Dark fixation of ¹⁴C: Variations related to biomass and productivity of phytoplankton and bacteria

Abstract

In various North Atlantic sites, the rate of dark 14 C fixation by microbial plankton was significantly correlated with the rate of light 14 C fixation, with the rate of bacterial [ 3 H]thymidine incorporation, and with chlorophyll biomass. For plankton sampled near the sea surface (<5 m) and at the depth of the maximum light 14 C fixation, the median value for the ratio of dark to light fixation was only 1%. At the depth of the subsurface chlorophyll maximum where less light was available, the median value for this ratio was still quite low at 7%. Our results confirm early claims that generally in the oceans, including oligotrophic regions where plankton biomass is low, dark 14 C fixation is high in relation to light fixation only at depths where photosynthesis is severely limited by light. We infer that direct 14 C fixation by both bacteria and phytoplankton are significant processes that contribute to measured 14 C fixation in opaque bottles during 12 h dawn‐to‐dusk incubations.