Abstract
The basaltic ocean crust is the largest aquifer system on Earth, yet the rates of biological activity in this environment are unknown. Low-temperature (<100°C) fluid samples were investigated from two borehole observatories in the Juan de Fuca Ridge (JFR) flank, representing a range of upper oceanic basement thermal and geochemical properties. Microbial sulfate reduction rates (SRR) were measured in laboratory incubations with 35S-sulfate over a range of temperatures and the identity of the corresponding sulfate-reducing microorganisms (SRM) was studied by analyzing the sequence diversity of the functional marker dissimilatory (bi)sulfite reductase (dsrAB) gene. We found that microbial sulfate reduction was limited by the decreasing availability of organic electron donors in higher temperature, more altered fluids. Thermodynamic calculations indicate energetic constraints for metabolism, which together with relatively higher cell-specific SRR reveal increased maintenance requirements, consistent with novel species-level dsrAB phylotypes of thermophilic SRM. Our estimates suggest that microbially-mediated sulfate reduction may account for the removal of organic matter in fluids within the upper oceanic crust and underscore the potential quantitative impact of microbial processes in deep subsurface marine crustal fluids on marine and global biogeochemical carbon cycling.
Highlights
Exploratory studies have revealed that low-temperature fluids flowing within the uppermost basaltic ocean crust harbor freeliving microorganisms that are only distantly related to any cultivated strains or communities attached to basaltic surfaces (Jungbluth et al, 2013)
In order to further characterize the microbial communities within fluid samples from boreholes U1301A and 1025C, we used an approach that involves amplifying a region of the 16S rRNA gene from environmental DNA using primers that are specific to bacteria and archaea and sequencing on the Illumina MiSeq platform (Caporaso et al, 2011)
SAMPLING AND HYDROGEOCHEMICAL SETTING High integrity basaltic fluid samples were collected from Circulation Obviation Retrofit Kit (CORK) installed at increasing distance from the spreading ridge axis fitted within boreholes 1025C and U1301A (Figure 1)
Summary
Exploratory studies have revealed that low-temperature fluids flowing within the uppermost basaltic ocean crust harbor freeliving microorganisms that are only distantly related to any cultivated strains or communities attached to basaltic surfaces (Jungbluth et al, 2013). Microbial sulfate reduction rates (SRR) were measured in laboratory incubations with 35S-sulfate over a range of temperatures and the identity of the corresponding sulfate-reducing microorganisms (SRM) was studied by analyzing the sequence diversity of the functional marker dissimilatory (bi)sulfite reductase (dsrAB) gene.
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