Paper No. 4
Presentation Time: 9:50 AM
FROM BACTERIAL FINCHES TO BACTIVOROUS MEIOFAUNA: MICROBIAL BIOGEOGRAPHY OF THE DEEP CRUSTAL BIOSPHERE
The biogeography of subsurface microorganisms in the fractured rock of the continental crust is controlled by interconnected, fluid-filled fractures, which occupy ~0.01%. The “habitable” matrix porosity ranges from 0.005 to 0.0005% and is much less interconnected. For the past 10 years the cellular abundance and the phylogenetic diversity of the occupants of this environment have been examined in the Witwatersrand Basin and Bushveld Igneous Complex in South Africa. From the 37 fractures intersected by sampled boreholes, 719 operational taxonomic units (OTUs) have been identified based upon 3% identity in the 16S rRNA gene sequences. For shallow fractures (0.6 – 1.5 km depth) Proteobacteria dominate the phylotypes, and for deeper fractures (2.0 – 3.8 km depth) Firmicutes dominate the phylotypes. The bacterial abundance ranges from 104 to 105 cells mL-1 with negligible correlation with depth. Archaea comprise 3% of the diversity and ~0.1% of the cellular abundance. The number of OTUs within a single fracture range from 1 to 20 with the greatest diversity associated with those fractures with the youngest fracture water ages (<35 kyr). Fifty OTUs occur in more than one fracture and of these only 3 OTUs occur across the Witwatersrand Basin, and they are restricted to great depth. These deep, yet widely distributed OTUs, one of which is candidatus D. audaxviator, may represent subsurface bacterial “finches” whose genomes enable self-sustainability under a broad environmental range. The lack of overlap in OTUs between fractures suggests an “desert oases” biogeographical pattern that could encompass ~8,000 OTUs within a 1016 cm3 volume of the Witwatersrand Basin containing ~1016-17 cells. Compared to soils, which typically contain 109 cells and ~8,000 OTUs per cm3, the diversity of deep subsurface biosphere is very low. Eukaryotes, including bactivorous nematodes, have been detected in deep fractures with young fracture water but at an extremely low abundance of one per ~104 liters of fracture water. Major gaps remain in our understanding of deep life biogeography including whether horizontal subsurface migration or vertical migration from the surface is more important in explaining the observed spatial distribution of phylotypes. We hope to fill these gaps by making use of a Network of Inner Space Observatories (NISO).