Phytoplankton release of extracellular organic carbon, molecular weight composition, and bacterial assimilation

Phytoplankton release of extracellular dissolved organic carbon (EOC) and its subsequent assimilation by planktonic bacteria was quantified using the procaryotic inhibitor streptomycin. The carbon flow was assayed in the Danish estuary. Randers Fjord, and in laboratory experiments. From 34 to 90 % of the released carbon was transported to the bacteria; the bacterial metabolism of EOC ranged from 3 to 30 % of total primary production. If no correction for bacterial respiration (20 to 50 Oh) is made the EOC transport can be seriously underestimated. The released products were predominantly of low molecular weight (< 900 Daltons). The bacteria showed distinct selectivity for these small molecules. Streptomycin, an active bactericidal agent, also affected algal photosynthesis in some cases. In carbon flow experiments antibiotics must be used with great care.