Microbial carbonation in natural and engineered environments

Abstract

Microbial carbonation, the ability of microorganisms to promote carbonate mineral precipitation, has been documented in a variety of environments. The purpose of this thesis was to investigate the biogeochemical processes by which cyanobacteria aid carbonate precipitation in two contrasting environments as a means of determining if the fundamental controls on cyanobacterial carbonation are consistent across environmental regimes.The first of these environments was beachrock in the intertidal zone of Heron Island (Capricorn Group, Great Barrier Reef). In this investigation, carbonate microbialites and cements in the beachrock were characterized using scanning electron microscopy (SEM) and X-ray fluorescence microscopy (XFM). Mapping strontium using XFM proved to be a useful technique for revealing structures in beachrock that are otherwise difficult to see, such as laminations in aragonite (CaCO 3 ) microbialites. Characterization using SEM suggested that extracellular polymeric substances (EPS) play an important role in carbonate mineral nucleation. These results indicate that microbial carbonate mineral dissolution and re-precipitation over time is crucial to beachrock lithification.The role of microorganisms, particularly alkalinity generating cyanobacteria, in beachrock formation was confirmed in an 8-week beachrock synthesis experiment. Aquaria containing beach sand and fragmented beachrock from Heron Island were maintained under conditions simulating the intertidal zone environment in which beachrock forms. The seawater added to the aquaria was enriched in strontium so that any new carbonate precipitates could be identified using XFM. Beachrock was successfully synthesized and contained fossiliferous microbialites comparable to those in natural beachrock. Cement nucleation occurred on EPS within biofilms and exhibited co-precipitation of aragonite and calcite. No beachrock formed in the aquarium controlled by evaporation and tidal wetting and drying. These findings suggest that microbial activity is necessary for instigating beachrock cementation, which may become important as a means of stabilizing reef sand cays against sea-level rise.The ability of cyanobacteria to induce materials stabilization was subsequently applied to a more industrial setting, the derelict Woodsreef Asbestos Mine (New South Wales, Australia). Woodsreef Mine hosts ~24 Mt of tailings rich in asbestiform chrysotile [Mg 3 (Si 2 O 5 )(OH) 4 ]. Cyanobacteriadominated microbial mats can be found in the open pits lakes, and were used as an inoculum for mineral carbonation experiments. These experiments aimed to contain the tailings by producing a magnesium carbonate crust, or 'magcrete', while also sequestering atmospheric carbon dioxide.iii Columns of tailings were leached using sulfuric acid to release magnesium from the tailing minerals, after which some columns were inoculated with the microbial consortium. In 4 weeks, a mm-scale crust of dypingite [Mg 5 (CO 3 ) 4 (OH) 2 ·5H 2 O] formed on the surface of the inoculated col...