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 (CaCO3) 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 [Mg3(Si2O5)(OH)4]. Cyanobacteria-dominated 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. 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 [Mg5(CO3)4(OH)2·5H2O] formed on the surface of the inoculated columns. When characterized using SEM, the platy dypingite crystals were observed nucleating on cyanobacteria cell exteriors. The columns that received the microbial inoculum exhibited better cementation than the control columns, resulting in carbon storage rates of 478 g/m2 (mineral and biomass) and 27 g/m2 (mineral), respectively. The column experiment results were used to design a carbonation trial at Woodsreef Mine. Two plots of tailings (1 m × 1 m × ½ m) were leached for 2 weeks using sulfuric acid, after which one plot was inoculated using the microbial consortium sourced from the mine pit lakes. After 9 weeks, depth profiles of the tailings in both plots and unreacted tailings were sampled for mineral abundance quantification by Rietveld refinement of powder X-ray diffraction data. Both plots contained a horizon of pyroaurite [Mg6Fe2(CO3)(OH)16·4H2O] at 2 cm depth. The inoculated plot showed an enrichment in hydromagnesite [Mg5(CO3)4(OH)2·4H2O] between 2-4 cm depth, suggesting that the microbial amendment enabled more carbonate precipitation than abiotic processes alone. Maximum mineral carbonation, however, was not achieved due to carbon and water limitations in the surface of the tailings pile. These results suggest that in situ mineral carbonation in the surface of the tailings is likely not the best strategy for deploying microbial carbon sequestration in mine tailings, and wetland-based bioreactors may be a better approach. In order to better understand mineral nucleation and the microbe-mineral relationships occurring in biofilms, a final investigation was conducted to explore new preservation and staining techniques for transmission electron microscopy (TEM) of biofilms. These methods provided better visualization of the extracellular matrix architecture than those traditionally used for TEM of microorganisms, suggesting that biofilms exhibiting three-dimensional heterogeneity are better suited to sample processing protocols designed for complex tissues. Identifying functional commonalities between the mineral structures produced in contrasting settings made it possible to identify the fundamental biogeochemical controls on cyanobacteria carbonation that are consistent across environmental regimes. These consisted of: 1) cation production through mineral dissolution, 2) photosynthesis generated alkalinity, 3) nucleation site production via EPS generation, resulting in 4) carbonate mineral precipitation on EPS. These findings make it possible to use natural mineral carbonation systems as analogues for industrial settings. Developing a functional knowledge of microbial carbonation may make it possible to utilize this biogeochemical process in carbon sequestration and climate change mitigation technologies.