Composition and Activity of Marine Alkane-Degrading Bacterial Communities in the Transition from Suboxic to Anoxic Conditions.

The impact of the oxygen supply rate (OSR) on the metabolic activity and on the composition of hexadecane-degrading bacterial communities in a quasi-anoxic milieu (nominal DOT = 0%) was studied in continuous cultures containing intertidal sediment. The dilution rate was kept constant at 0.035 h-1. The OSR was stepwise reduced from 3.5 mmol O2 L-1 h-1 to 0.06 mmol O2 L-1 h-1. Activity was determined by analyzing the respiration quotient (RQ) and the rates of hexadecane degradation (QHex), of hexadecane mineralization, and of protein production (PPR). The community composition and size were investigated by fluorescence in situ hybridization (FISH), by dilution plating (colony forming units or CFU), and by most probable number (MPN). The culture showed an aerobic hexadecane metabolism down to an OSR of 0.35 mmol O2 L-1 h-1. Below this OSR, anaerobic metabolism was initiated. The relationship among the RQ, PPR, QHex, and the OSR can be approximated by hyperbola (Michaelis-Menten kinetics). We suggest that the metabolic adaptation of the culture to low OSRs is due to regulation of protein expression and enzyme activity. Reducing the OSR resulted in minor but significant changes in the concentration of different physiological and phylogenetic groups. This means that, in addition to protein expression and activity regulation, the adaptation of the population to low OSRs is due to changes in the community composition.