||A hyperthermophilic deep-sea methanogen, Methanocaldococcus strain JH146, was isolated from 26 degrees C hydrothermal fluid at Axial Volcano to model high temperature methanogenesis in the subseafloor. Emphasis was placed on defining growth kinetics, cell yields and growth energy demand (GE) across a range of conditions. The organism uses H-2 and CO2 as its sole carbon and energy sources. At various temperatures, pHs, and chlorinities, its growth rates and cell yields co-varied while GE remained uniform at 1.69x10(-11) J cell(-1)s(-1)+/- 0.68x10(-11) J cell(-1)s(-1) (s.d., n=23). An exception was at superoptimal growth temperatures where GE increased to 7.25x10(-11) J cell(-1)s(-1) presumably due to heat shock. GE also increased from 5.1x10(-12) J cell(-1)s(-1) to 7.61x10(-11) J cell(-1)s(-1) as NH4+ concentrations decreased from 9.4mM to 0.14mM. JH146 did not fix N-2 or assimilate NO3-, lacked the N-2-fixing (cluster II) nifH gene, and became nitrogen limited below 0.14mM NH4Cl. Nitrogen availability may impact growth in situ since ammonia concentrations at Axial Volcano are lt;18M. Our approach contributes to refining bioenergetic and carbon flux models for methanogens and other organisms in hydrothermal vents and other environments.