Microbial dark assimilation of CO₂ in the Cariaco Trench1

Dark assimilation of carbon dioxide has been measured in water from the oxygen‐sulfide interface and the deep anoxic zone in the Cariaco Trench. Comparison of rates of interface‐associated dark assimilation with primary production and organic carbon flux to the deep anoxic water indicates that chemoautotrophic sulfur bacteria cannot be wholly responsible for carbon dioxide assimilation at the observed magnitude. It is suggested that dark assimilation of carbon dioxide near the interface chiefly represents the metabolic activities of facultatively and obligately anaerobic bacteria which use inorganic sulfur compounds as electron acceptors, and sulfide or thiosulfate‐oxidizing bacteria whose growth is dependent upon organic carbon.