The Role of Exopolymeric Substances (EPS) in Dolomite Biomineralization by Microbial Isolates from the Coastal Sabkhas of Abu Dhabi

Abstract

Dolomite, CaMg(CO3)2, is an ancient form of carbonate constituting a wide variety of geological 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. Its otherwise common components (calcium, magnesium and carbonate ions) are not known to come readily into its characteristic layered, alternating organization except via microbial mediation, observed only in a few unique geologic sites. 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 is still unclear. To address this, the biofilm aspect ofnatural SRB growth was explored within the setting of a dolomitizing culture. The study sought to investigate the potential role of exopolymeric substances (EPS)—the complex mix of compounds in a biofilm matrix—in promoting favorable conditions that drive the elusive reaction forward. 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. EPS content was shown to influence the experimental cultures and the biomineralization process, based on periodic analyses of the media chemistry, community behavior, biofilm development and mineral formation. 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.

Date of Award	2012
Original language	American English
Supervisor	Farrukh Ahmad (Supervisor)