve as a foundational food source, providing organic matter for invertebrates and other organisms, supporting higher trophic levels within the aquatic food web.
Presence Across Earth’s Environments
Benthic mats demonstrate adaptability across Earth’s environments. They are found in shallow coastal areas, such as intertidal flats and hypersaline lagoons, where they form extensive visible layers. Freshwater lakes and rivers also host these microbial communities, appearing as dark green or brown clumps on rocks or floating. Their ability to withstand fluctuations in temperature, salinity, and desiccation allows them to colonize diverse aquatic habitats.
Beyond temperate zones, benthic mats are found in extreme environments. They flourish in hot springs, where thermophilic microbes form colorful layers adapted to high temperatures. Deep-sea hydrothermal vents, devoid of sunlight, support chemosynthetic mats fueled by chemical energy from Earth’s interior. These mats represent some of the most ancient forms of life on Earth, with fossilized microbial mats, known as stromatolites, dating back 3.5 billion years. Studying these ancient structures and their modern counterparts provides insights into early Earth’s biosphere and offers analogs for potential life on other planets.