Hydrogen Limitation, Competition and Syntrophy Among Thermophiles in the Hot Subseafloor

To understand the biogeochemical impact of subseafloor life on surrounding environments and the overlying ocean, the rates of and constraints on growth and metabolite production must be determined for subseafloor microbes. The diversity of thermophiles and hyperthermophiles in the hot subseafloor is low, making them amenable to biogeochemical modeling. At hydrothermal vents, Methanothermococcus and Methanocaldococcus are common thermophilic and hyperthermophilic methanogens; Desulfurobacterium, common thermophilic autotrophic sulfur reducers; and Thermococcus, common hyperthermophilic H₂-producing heterotrophs. Diffuse hydrothermal fluids (7-40°C) from Axial Seamount were incubated anaerobically at 55°C and 80°C. Microcosms with 1.2 mM added H₂ produced 30-50 times more CH₄ than those with 20 µM H₂, demonstrating that H₂ and heat were the primary constraints on the growth of thermophilic and hyperthermophilic methanogens. For Methanothermococcus thermolithotrophicus and Methanocaldococcus jannaschii, the lower H₂ thresholds and half-saturation constants (K_s) for growth in a chemostat were 9-20 µM and 47-69 µM, respectively, near their optimum growth temperatures. The lower H₂ threshold and K_s for Desulfurobacterium sp. strain HR11 grown in a batch reactor at 72°C were <3 µM and 29 µM, respectively. Its catalytic efficiency (µ_max/K_s) for H₂ use was 4 times higher than those of M. thermolithotrophicus and M. jannaschii, but its K_s for thiosulfate was 19 µM, suggesting that it will only outcompete methanogens when sulfur is not growth limiting. Microcosm incubations at 55°C and 80°C supplemented with peptides in lieu of H₂ produced the same amount of CH₄ as microcosms with 1.2 mM H₂. Co-culture incubations showed that H₂ production by T. paralvinellae supported the growth of Methanothermococcus sp. strain BW11, Methanocaldococcus bathoardescens, and strain HR11 in H₂-free medium, suggesting that syntrophy is a possible alternative H₂ source in the subseafloor.