We investigated changes in quality and quantity of extracellular and biomass-derived organic matter (OM) from three axenic algae (genera Rhodomonas, Chlamydomonas, Coelastrum) during growth of Limnohabitans parvus, Limnohabitans planktonicus and Polynucleobacter acidiphobus representing important clusters of freshwater planktonic Betaproteobacteria. Total extracellular and biomass-derived OM concentrations from each alga were approximately 20 mg l-1and 1 mg l-1respectively, from which up to 9% could be identified as free carbohydrates, polyamines, or free and combined amino acids. Carbohydrates represented 54%-61% of identified compounds of the extracellular OM from each alga. In biomass-derived OM of Rhodomonas and Chlamydomonas 71%-77% were amino acids and polyamines, while in that of Coelastrum 85% were carbohydrates. All bacteria grew on alga-derived OM of Coelastrum, whereas only Limnohabitans strains grew on OM from Rhodomonas and Chlamydomonas. Bacteria consumed 24%-76% and 38%-82% of all identified extracellular and biomass-derived OM compounds respectively, and their consumption was proportional to the concentration of each OM compound in the different treatments. The bacterial biomass yield was higher than the total identifiable OM consumption indicating that bacteria also utilized other unidentified alga-derived OM compounds. Bacteria, however, also produced specific OM compounds suggesting enzymatic polymer degradation or de novo exudation.

Biology Centre of the Academy of Sciences of the Czech Republic v v i Institute of Hydrobiology Na Sádkách 7 České Budějovice CZ 37005 Czech Republic

Leibniz Institute of Freshwater Ecology and Inland Fisheries Limnology of Stratified Lakes Alte Fisherhütte 2 OT Neuglobsow Stechlin D 16775 Germany Institute for Biochemistry and Biology Potsdam University Maulbeerallee 2 Potsdam D 14468 Germany

Limnological Station Department of Plant and Microbial Biology University of Zurich Seestrasse 187 Kilchberg CH 8802 Switzerland

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