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Appl Environ Microbiol. 1992 September; 58(9): 2938-2947

Biochemical Composition of Dissolved Organic Carbon Derived from Phytoplankton and Used by Heterotrophic Bacteria

Ingvar Sundh

Uppsala University, Institute of Limnology, Box 557, S-751 22 Uppsala, Sweden

ABSTRACT

The molecular size distribution and biochemical composition of the dissolved organic carbon released from natural communities of lake phytoplankton (photosynthetically produced dissolved organic carbon [PDOC]) and subsequently used by heterotrophic bacteria were determined in three lakes differing in trophic status and concentration of humic substances. After incubation of epilimnetic lake water samples with H¹⁴CO₃^- over one diel cycle, the phytoplankton were removed by size-selective filtration. The filtrates, still containing most of the heterotrophic bacteria, were reincubated in darkness (heterotrophic incubation). Differences in the amount and composition of PDO¹⁴C between samples collected before the heterotrophic incubation and samples collected afterwards were considered to be a result of bacterial utilization. The PDO¹⁴C collected at the start of the heterotrophic incubations always contained both high (>10,000)- and low (<1,000)-molecular-weight (MW) components and sometimes contained intermediate-MW components as well. In general, bacterial turnover rates of the low-MW components were fairly rapid, whereas the high-MW components were utilized slowly or not at all. In the humic lake, the intermediate-MW components accounted for a large proportion of the net PDO¹⁴C and were subject to rapid bacterial utilization. This fraction probably consisted almost entirely of polysaccharides of ca. 6,000 MW. Amino acids and peptides, other organic acids, and carbohydrates could all be quantitatively important parts of the low-MW PDO¹⁴C that was utilized by the heterotrophic bacteria, but the relative contributions of these fractions differed widely. It was concluded that, generally, low-MW components of PDOC are quantitatively much more important to the bacteria than are high-MW components, that PDOC released from phytoplankton does not contain substances of quantitative importance as bacterial substrates in all situations, and that high-MW components of PDOC probably contribute to the buildup of refractory, high-MW dissolved organic carbon in pelagic environments.

Present address: Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, S-750 07 Uppsala, Sweden.

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