Influence of incubation conditions on bacterial production estimates in an estuarine system

Abstract

This study aimed to assess the influence of incubation conditions in the determination of bacterial production (BP). In order to achieve that goal, experimental setups were performed in situ and in the laboratory under both dark and light conditions. To test spatial and seasonal variations and the different natural light exposure of microorganisms, sampling was performed in two distinct zones of the estuary Ria de Aveiro (Portugal), typifying the marine and brackish water zones of the estuarine system. Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments was used to monitor possible alterations in bacterial community composition induced by the incubation conditions. The results showed that BP determined in situ conditions significantly differed from in the laboratory. In the marine zone, a defined pattern of variation was detected, with consistent higher values of BP in laboratory dark conditions. This trend was not present in the brackish water zone. The seasonal and spatial variability of BP observed in field incubations was related to the physical–chemical proprieties of the water column, irradiance levels and the original community composition. The metabolic active profiles of bacteria were substantially different in the several incubation conditions, suggesting that methodological procedure influences the bacterial community composition, and the values of BP reported for aquatic ecosystems could be quite different from the real ones. In the light of these results, we suggest that BP determinations should be conducted under in situ conditions. However, due to execution limitations, BP needs to be frequently determined in the laboratory, and in this case, dark incubations provide more approximate values. This is the method routinely used, and although this incubation condition can cause stimulation of BP, the structure of the bacterial community is more similar to the one obtained with the in situ incubations.