Deep-sea vent mussels (Bathymodiolus) and other chemosynthetic symbioses were shown in 2011 to carry and use hydrogenase genes, with shipboard incubations indicating that symbiont H2 oxidation can power chemosynthetic primary production at H2-rich ultramafic vents. The problem is to establish, in situ and at ecosystem scale, the actual role of symbiont-mediated hydrogen oxidation: whether measured H2 uptake and H2-stimulated CO2 fixation hold under ambient pressure and temperature, how hydrogenase prevalence and expression are regulated across spatial, temporal and taxonomic gradients (including low-H2 basalt-hosted sites, and cold-seep mussels in which hydrogenase genes have not been detected), and how much symbiont H2 oxidation contributes to local and regional hydrogen biogeochemical cycles. Subsequent work reports taxon-specific progress (high-pressure respirometry refuting major H2 use in Riftia; environment-dependent H2 dependence in Alviniconcha; genomic evidence for regulatory hydrogenases in some mussel symbiont strains) but the mussel in-situ rate question, the regulation question, the cold-seep absence question, and the ecosystem flux question all remain open.

Background

Chemosynthetic symbioses are associations in which marine invertebrates host chemolithoautotrophic bacteria that fix CO2 into biomass using energy from reduced chemical species; until 2011 only two energy sources were known to power them: reduced sulfur compounds and methane (Cavanaugh 1981; Felbeck 1981; Dubilier et al. 2008). The vent mussel genus Bathymodiolus hosts intracellular sulfur-oxidizing (thiotrophic) and/or methane-oxidizing (methanotrophic) Gammaproteobacteria in gill cells. At hydrothermal vents, molecular hydrogen (H2) is abundant where ultramafic, serpentinization-influenced basement reacts with seawater (serpentinization is the water-rock reaction that generates H2 from iron-bearing minerals such as olivine), and H2 concentrations can exceed those of sulfide, whereas basalt-hosted vent sites are comparatively H2-poor. Petersen et al. (2011, Nature 476:176-180) combined metagenome sequencing, geneFISH (a fluorescence in-situ-hybridization technique that localizes specific genes inside individual microbial cells), immunohistochemistry, shipboard incubations, in situ mass spectrometry (underwater mass spectrometers measuring dissolved-gas concentrations directly at the seafloor) and thermodynamic modeling to show that Bathymodiolus puteoserpentis symbionts at the H2-rich Logatchev field (Mid-Atlantic Ridge) take up H2 and that H2 stimulates CO2 fixation; thermodynamics predicted roughly an order of magnitude more energy per kilogram of fluid from H2 oxidation than from sulfide oxidation at ultramafic sites. They further detected hupL, the large-subunit gene of the group 1 [NiFe] hydrogenase (the membrane-bound enzyme catalyzing H2 oxidation), in symbionts of Pacific Bathymodiolus species and of other dominant vent animals including the tubeworm Riftia pachyptila and the shrimp Rimicaris exoculata, proposing that H2 is a widespread energy source for vent symbioses (Orphan and Hoehler 2011). Free-living relatives of these symbionts, the SUP05 clade of sulfur-oxidizing Gammaproteobacteria, likewise carry and dynamically express hydrogenase genes in hydrothermal plumes (Anantharaman et al. 2013). The initial demonstration, however, rested on shipboard incubations performed at atmospheric pressure and non-in-situ temperatures, on gene-presence surveys rather than expression or activity measurements, and on flux estimates extrapolated from laboratory rates and population-density estimates; these limitations define the open problem addressed here, and the corresponding gaps have since been restated by the field's dedicated review of microbial hydrogen cycling at vents (Adam and Perner 2018) and by later genomic and physiological studies.

Problem Statement

Determine the in-situ ecological and biogeochemical role of symbiont-mediated hydrogen oxidation in chemosynthetic vent symbioses, through five questions that remain open after the 2011 demonstration of Petersen et al. and the work that followed (Adam and Perner 2018; Mitchell et al. 2019; Patra et al. 2022): (1) whether the measured H2 uptake rates and H2-stimulated CO2 fixation of Bathymodiolus symbionts, obtained in shipboard incubations at atmospheric pressure and non-in-situ temperature, represent in-situ rates under ambient hydrostatic pressure and temperature (no such measurement has yet been made for Bathymodiolus, although incubation of an intact vent holobiont at in-situ temperature, pressure and water chemistry has been demonstrated for Riftia, Mitchell et al. 2019); (2) the prevalence, regulation (constitutive versus inducible expression) and environmental control of hydrogenase (hupL, the large-subunit gene of the membrane-bound H2-oxidizing group 1 [NiFe] hydrogenase) expression across spatial, temporal and taxonomic gradients, including assessment of in-situ hydrogenase activity and its ecological significance at basalt-hosted, low-H2 vent sites where hupL is present but H2 is scarce; (3) the reasons hydrogenase genes are absent or undetected in cold-seep mussel symbionts and in several sequenced vent Bathymodiolus symbiont genomes, whether host-symbiont phylogeny, local geochemistry, horizontal gene transfer and gene-loss dynamics, or methodological detection limits and assembly incompleteness; (4) the magnitude of symbiont-mediated H2 oxidation as a contribution to local and regional hydrogen biogeochemical cycles, in terms of in-situ H2 concentrations, in-situ fluxes and mussel population densities rather than extrapolations from laboratory rates (in-situ campaigns combining H2 and CO2 monitoring, cell counts and sample fixation for metagenomic and metatranscriptomic analysis have been called for but not yet carried out for vent symbioses, Adam and Perner 2018); and (5) the functional contribution of hupL-bearing epibionts and endosymbionts of other dominant vent animals, such as Rimicaris shrimp epibionts, to host nutrition and community-scale H2 turnover.

These five questions are separately motivated and independently open, and none of them is a prerequisite for the others. As a framing choice of this statement they are grouped here because they bear on one broader aim: establishing whether, and how much, hydrogen actually powers chemosynthetic symbioses, rather than merely being encoded by their symbionts.

References

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