AbstractMicrobial ecology within oligotrophic marine sediment is poorly understood, yet is critical for understanding geochemical cycles. Here, 16S rRNA sequences from RNA and DNA inform the structure of active and total microbial communities in oligotrophic sediment on the western flank of the Mid-Atlantic Ridge. Sequences identified as Bacillariophyta chloroplast were detected within DNA, but undetectable within RNA, suggesting preservation in 5.6-million-year-old sediment. Statistical analysis revealed that RNA-based microbial populations correlated significantly with nitrogen concentrations, whereas DNA-based populations did not correspond to measured geochemical analytes. Bioenergetic calculations determined which metabolisms could yield energy in situ, and found that denitrification, nitrification, and nitrogen fixation were all favorable. A metagenome was produced from one sample, and included genes mediating nitrogen redox processes. Nitrogen respiration by active bacteria is an important metabolic strategy in North Pond sediments, and could be widespread in the oligotrophic sedimentary biosphere.
AbstractThe rock-hosted, oceanic crustal aquifer is one of the largest ecosystems on Earth, yet little is known about its indigenous microorganisms. Here we provide the first phylogenetic and functional description of an active microbial community residing in the cold oxic crustal aquifer. Using subseafloor observatories, we recovered crustal fluids and found that the geochemical composition is similar to bottom seawater, as are cell abundances. However, based on relative abundances and functional potential of key bacterial groups, the crustal fluid microbial community is heterogeneous and markedly distinct from seawater. Potential rates of autotrophy and heterotrophy in the crust exceeded those of seawater, especially at elevated temperatures (25 °C) and deeper in the crust. Together, these results reveal an active, distinct, and diverse bacterial community engaged in both heterotrophy and autotrophy in the oxygenated crustal aquifer, providing key insight into the role of microbial communities in the ubiquitous cold dark subseafloor biosphere.
It has been long known that the oceanic crust is the largest aquifer on Earth. However, relatively little is known about how this aquifer influences biogeochemical cycles in the deep ocean. Recent studies suggest that in some seafloor settings, crustal aquifer fluids are replete with oxygen or nitrate, and provide the overlying sediments with additional oxidants. These environments provide a unique opportunity to examine how the availability of oxidants might influence sedimentary biogeochemical processes, in particular recalcitrant organic matter degradation. Specifically, deeper sediments at the North Pond appear to be replenished by oxygen through an upward flux of recharged crustal fluids. Thus, the North Pond represents an ideal natural laboratory to study the fate of recalcitrant organic carbon present in deep anoxic sediments that are being replenished with oxygen and other electron acceptors from below through recharged basement fluids. The proposed project will shed light on the degree to which the availability of oxidants supports the transformation of recalcitrant organic carbon by microbes in these deep subsurface sediments. The assessment of the metabolic potential of crustal microbial communities solidly aligns with the C-DEBI research themes, and our studies in particular are aimed at furthering our understanding of metabolic activity in the deep subseafloor biosphere and the limits of life across the range of habitats encompassing the subsurface biosphere.