Bacterial Uptake of Photosynthetic Carbon from Freshwater Phytoplankton

Microheterotrophic uptake of algal extracellular products was studied in two eutrophic lakes in southern Sweden. Size fractionation was used in H14CO3 uptake experiments to measure l4C fixation in total particulate, small particulate and dissolved organic fractions. Carbon fixed in algal photosynthesis was recovered as dissolved and small particulate 14C, representing excretion and bacterial uptake of algal products. Estimated gross extracellular release was low in these eutrophic systems, 1 to 7% of total 14C uptake per m2 lake surface. From 28 to 80% of 14C released was recovered in the small particulate fraction after ca. 4 h incubation. This percentage was uniform within each depth profile, but varied directly with in situ water temperature. Laboratory time-series incubations indicated steady state for the pool of algal extracellular products on one occasion, while increasing pool size was indicated in the remaining two experiments. Uptake of photosynthetic carbon to small particles in situ was 32 to 95% of estimated heterotrophic bacterial production (as dark '4C02 uptake) on four occasions. While excretion apparently was not an important loss of carbon for phytoplankton, it may have represented an important carbon source for planktonic bacteria.