| Thrombolites are unlaminated carbonate structures resulting from the trapping, binding, and mineral precipitation activities of complex microbial mat communities. Thrombolites have a long geologic record representing one of Earth's earliest ecosystems and harbor an unexplored area of genetic diversity associated with biologically induced carbonate precipitation. The underlying functional complexity in the thrombolitic mats of Highborne Cay, The Bahamas was explored using high-throughput sequencing techniques to determine microbial diversity, metabolic potential, and community gene expression. Bacterial, cyanobacterial, and archaeal diversity within four morphologically distinct thrombolitic mat types was assessed through 16S rRNA gene pyrosequencing. The most prominent differences between the four communities were within the cyanobacterial and archaeal populations. The "button mats" were identified as the most abundant and productive thrombolitic mat type and were characterized by an enrichment of Dichothrix spp., a cyanobacterium identified as a "hot spot" of carbonate deposition. Due to prevalence and high metabolic activity, the button thrombolitic mat type was selected for further functional analysis. The metabolic potential of these mat were delineated through metagenomic sequencing and community-level physiological profile assays. Functional protein analysis of the metagenome indicated the presence of several key metabolic pathways known to influence carbonate mineralization including photosynthesis, aerobic heterotrophy, nitrogen and sulfur cycling. Spatial profiling of metabolite utilization suggested that the upper zone of the thrombolitic mat (0-5 mm) contained a more metabolically active community than the deeper regions of the mat. Additionally, metatranscriptomic sequencing was used to further examine spatial differences within the thrombolitic mats. The spatial regions were delineated using oxygen microelectrode profiling and identified as oxic (0-3 mm), transitional (3-5 mm), and anoxic (5-9 mm) zones. The resulting gene expression profile revealed discrete gradients of microbial activity occurring within the unlaminated thrombolitic mat. This study showed that cyanobacterial photosynthesis is a major driver of energy production and carbon fixation in upper oxic and transitional zones, and that bacteria, eukaryotes, and archaea show spatial differences in metabolic activity. Together, this research represents the first in-depth genetic analysis of thrombolitic mat metabolisms, and provides the foundation for delineating the molecular mechanisms associated with carbonate mineralization in lithifying mat communities. |
