Julie Huber’s research program investigates deep-sea microbial ecosystems with an emphasis on using crustal fluids to interrogate the rocky subseafloor habitat. The potential for production of new biomass within the seafloor is rarely considered in traditional oceanographic paradigms of carbon cycling or microbial food webs due to how little we know about this under-explored and potentially ubiquitous microbial habitat. The functional consequences of an extensive population of microbes living in the subseafloor remains unknown, as does our understanding of how these organisms interact with one another and influence the biogeochemistry of the oceans. Due to the inherently risky nature of deep-sea field investigations, there is often limited access to samples and the best available technology and experimental strategies must be leveraged in order to maximize return of information from those precious few samples. This includes using next-generation sequencing, high temperature cultivation, stable isotope experimentation, geochemical measurements, and novel sampling and seafloor instrumentation for these subseafloor studies.

Figure X. A, B show DAPI-stained cells from vent fluids from the Mid-Cayman Rise and C shows methane generation in vent fluids labeled with various stable carbon isotope substrates. The shallow ultramafic site (VD) appears to have more autotrophic metabolism at 70°C than the deep basalt-hosted site (PD). No methane generation is seen in background seawater.

In 2012, the Huber lab participated in cruises to the worlds’ deepest hydrothermal vents (Mid-Cayman Rise), a recently-erupted volcano in the Northeast Pacific (Axial Seamount), and subseafloor observatories in the middle of the Atlantic Ocean (North Pond). At the Mid-Cayman Rise (MCR), we sampled vents we first discovered in 2009 as part of our NASA team, with leveraging funds from NSF and the Sloan Foundation’s Deep Carbon Observatory. The MCR represents an ideal natural laboratory in which to examine two key understudied aspects of vent microbiology: the role of pressure and of ultra-mafic substrates. A relatively large literature exists for the microbial ecology of basalt- and sulfide hosted hydrothermal systems shallower than 3500 m, but much less is known regarding microbial populations in ultramafic-hosted or extremely deep hydrothermal settings. To that end, the goal of our field work on the Mid-Cayman Rise was to collect seafloor samples to assess the phylogenetic, functional, and physiological diversity of microbial communities in geochemically diverse habitats at the MCR vent fields. Fluids for geochemical and microbial analysis were collected from the Von Damm and Piccard vent fields, which are located within 20 km of one another, yet have extremely different thermal, geological, and depth regimes. Geochemical data indicates that both fields are highly enriched in volatiles, in particular hydrogen and methane, important energy sources for and by-products of microbial metabolism. At both sites, total microbial cell counts in the fluids ranged in concentration from 5 x 104 to 3 x 105 cells ml-1, with background seawater concentrations of 1-2 x 104 cells ml-1. In addition, distinct cell morphologies and clusters of cells not visible in background seawater were seen, including large filaments and mineral particles colonized by microbial cells (Fig X A, B). These results indicate local enrichments of microbial communities in the venting fluids, distinct from background populations, and are consistent with previous enumerations of microbial cells in venting fluids. Stable isotope tracing experiments were used to detect utilization of acetate, formate, and dissolve inorganic carbon and generation of methane at 70 °C under anaerobic conditions (Fig. X, C). At Von Damm, a putatively ultra-mafic hosted site located at ~2300 m with a maximum temperature of 226 °C, stable isotope tracing experiments indicate methanogenesis is occurring in most fluid samples. No activity was detected in Piccard vent fluids, a basalt-hosted black smoker site located at ~4950 m with a maximum temperature of 403 °C. However, hyperthermophilic and thermophilic heterotrophs of the genus Thermococcus were isolated from Piccard vent fluids, but not Von Damm. These obligate anaerobes, growing optimally at 55-90 °C, are ubiquitous at hydrothermal systems and serve as a readily cultivable indicator organism of subseafloor populations. Finally, molecular analysis of vent fluids is on-going and will define the microbial population structure in this novel ecosystem and allow for direct comparisons with other deep-sea and subsurface habitats as part of our continuing efforts to explore the deep microbial biosphere on Earth.