Abstract
Microbial mats are taxonomically and metabolically diverse microbial ecosystems, with a characteristic layering that reflects vertical gradients in light and oxygen availability. Silicified microbial mats in Proterozoic carbonate successions are generally interpreted in terms of the surficial, mat building community. However, information about biodiversity in the once-surface-layer can be lost through decay as the mats accrete. To better understand how information about surface microbial communities is impacted by processes of decay within the mat, we studied microbial mats from Little Ambergris Cay, Turks and Caicos Islands. We used molecular techniques, microscopy and geochemistry to investigate microbial mat taphonomy— how processes of degradation affect biological signatures in sedimentary rocks, including fossils, molecular fossils and isotopic records. The top <1 cm of these mats host cyanobacteria-rich communities overlying and admixed with diverse bacterial and eukaryotic taxa. Lower layers contain abundant, often empty, sheaths of large filamentous cyanobacteria, preserving their record as key mat-builders. Morphological remains and free lipid biomarkers of several bacterial groups, as well as diatoms, arthropods, and other eukaryotes also persist in lower mat layers, although at lower abundances than in surface layers. Carbon isotope signatures of organic matter were consistent with the majority of the biomass being sourced from CO2-limited cyanobacteria. Porewater sulfide sulfur isotope values were lower than seawater sulfate sulfur isotope values by ~ 45 to 50‰, consistent with microbial sulfate reduction under sulfate-replete conditions. Our findings provide insight into how processes of degradation and decay bias biosignatures in the geological record of microbial mats, especially mats that formed widely during the Proterozoic (2,500-541 million years ago) Eon. Cyanobacteria were the key mat-builders, their robust and cohesive fabric retained at depth. Additionally, eukaryotic remains and eukaryotic biosignatures were preserved at depth, which suggests that microbial mats are not inherently biased against eukaryote preservation, either today or in the past.
Highlights
Microbial mat ecosystems contain diverse consortia of primarily microscopic organisms along with their metabolic products and, occasionally, trapped sediment or authigenic minerals
In addition to testing the hypothesized anti-eukaryote bias suggested by the “mat-seal effect” and the role of cyanobacteria as mat-builders, we investigated more broadly in what ways fossilized mats act as reliable narrators of history and which elements of microbial history are lost through processes of degradation and decay as the mat accretes
Microbial mats that occur in shallow bays and channels in the interior of the island can be divided into three classes based on surface morphology and texture (Figure 2), which vary as a function of elevation and the duration of subaerial exposure over tidal cycles: flat mats lie at low elevations and are persistently submerged; polygonal mats are found at intermediate elevations and are submerged during high-tide; and blister mats with knobbly, low-relief morphologies occur at higher elevations and are subaerially exposed except during storms (Stein et al, 2016; Trembath-Reichert et al, 2016)
Summary
Microbial mat ecosystems contain diverse consortia of primarily microscopic organisms along with their metabolic products and, occasionally, trapped sediment or authigenic minerals. A green to deep blue-green, mm- to cm-scale surface layer gets its color primarily from cyanobacteria – the predominant primary producers and agents of mat accretion. The mat surface hosts a variety of other mat-dwelling photoautotrophs and heterotrophs, their diversity and abundance reflecting physical conditions within and around the mat. The surface, where light penetrates but oxygen does not, a variably thick, pink layer is often present, populated by anoxygenic phototrophic bacteria (predominantly proteobacteria) as well as anaerobic heterotrophs and chemoautotrophs. Biomass produced in surface layer becomes the substrate for respiration and fermentation in the lower layers. The morphology and biogeochemistry of lower layers reflects both the nature and growth of mat-building communities as well as processes within the mat that may overwrite some or all of the surface record
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