The use of oxygen microprobes to measure bacterial respiration for determining bacterioplankton growth efficiency

Heterotrophic bacterial growth efficiency (BGE), the ratio between the carbon consumed and the bacterial biomass produced, is a key factor in understanding flows of organic matter in aquatic ecosystems. Methods generally used to estimate bacterial respiration require long incubations ( 24 to 36 h) to measure significant rates during which nonlinear patterns of oxygen decrease may bias BGE computation. These respiration estimates are generally compared to bacterial production rates determined from radiotracer incorporation from much shorter incubations. The aim of this study was to improve the determination of bacterial respiration to better estimate BGE. For this purpose, we employed oxygen microprobes in predator free ( 0.6 mum filtered) seawater samples and determined in parallel bacterial abundance. The use of oxygen microprobes allowed us to continuously monitor oxygen concentration during the incubation. Hence, the length of incubation can be adjusted as soon as a significant decrease of oxygen is observed. At the most productive sites, respiration was measurable from the beginning of the incubation and varied with time. In contrast, at the oligotrophic sites, respiration was often detectable only after a lag-phase of 5 to 10 h and remained constant thereafter. BGE was computed from the changes in bacterial abundance observed during the respiration measurements. This way, both processes were determined in similar incubation conditions. In comparison, the use of radiotracer derived bacterial production systematically resulted in an underestimation of BGE.