Microbial diversity of two natural CO2-rich springs with contrasting hydrochemical features

Abstract

Microbial communities in CO2-rich springs have adapted to environmental pressure including low pH resulting from high concentrations of CO2. Microbial community composition and hydrochemical characteristics in the CO2-rich springs are important features affected by high levels of CO2 in groundwater ecosystems, and they may be useful indicators of CO2 leakage from carbon capture and storage (CCS) sites where CO2 is injected into deep geological reservoirs for the mitigation of climate change. To determine how long-term CO2 leakage affects the groundwater ecosystems, we analyzed the bacterial and archaeal community composition and hydrochemical features of two different natural CO2-rich springs, Hansil (HS) and Chosukol (CS), located in south-eastern Korea which are natural analog sites for groundwater ecosystems affected by CO2 leakage from geological CO2 storage sites. Analyses of hydrochemical characteristics and 16S rRNA gene amplicon sequencing of bacteria and archaea revealed a marked distinction between the two springs. The pH, the concentrations of bicarbonate ion (HCO3-), major cations including Ca2+ and Mg2+, and specific trace elements including Fe and Mn were higher in HS as compared to CS while no distinction in pCO2 between the two springs was observed. Microbial community composition was distinct between the two springs at the genus level. Specifically, the relative abundance of Gallionella, an iron-oxidizer, was significantly higher in HS whereas that of Albidiferax known as dissimilatory iron-reducing bacteria was higher in CS than HS, reflecting the difference of iron concentration in the two springs. In our study, microbial community composition appeared to be more strongly affected by the concentration of iron than that of CO2. Our results enhance the understanding of hydrochemical features and microbial communities in natural CO2-rich springs and suggest that microbial communities may be useful indicators of CO2 leakage from CCS sites.