Microbial diversity within basement fluids of the sediment-buried Juan de Fuca Ridge flank

Abstract

Despite its immense size, logistical and methodological constraints have largely limited microbiological investigations of the subseafloor basement biosphere. In this study, a unique sampling system was used to collect fluids from the subseafloor basaltic crust via a Circulation Obviation Retrofit Kit (CORK) observatory at Integrated Ocean Drilling Program borehole 1301A, located at a depth of 2667 m in the Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Here, a fluid delivery line directly accesses a 3.5 million years old basalt-hosted basement aquifer, overlaid by 262 m of sediment, which serves as a barrier to direct exchange with bottom seawater. At an average of 1.2 × 104 cells ml−1, microorganisms in borehole fluids were nearly an order of magnitude less abundant than in surrounding bottom seawater. Ribosomal RNA genes were characterized from basement fluids, providing the first snapshots of microbial community structure using a high-integrity fluid delivery line. Interestingly, microbial communities retrieved from different CORKs (1026B and 1301A) nearly a decade apart shared major community members, consistent with hydrogeological connectivity. However, over three sampling years, the dominant gene clone lineage changed from relatives of Candidatus Desulforudis audaxviator within the bacterial phylum Firmicutes in 2008 to the Miscellaneous Crenarchaeotic Group in 2009 and a lineage within the JTB35 group of Gammaproteobacteria in 2010, and statistically significant variation in microbial community structure was observed. The enumeration of different phylogenetic groups of cells within borehole 1301A fluids supported our observation that the deep subsurface microbial community was temporally dynamic.

Oxygen consumption rates in subseafloor basaltic crust derived from a reaction transport model

“…Recent studies have documented the presence of active microbial communities in the JFR flank setting 11,13,38 , although this is a warm and chemically reduced environment that is not representative of global upper crustal conditions. Confirmation of oxygen consumption by active microbial communities in cool temperature (o25°C) upper oceanic crust still remains to be done.…”

“…Although the JFR system provides a highly reacted, warm end-member for studying basaltic basement biogeochemistry and microbial ecology [11][12][13] , the majority of the upper oceanic crust experiences much cooler (o20°C) and presumably oxic conditions 2 . Relative to the warm JFR system, oxygen in cooler systems probably persists for longer periods of time, as abiotic removal of oxygen is sluggish kinetically 2 ; however, oxygen removal rates in oceanic crustal aquifers are unconstrained.…”

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“…Evidence for active microbial life has been detected as deep as 2458 mbsf (Inagaki et al, 2015), and the introduction of molecular biology into marine ecology has led to great advances in our understanding of microbial life below the seafloor (Biddle et al, 2008;Cowen et al, 2003;Inagaki et al, 2006;Mason et al, 2010;Edgcomb et al, 2011;Orsi et al, 2013aOrsi et al, , 2013b. Much of the microbiology performed during ODP and Integrated Ocean Drilling Program expeditions has concentrated on sediment (e.g., ODP Leg 201 to the Peru margin), with the notable exception of expeditions to the Juan de Fuca Ridge (Cowen et al, 2003;Lever et al, 2013;Jungbluth et al, 2013), Atlantis Massif (Mason et al, 2010), North Pond (Orcutt et al, 2013), and Louisville Seamounts (Sylvan et al, 2015).…”

Dissolved amino acids in oceanic basaltic basement fluids

“…There are several lines of evidence, including the texture, chemical, and isotopic composition of both basaltic glass and rocks, suggesting the existence of biological activity within the ridge-flank basement (Giovannoni et al, 1996;Fisk et al, 1998;Furnes et al, 2006;Rouxel et al, 2008;Wheat et al, 2010;Alford et al, 2011;Alt and Shanks, 2011;Ono et al, 2012). Fluids sampled from the ridge-flank aquifer have yielded ribosomal RNA gene sequences related to chemolithoautotrophs and heterotrophs, and functional genes of sulfate reducers, indicating the presence of a diverse bacterial and archaeal community (Cowen et al, 2003;Huber et al, 2006;Jungbluth et al, 2013;Robador et al, 2015). However, these genomic data do not necessarily reveal what, if any, heterotrophic activity is ongoing in this environment.…”