Kinetics of Extracellular Release of 14 C-Labelled Organic Carbon by Submerged Macrophytes

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Abstract
The release of extracellular organic C (EOC) by 6 submerged freshwater macrophytes [Littorella uniflora, Elodea canadensis, Fontainea antipyretica, Ceratophyllum demerusum, C. submersum and Nitella sp.] was measured in time course studies with a 14C-technique. Incubation in light in an open water-flow system made it possible to assay the time courses of 14C-fixation and the simultaneous release of labeled EOC. Heterotrophic utilization of the released products by epiphytic communities was measured. Two patterns of release kinetics were found: constant rates of release occurred during the incubations; the rates still increased after 24 h of incubation. During the first hours of incubation the rates of relase increased in all species. Elodea reached constant rates after 2-4 h and Littorella and C. demersum after .apprx. 20 h. In the experiments with C. submersum and Nitella the rates of release increased almost linearly during the the entire incubation period. The kinetics of release were in agreement with the molecular weight distribution of the dissolved EOC measured with gel chromatography. Low MW products (< 1000 daltons) dominated in experiments with Elodea, Littorella and C. demersum. High MW compounds (> 10,000 daltons) dominated the dissolved EOC released by C. submersum and Nitella. A large fraction (18-60%) of the total EOC could be recovered on filters with a pore size of 0.2 .mu.m. This particulate fraction probably represents some abiotic removal. The quantities of release were low in all species and did not exceed 0.9% of the photosynthetic C fixation. Heterotrophic uptake by the epiphytic communities was less than 10% of the EOC released. The 14C-labeling of extracellular products apparently is a time dependent process.