His research is focused primarily on microorganisms that grow optimally above 80°C, or so-called ‘hyperthermophiles’, from geothermal environments in the deep sea and on land. Dr. Holden’s field research examines microbial life in the subsurface biosphere, the distribution and abundances of specific types of microorganisms and their relation with environmental factors, numerical modeling of methane production, microbial responses to volcanic eruptions, and the upper temperature limit for life. His laboratory research is interested in physiological mechanisms of anaerobic respiration and CO2 assimilation in hyperthermophilic archaea.