Role of EPS in dolomite biomineralization by coastal sabkha microbial isolates

Dolomite, CaMg(CO3)2, is an ancient form of carbonate constituting a wide variety of geologic formations. Despite its abundant deposits in geologic record, a longstanding mystery termed 'the Dolomite Problem' surrounds the formation of this mineral under present-day Earth conditions. Attempts to precipitate dolomite abiotically out of supersaturated solutions, and at ambient temperatures were not successful, particularly due to the kinetic barrier of strong ion pairs formed by calcium and magnesium with sulfate. The coastal sabkhas of western Abu Dhabi, alongside locations like Lagoa Vermelha, Brejo de Espinho, the Coorong, and Qinghai Lake were reported to exhibit modern dolomite formation, driven by the activity of sedimentary microbes well-adapted to these highly evaporative, hypersaline and sulfur-rich environments. Our study investigated one group of microbes, the sulfate-reducing bacteria (SRB) hypothesized to overcome the kinetic barrier through cation-sulfate pair dissociation and subsequent sulfate reduction into sulfide. While previous studies have successfully precipitated dolomite at ambient temperatures when incubated with SRB, the exact mechanism remains unclear. To address this, the biofilm aspect of natural SRB growth was explored within the setting of a dolomitizing culture. The study sought to investigate the potential role of exopolymeric substances (EPS) in promoting favorable conditions for dolomite biomineralization. SRB isolated from a coastal sabkha (western Abu Dhabi) were cultured and their EPS was extracted for characterization studies. To investigate the influence of EPS on dolomite biomineralization, sabkha SRB were cultured on dynamic hypersaline medium in the presence of varying EPS concentrations. Periodic chemical analysis of the growth medium, and HVSEM-EDS studies of the generated biofilm and biominerals suggest possible EPS influence on the microbial cultures and biomineralization process. Direct imaging of emerging Ca-Mg biominerals in association with preserved EPS was also made possible with our approach. This work sheds more light on the role of microbial community structures in driving geochemical processes, which can potentially support novel strategies in Carbon Capture and Sequestration (CCS) and self-assembly of materials.