Modeling the growth of hyperthermophiles in deep-sea hydrothermal diffuse fluids and sulfide deposits

H.C. Ver Eecke¹, D.M. Oslowski¹, D.A. Butterfield², E.J. Olson³, M.D. Lilley³, & J.F. Holden¹*

Corresponding author: jholden@microbio.umass.edu
¹University of Massachusetts, Department of Microbiology, Amherst, MA, 01003
²University of Washington, JISAO, Seattle, WA, 98195
³University of Washington, School of Oceanography, Seattle, WA, 98195

Abstract:
In 2008 and 2009, 534 hydrothermal fluid samples and 5 actively-venting black smoker chimneys were collected using Alvin for correlative microbiological and chemical analyses as part of the Endeavour Segment and Axial Volcano Geochemistry and Ecology Research (EAGER) program. Hyperthermophilic, autotrophic Fe(III) oxide reducers, methanogens, and sulfur-reducing heterotrophs were enriched for at 85 and 95°C using most-probable-number estimates from 28 diffuse fluid and 8 chimney samples. Heterotrophs were the most abundant of the three groups in both diffuse fluids and black-smoker chimneys. Iron reducers were more abundant than methanogens, and more abundant in sulfide-hosted vents than in basalt-hosted vents. Fluid chemistry suggests that there is net biogenic methanogenesis at the Marker 113/62 diffuse vent at Axial Volcano but nowhere else sampled. The growth of hyperthermophilic methanogens and heterotrophs was modeled in the lab using pure cultures. Methanocaldococcus jannaschii grew at 82°C in a 2-liter reactor with continuous gas flow at H2 concentrations between 20 and 225 μM with a H2 km of 100 μM. Correlating H2 end-member mixing curves from vent fluids and seawater with our laboratory modeling study suggests that H2 concentrations are limiting for Methanocaldococcus growth at most Mothra, Main Field, and High Rise vent sites at Endeavour but sufficient to support growth at some Axial Volcano vents. Therefore, hyperthermophilic methanogens may depend on H2 syntrophy at low H2 sites. Twenty-one pure hyperthermophilic heterotroph strains each grew on α-1,4 and β-1,4 linked sugars and polypeptides with concomitant H2 production. The H2 production rate (cell-1 doubling-1) for Pyrococcus furiosus at 95°C without sulfur was 29 fmol, 36 fmol, and 53 fmol for growth on α -1,4 sugars, β -1,4 sugars, and peptides, respectively. The CH4 production rate for M. jannaschii was 390 fmol cell-1 doubling-1; therefore, we estimate that it would take approximately 40 heterotroph cells to provide all of the H2 necessary to support the growth of a single methanogen. In contrast to methanogens, autotrophic Fe(III) oxide reducers consume far less H2 during growth and reach cell concentrations similar to methanogens in pure culture. Thermodynamic predictions suggest that they would grow at H2 concentrations lower than those needed by methanogens.

Keywords:
Endeavour ISS, hyperthermophiles, methanogenesis, heterotrophs, hydrogen

Contributions to Integration and Synthesis:
The results of our research would contribute to modeling the growth of hyperthermophiles and other microorganisms at hydrothermal vent sites and predictions of the types of microbial processes that would occur following a volcanic eruption when H2 concentrations increase in vent fluids.