Environmental Dynamics, Community Structure and Function in a Hypersaline Microbial Mat

Microbial communities are often organized in surficial laminated matrices called mats, which provide both a substrate for attachment and chemical gradients. Within the mats are distinct microhabitats in which specific members of the community are able to optimize biogeochemical cycling and growth. These gradients also provide protection against external environmental factors, including extremely high irradiance (including UV), nutrient deprivation, and periodic desiccation, which potentially inhibit growth and reproduction. Hypersalinity (>45‰) represents a particular challenge to microbes, as this condition can adversely affect microbial community function and structure. Benthic microbial mats provide a protective habitat in systems experiencing large fluctuations in salinity, including hypersalinity, by enabling microbial taxa to produce and exchange osmotically active metabolites, including intracellular and extracellular polymers that can counter such fluctuations; these polymers can also form part of the mat matrix. This structure enables a taxonomically diverse, productive community to thrive under conditions that would otherwise be too hostile for life to flourish, as in the overlying water column. As such, microbial production, nutrient cycling, and growth are often optimized in mats in aquatic ecosystems experiencing large swings in salinity. Microbial communities in these mats are surprisingly diverse and exhibit a high degree of redundancy. They include a wide spectrum of metabolically flexible prokaryotes (Eubacteria—including Cyanobacteria, photosynthetic and heterotrophic bacteria, and Archaea) and eukaryotic microalgae and micrograzers. Together, these organisms carry out primary production, nutrient cycling, and trophic transfer in lakes, lagoons, and other shallow-water ecosystems under both fluctuating and extreme conditions. We examined a highly dynamic and diverse community in a benthic mat system of hypersaline Salt Pond, located on San Salvador Island, the Bahamas. This mat community exhibits remarkable diversity in the face of extreme variations in salinity, brought about by long periods of drought and desiccation (leading to salinities in excess of 200%) and periodic tropical storms and hurricanes that can reduce the salinity to less than 40%. The high microbial diversity serves to provide a rapid response to environmental variability, in that specific members of this community can assume key production and nutrient cycling functions depending on their salinity optima. This mat system illustrates the functional roles and significance of a high degree of taxonomic and metabolic diversity in response to either sort of long-term climatic change, which characterized past and present environmental conditions on Earth.