Investigating Chemotactic Potential Within Crustal Fluid Communities

Abstract:

The oceanic crust constitutes, possibly, the largest but most inaccessible habitat on Earth. Exchange of fluid between the permeable crustal environment and overlying sediments and bottom seawater transports electron donors and acceptors, which create redox gradients exploitable by microbial life. While the presence of microbial communities within the oceanic crust is strongly suggested, the structure of these communities, and survival mechanisms used within the hydrothermally-active basement aquifer remain unclear. Recently, crustal fluids from two subsurface borehole observatories (IODP CORKs U1362A and U1362B), located on the eastern flank of Juan de Fuca Ridge, were collected for both single cell genomic and metagenomic analyses. Both techniques revealed an abundance of motility and chemotactic genes. Single-cell amplified genomes (SAGs) classified as Marine Benthic Group E had relatively more motility and taxis genes than any other publically available archaeal SAG. Furthermore, metagenomes from these sites had 3.5 times as many motility and taxis genes than those from sedimentary environments. Many of the detected chemotactic genes (such as tsr and aer) are known to monitor electron flow through the electron transport system, thereby serving as “energy receptors,” which direct organisms to the most fitting redox zone. Considering fluid advection occurring within the oceanic crust, the observation of chemotaxis suggests an adaptive lifestyle for crustal microbes.