The aerobic oxidation of molecular hydrogen (H2) is a thermodynamically favorable energy-yielding process for microbial communities, but how H2 oxidation is connected to anabolic processes in marine microbial communities is poorly understood. We quantified the effects of H2 on carbon utilization using quantitative DNA stable isotope probing (DNA-qSIP), which revealed that H2 utilization increased carbon assimilation by diverse microbial taxa synthesizing 13C-labeled DNA associated with ultramafic rocks at the Mid-Atlantic Ridge.