Abstract
BackgroundThis study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg5(CO3)4(OH)2·5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2·4H2O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared.ResultsField observations show that evaporation of wetland water produces carbonate films of nesquehonite (MgCO3·3H2O) on the water surface and crusts on exposed surfaces. In contrast, benthic microbial mats possessing filamentous cyanobacteria (Lyngbya sp.) contain platy dypingite (Mg5(CO3)4(OH)2·5H2O) and aragonite. Bulk carbonates in the benthic mats (δ13C avg. = 6.7‰, δ18O avg. = 17.2‰) were isotopically distinguishable from abiotically formed nesquehonite (δ13C avg. = 9.3‰, δ18O avg. = 24.9‰). Field and laboratory experiments, which emulated natural conditions, were conducted to provide insight into the processes for magnesium carbonate precipitation in this environment. Field microcosm experiments included an abiotic control and two microbial systems, one containing ambient wetland water and one amended with nutrients to simulate eutrophic conditions. The abiotic control developed an extensive crust of nesquehonite on its bottom surface during which [Mg2+] decreased by 16.7% relative to the starting concentration. In the microbial systems, precipitation occurred within the mats and was not simply due to the capturing of mineral grains settling out of the water column. Magnesium concentrations decreased by 22.2% and 38.7% in the microbial systems, respectively. Laboratory experiments using natural waters from the Atlin site produced rosettes and flakey globular aggregates of dypingite precipitated in association with filamentous cyanobacteria dominated biofilms cultured from the site, whereas the abiotic control again precipitated nesquehonite.ConclusionMicrobial mats in the Atlin wetland create ideal conditions for biologically induced precipitation of dypingite and have presumably played a significant role in the development of this natural Mg-carbonate playa. This biogeochemical process represents an important link between the biosphere and the inorganic carbon pool.
Highlights
This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, dypingite (Mg5(CO3)4(OH)2·5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia
We examine the microbial life and carbonate mineralogy of benthic microbial mats in an alkaline wetland that is part of a hydromagnesite (Mg5(CO3)4(OH)2·4H2O) playa near Atlin, British Columbia, Canada
The greater decreases of Mg and Ca in the biotic microcosms suggest that carbonates are forming in the presence of cyanobacteria at lower concentrations than that needed for purely abiotic precipitation to occur
Summary
This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, dypingite (Mg5(CO3)4(OH)2·5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2·4H2O) playa. The contribution to carbonate precipitation and sedimentation by microorganisms has been substantial since the Archaean. Field studies of stromatolites in Salda Gölü (Lake) in southwestern Turkey [8,9,10] and the hydromagnesite playas of the Cariboo Plateau in British Columbia, Canada [11] have implicated cyanobacterial activity in the precipitation of magnesium carbonate, hydromagnesite. Direct experimental evidence connecting microbial activity to the precipitation of magnesium carbonates is lacking
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