GEOCHEMICAL CYCLES REFLECT DIVERSITY IN MODERN BENTHIC MICROBIAL MATS

Benthic photosynthetic microbial mats are productive ecosystems and biogeochemical hotspots in which physicochemical gradients and microbes are structured on micrometer scales. Such mats likely played a key role in the productivity and biogeochemistry of Archean and Proterozoic ecosystems, but their microbial composition and metabolisms are difficult to discern from the fossil record. Investigations of extant mats with settings similar to ancient Earth can reveal the links between mat geochemistry and microbiology, and assist interpretation of the geological record. In this study, we analyzed microbial community structure, function, and cycling of sulfur and oxygen in four different mat morphotypes (flat purple mat, finger mat, white mat, and giraffe mat) observed in Middle Island Sinkhole, a submerged sinkhole in which the benthic environment is impacted by low-O₂, sulfur-rich groundwater. Purple mats that were flat, as well as columnar protrusions ("fingers") that were buoyed from the sediment by trapped gases, hosted Phormidium sp. These cyanobacteria flexibly switch between oxygenic photosynthesis (OP) and anoxygenic photosynthesis (AP). Flat white and purple mats were enriched in sulfide-oxidizing bacteria (Beggiatoa sp.). We observed AP and OP in flat as well as finger mats, but fingers tended to produce O₂ throughout the day, whereas light intensity, chemoclines, and other bacteria dictated the balance of OP and AP in flat mat. Sulfide oxidizers covered cyanobacteria, reducing light and limiting OP and O₂ release in flat purple and white mats. Preliminary exploration of giraffe mats, which had darker pigmented ridges surrounding 10-30 cm wide lighter purple pits, uncovered higher relative abundances of cyanobacteria Pseudanabaena sp. in the pits and Spirulina sp. in the ridges. Overall these results show that microbial community composition, function, and geochemical cycles in mats are closely related, ultimately affecting their physiognomy. Taken together with the intriguing similarity of these morphotypes to microbial mats preserved in the geologic record, this study highlights how research on modern microbial mats can inform the interpretation of the chemical environment, metabolisms, and biogeochemical impact of microbial mats through Earth history.