Oceanic crust is the largest aquifer on Earth, with a massive volume of seawater advecting through the crust. In contrast to high-temperature ridge axis and ridge flank hydrothermal systems, little is known about the geochemistry and microbiology of low-temperature ridge flanks systems. Such systems are globally abundant and thus may have a major influence on oceanic geochemistry. The microbiome of these deep marine subsurface habitats has been estimated to be substantial, and consequently microbial metabolic activity may have major influences on global element cycles. Nevertheless, information on the microbial ecophysiology is broadly missing. In order to address this question, venting fluids were sampled at the Dorado outcrop and crustal fluids were sampled from two neighboring CORK (Circulation Obviation Retrofit Kit) observatories at the North Pond sediment pond. This study provides potential respiration rates and assesses the fraction of actively protein synthesizing microbes within the microbial community in low-temperature crustal fluids. Furthermore, these data provide a first insight into the microbial conversion of sulfur and nitrogen species at low-temperature ridge flanks.