Distribution and Capacity for Utilization of Lower Fatty Acids of Phototrophic Purple Nonsulfur Bacteria in Wastewater Environments

Abstract

Populations of phototrophic purple nonsulfur bacteria (PNSB) of up to 104-107 CFU ml-1 were found in photosynthetic sludge and activated sludge plants treating wastewater having different biological oxygen demand (BOD) levels and containing acetate as a main BOD source. There was a positive correlation between the population density of PNSB and the strength of BOD in the aeration tanks. All of the PNSB strains isolated were identified as species of the genera Rhodobacter and Rhodopseudomonas by 16S rRNA gene sequencing. The Rhodobacter species were abundant in high BOD-loaded wastewater environments, while the Rhodopseudomonas species increased at lower BOD levels. Therefore, the PNSB population structure appeared to be greatly affected by the concentration of lower fatty acids as major BOD sources, varying over time and space. The utilization of lower fatty acids with different carbon numbers (C2 to C6) by aerobically or semi-aerobically grown cells of authentic and isolated PNSB strains was evaluated by monitoring substrate-dependent oxygen uptake. The oxidation of the fatty acids by the Rhodobacter strains depended upon the number of carbons in the substrate, while the Rhodopseudomonas strains utilized all substrates equally. A low carbon number was much preferred by the Rhodobacter strains. The affinity for acetate of the Rhodobacter and Rhodopseudomonas strains ranged from 0.14 to 3.0 mM and 0.032 to 0.096 mM, respectively. These results suggest that the concentration and kind of lower fatty acids as major BOD sources are important factors affecting not only the population level but also the species composition of PNSB in wastwater environments.