Comparison between the carbon-14 and oxygen consumption method for the determination of the activity of heterotrophic bacterial populations

The activity of the heterotrophic microbial population in the saline Lake Grevelingen (The Netherlands) and the Mediterranean Etang Salses Leucate (France) was determined by measuring the oxygen consumption rate, and the uptake of ¹⁴C-labelled glycollate, pyruvate and an amino acid mixture. The maximum uptake rate of the applied organic compounds in Lake Grevelingen was generally less than 10% of the carbon mineralization rate calculated from the oxygen consumption experiments. Only for pyruvate and glycollate higher values were found of about 30 to 40% with one exceptionally high value for pyruvate of 149%. However, these higher percentages were found in winter, when the activity of the heterotrophic microbial population was very low. In Etang Salses Leucate higher maximum uptake rates of the ¹⁴C-labelled compounds were found, relating this uptake to the oxygen consumption rate. Yet the maximum uptake rate is still always lower than 35% of the carbon mineralization calculated from the oxygen uptake rate. Taking into account that maximum uptake rates were considered, the results demonstrate that the uptake of ¹⁴C-labelled organic compounds represents a serious underestimation of the activity of the bacterial population in situ. The extent of the underestimation depends on the water type. It was concluded that the determination of the heterotrophic activity by measuring oxygen consumption rates offers a better insight into the carbon mineralization process in natural waters than the uptake experiments with ¹⁴C-labelled substrates.Communication no. 228 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands