Exploring the deep subsurface microbial life in impact-altered Late Paleozoic granitoid rocks from the Chicxulub impact crater
In 2016, IODP-ICDP Expedition 364 recovered an 829-meter-long core within the peak ring of the Chicxulub impact crater (Yucatan, Mexico), allowing us to investigate the post-impact recovery of the heat-sterilized deep continental microbial biosphere at the impact site. We recently reported increased cell biomass in the impact suevite, which was deposited within the first few hours of the Cenozoic, and that the overall microbial communities differed significantly between the suevite and the other main core lithologies (i.e., the granitic basement and the overlying Early Eocene marine sediments; Cockell et al., 2021). However, only a few rock intervals (n=7) were analyzed from the geologically heterogenic and impact-deformed 587-m-long granitic core section below the suevite interval. Here, we used 16S rRNA gene profiling to study the microbial community composition in impact-shocked granites (n=31), non-granitic rocks consisting of strongly serpentinized pre-impact sub-volcanic, ultramafic basanite/dolerite, and where suevite and impact melt rocks were intercalated into the granites during crater formation (n=7), and within cross-cut mineral veins of anhydride and silica (n=7). Our results revealed significantly different microbial communities between impact-shocked granites and non-granitic rocks, confirming that postimpact microbial niche separation has occurred in the granitic basement lithologies. Many of the recovered taxa resembled those found in other hydrothermal systems. Pearson correlation analysis (PCA) between relative changes in the microbial community composition and bioavailable chemical compounds indicated the presence of chemolithoautotrophs, which most likely still play an active role in Fe, Mn, and S cycling. Furthermore, a subset of the community was most strongly shaped by borehole temperature and, to a lesser extent, downcore changes in porosity. These results showed that in addition to the newly formed lithologies, the impact-induced geochemical boundaries continue to shape the modern-day deep biosphere in the granitic basement underlying the Chicxulub crater.