Excretion of dissolved organic carbon by phytoplankton of different sizes and subsequent bacterial uptake

Abstract

Identical techniques and laboratory equipment were used to compare photosynthetic particulate assimilation and excretion of dissolved organic compounds (EOC) by different size classes of phytoplankton from marine and freshwater ecosystems. The subsequent assimilation and respiration of these algal exudates by bacteria were measured. The percentage of extracellular release of dissolved organic compounds was lower (4 to 5%) for larger algal cells than for picoeukaryotes (29%). Bacterial assimilation of the EOC released by the algae under these conditions seemed to be related more to algal species than to biovolume, although there may also be an effect on the specific bacterial assemblages. Bacterial uptake and utilization of EOC was 19% for freshwater picoeukaryotes, and 18 to 70% for marine nanoplankton. Great variations were found in the respiration rates of natural bacteria on EOC released by the freshwater picoalgae (95%) and nanoplanktonic salt pond algae (20 to 41%).