Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

Abstract

The “Dolomite Problem” has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth’s history may explain important aspects of “The Dolomite Problem”. Our results provide new context for the interpretation of dolomite throughout Earth’s history.

High Mg/Ca Molar Ratios Promote Protodolomite Precipitation Induced by the Extreme Halophilic Bacterium Vibrio harveyi QPL2

“…Mounting evidence confirms that dolomite is more common in modern evaporitic saline environments than it is in modern, open-marine sediments. In the past decades, dolomite has been frequently found in modern saline lakes ( Deckker and Last, 1988 ; Deng et al, 2010 ; Samylina and Zaytseva, 2019 ; Li et al, 2020 ), coastal evaporitic sabkhas ( Bontognali et al, 2010 ; Shalev et al, 2020 ; Diloreto et al, 2021 ), and saline soils ( Sherman et al, 1947 ; Kohut et al, 1995 ), while by contrast, almost no dolomite precipitation has been reported in the ocean ( Gregg et al, 2015 ). The concentrations of various ions in these evaporative saline systems are significantly higher than those in surface seawater, including Na + , Ca 2+ , Mg 2+ , Cl – , and ( Deng et al, 2010 ; Kolpakova et al, 2019 ), leading to the inference that dolomite precipitation may be linked to high salinity, high sulfate concentration, and increasing Mg/Ca molar ratios.…”

Controlling pore-scale processes to tame subsurface biomineralization

“…Saline environments are favorable for microbial formation of Mg-rich carbonates (Al Disi et al 2019 ). Indeed, environmental shifts from low- to high- salinity has been shown to increase the fraction of carboxylic groups on EPS, suggesting that such shifts could increase the Mg-consuming precipitation of dolomite (Diloreto et al 2021 ). At low pH, EPS from Bacillus megaterium shows a more dense and compact structure due to altered interactions of intermolecular hydrogen bonds (Wang et al 2012a ).…”

Systematic laboratory approach to produce Mg-rich carbonates at low temperature

“…25 Sabkhas occur in regions characterized by extreme climatic and environmental conditions, such as high temperature, salinity, light intensity, etc. 26,27 Since long, sabkhas have been studied as modern analogues for interpreting some ancient dolomite-rich sedimentary sequences, and several models for dolomite formation are based on the study of these depositional settings. 28 More recent studies indicate that not only evaporation, but also microbial processes may play a key role in these evaporitic environments.…”

“…Recent works have focused on the community composition of microbial mats (with top layers comprised mostly of cyanobacteria and anoxygenic phototrophs) that promote the formation of dolomite in a sabkha environment, revealing that anoxygenic phototrophic microbes are particularly important for the mineralization process. 10,27 Specically, a cyclic shi in microbial community between cyanobacteria and anoxygenic phototrophs results in the production of EPS with an increased concentration of carboxylic functional groups, which in turn favors dolomite formation.…”

“…ref. 15 and 27 ), or that EPS need to co-occur with other factors. The identification of co-occurring factors essential for dolomite formation cannot be easily inferred by reviewing and comparing the results of published studies, which are often conducted using different methodologies.…”