MICROBIAL METABOLISMS INFLUENCE CARBONATE PRECIPITATION IN A LAMINATED MICROBIAL MAT

Investigating mineralization within modern mat systems informs our interpretation of ancient microbialites and the mineralization process. Microbial mats in Little Hot Creek (LHC), California contain 4 distinct layers with different microbiota. Each layer of the mat is supersaturated with regard to calcium carbonate (CaCO₃) that increases with depth, while total organic carbon decreases with depth.

To better understand what control the microbial community has on carbonate precipitation, we characterized the functional and phylogenetic diversity of each layer using high throughput sequencing. We also analyzed the impacts of Mn on carbonate precipitation using ¹³C-bicarbonate labeling and Mn addition. Cyanobacteria dominate the top layer, followed by representatives of Bacteroidetes. Chloroflexi, Proteobacteria, Chlorobi, and Planctomycetes are abundant in the 2^ndlayer. Candidate division OP8 constitutes 60-70% of the community in the lower 2 layers, and members of Thaumarchaeota increase in abundance with depth.

We hypothesize that oxygenic photosynthetic cyanobacteria promote carbonate precipitation in the top layer. Mn is an essential cofactor in photosynthetic machinery, and we observed increased carbonate precipitation and organic carbon growth in the top layer amended with Mn. Mn amendment increased organic carbon production in the lowest layer to a small degree, but did not increase growth of CaCO₃. Mn addition had no effect on growth rates of the two intervening layers. We propose that as carbonate precipitated around them, oxygenic phototrophs, dominated by Cyanobacteria, gradually moved upwards for optimal access to sunlight. As the mat grew, “tenant” microorganisms such as Bacteroidetes and Proteobacteria inhabited the lower carbonate layers while the “builders” remained on top. OP8 is the most abundant division in the lower 2 layers, yet the functions and metabolisms of its members are not well known. Potential metabolisms in lower layers of the microbial mat that may affect carbonate formation, such as anoxygenic photosynthesis and anaerobic heterotrophy, require further study. These results contribute to the characterization of the role of microbes in the mineralization of microbial mats.