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Determination of specific metabolic activities of anammox bacteria and their sensitivity to oxygen
(2013)
The discovery of Anammox bacteria has bridged our knowledge gap in the nitrogen cycle. This is because it explained the direct conversion of nitrite and ammonium to nitrogen gas in many reducing natural environments. However, it has been found that the presence of oxygen is an important determinat factor for certain biochemical processes and distribution of organisms in the environment. In this work, two methodological approaches were used to investigate the specific metabolic activities of Anammox bacteria and their sensitivity to oxygen using a mixed biomass culture. Two laboratory-scale bioreactors were maintained and the biomass in the reactors served as the inoculum for the investigations. The molecular characterization of the biomass was done using the Fluorescence in situ hybridization (FISH) technique. The specific Anammox activity (SAA) and important inhibition kinetic parameter characterization were estimated while monitoring activities of the biomass using several batch experiments.
The results of the biochemical characterization revealed that several microbial activities such as denitrification, nitrification, dissimilatory nitrate reduction to ammonium and Anammox processes were competing in the bioreactors. These were observed in the analysis of effluent recirculation of the feed solutions in the bioreactors, however, the Anammox process was predominant. An optimum sonication time top of eight minutes was established used to determine the average optical density to dry mass (OD: DM) ratio of 0.87 in the biomass. The FISH characterization estimated 60.4 % ± 14.6% (mean ± standard deviation) Anammox bacteria concentration in the biomass of Bioreactor 1. The Anammox bacteria were made up C. Kuenenia Stuttgartiensis (̴ 78%) and the remaining C. Brocadia Fulgida.
The maximum SAA of 0.025 mgNH4-N (TSSAnammox-1. d-1) and 5.4 mgNH4–N (1011 CellAnammox-1. d-1) for ammonium, and 0.04 mg N2-N (TSSAnammox-1. d-1) and 9.4 mgN2–N (1011 CellAnammox-1. d-1) for nitrogen gas were found in the biomass. While a binding affinity constant Ki = 0.07 mgO2 /l, maximum half inhibition concentration IC50 ̴ 0.2 mg O2 /l and KM = 50 mg/l were estimated for the inhibition kinetics of oxygen in the biomass. The low Ki value found in this study indicates that oxygen is an efficient inhibitor for Anammox activities and concentrations between 0.4 - 0.64 mg O2 /l can lead to complete inhibition of Anammox activity. However, this process is reversible. Finally, the implication of this inhibition kinetic results might be important in tracing the distribution of Anammox processes in natural environments where fixed nitrogen is present.