Award: OCE-2048489

We have explored the composition and activity of the deep subsurface biosphere in the hydrothermal sediments of Guaymas Basin, following the successful conclusion of deep subsurface drilling expedition 385 (Sept. to Nov. 2019). In terms of itellectual merit, several trends became clear. With increasing temperature, the microbial community is gradually reduced in diversity. At the deepest and hottest samples that yielded sufficient DNA for metagenomic sequencing (near 60C), only specialized subsurface archaea persisted. The size of the genomes decreased with depth and increasing temperature, suggesting that persistent physiological stress and energy limitations selected for small microbial cells that were easier to maintain. Microbial gene expression follow a pattern where the entire cellular metabolism is fine-tuned towards efficient (energy-saving) regulation of protein synthesis and protein maintenance; in other words, microbial cells do not rely on particular "superpowers" or extreme physiological capabilities but show all-over adaptations to the rigors of subsurface life. The most abundant carbon compound in the Guaymas subsurface, methane, is produced and consumed by methanogenic and methane-oxidizing archaea, respectively, but these only represent a minor component of the subsurface microbial community (which is in fact dominated by microbes that degrade all kinds of buried organic matter). The massive stores of methane that seep from the sediment into the water column and the surface biosphere are the result of gradual production in the subsurface over geological time. In terms of broader implications, two project participants (entry-level scientists and faculty) have learned new skills, and a participating undergraduate (John E. Hinkle) has done his honors thesis on methane cycling in the Guaymas subsurface. Cruise blogs from the Guaymas deep drilling expedition were used in undergraduate teaching at UNC-Chapel Hill, and continue to be publicly available for everyone who takes an interest in this expedition (https://joidesresolution.org/expedition/385/). Last Modified: 01/30/2025 Submitted by: AndreasPTeske