DARK FIXATION OF C-14 - VARIATIONS RELATED TO BIOMASS AND PRODUCTIVITY OF PHYTOPLANKTON AND BACTERIA

In various North Atlantic sites, the rate of dark C-14 fixation by microbial plankton was significantly correlated with the rate of light C-14 fixation, with the rate of bacterial [H-3]thymidine incorporation, and with chlorophyll biomass. For plankton sampled near the sea surface (<5 m) and at the depth of the maximum light C-14 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 C-14 fixation is high in relation to light fixation only at depths where photosynthesis is severely limited by light. We infer that direct C-14 fixation by both bacteria and phytoplankton are significant processes that contribute to measured C-14 fixation in opaque bottles during 12 h dawn-to-dusk incubations.