Microbial mats are complex, multi-layered communities of microorganisms. These ancient structures represent some of the earliest forms of organized life on Earth, with fossil evidence dating back 3.5 billion years. They are found globally, often in environments with specific conditions that allow them to flourish. Though typically only a few centimeters thick, these mats create a range of internal chemical environments that support diverse microbial life.
What are Microbial Mats?
The distinct layers of microorganisms within microbial mats arise from gradients of light penetration, oxygen availability, and nutrient diffusion. The uppermost layers, exposed to sunlight, are often dominated by photosynthetic microbes like cyanobacteria, which use chlorophyll to capture light energy. These organisms produce oxygen during the day, creating an oxygen-rich zone.
Below the oxygenated surface, the mat transitions into layers with progressively lower oxygen levels. Here, facultative anaerobic bacteria may thrive, tolerating periods of both oxygen presence and absence. Deeper still, anaerobic conditions prevail, supporting microbes such as sulfate-reducing bacteria. These organisms utilize other chemical compounds in the absence of oxygen. The different microbial groups secrete slimy substances, known as extracellular polymeric substances (EPS), which help hold the mat together and provide a protective matrix.
Diverse Habitats of Microbial Mats
Microbial mats are found in a wide variety of environments, demonstrating their adaptability. They thrive in aquatic habitats, including hypersaline lakes and salt pans, where high salt concentrations limit competition from other organisms. Hot springs and hydrothermal vents also host extensive microbial mats, able to withstand extreme temperatures.
Beyond these more extreme locations, microbial mats can be found in common settings like freshwater lakes and rivers, marine sediments, and intertidal zones. They can also colonize less obvious places such as anoxic sediments and even survive in certain deserts. This broad distribution highlights their resilience to diverse physical and chemical conditions.
Ecological and Historical Significance
Microbial mats play a role in shaping current ecosystems by facilitating nutrient cycling. They participate in the carbon, nitrogen, and sulfur cycles, transforming these elements within their environments. For example, photosynthetic microbes in the mats fix carbon, converting carbon dioxide into organic matter. Nitrogen-fixing bacteria within the mats convert atmospheric nitrogen into usable forms for other organisms, influencing the nitrogen budget.
Ancient microbial mats, particularly those that formed stromatolites, are thought to have been responsible for the initial oxygenation of Earth’s atmosphere. The study of modern mats provides valuable insights into early Earth environments and helps scientists understand the evolution of life and potential signs of life on other planets. These ancient structures were abundant until the Cambrian period, when competition from newly evolved plants and animals reduced their widespread presence.
Remarkable Adaptations and Metabolism
Microbial mats exhibit diverse metabolic capabilities, allowing them to thrive in diverse conditions. They utilize a spectrum of energy acquisition strategies, including oxygenic photosynthesis, anoxygenic photosynthesis, and chemosynthesis. This metabolic flexibility enables different microbial groups to perform specialized functions, contributing to the mat’s resilience. For instance, some microbes can use alternative electron acceptors for respiration when oxygen is scarce.
The metabolic versatility within the mat community allows them to withstand environmental stressors. They can tolerate variations in light, oxygen, pH, and salinity. Some microbes in mats can even use “nanowires” to transfer electrons over distances of up to two centimeters, connecting metabolic reactions deep within the sediment to electron acceptors like oxygen in the water. This adaptability, including the ability to enter dormant states during desiccation, contributes to their persistence in harsh environments.