Chemosynthesis is an ancient process that uses energy from reduced chemical compounds to support microbial survival and growth. While traditionally associated with supporting life in the deep ocean, recent studies show that chemosynthesis is more widespread across marine environments. At euphotic depths, life is primarily supported by photosynthesis, however, even in shallow waters there are permanently dark environments. Among them, sea caves may have the conditions required for chemosynthetic communities to thrive. In this study, using a sea cave at 35 meters we have tested the hypothesis that chemosynthesis supports life in dark environments within the euphotic zone. Metagenome data show that dark adapted microbial communities encode enzymes for the chemosynthetic oxidation of reduced compounds including nitrogen, sulfur and trace gases. Microcosm activity assays demonstrate that ammonia, hydrogen and carbon monoxide were oxidized by the microbial community, albeit at difference rates. Altogether, our data show that chemosynthesis supports life in dark marine environments in the euphotic zone and may influence oceanic biogeochemistry and ecology.