Response of antioxidant defense to oxidative stress induced by H2O2 and NO in anammox bacteria

Exposure to the stressful environment results in excessive generation of reactive oxygen species (ROS) or reactive nitrogen species (RNS) in anaerobes, which causes deterioration of microbial activities in biological wastewater treatment systems. Although the genes involved in oxidative stress defense have been primarily identified in the genome of Candidatus Kuenenia stuttgartiensis (a typical anammox species), their function is still not verified. Therefore, the expression of putative antioxidation genes kat, sor, and sod in anammox bacteria was studied by in situ transcription and function validated by heterologous expression under the typical ROS (H2O2) and RNS (NO) stress. After H2O2 and NO additions, the genes involved in the anammox central metabolism (nirS, hzsB, and hdh) were immediately down expressed consistent with the decreased anammox activity. However, the expression of putative antioxidation gene kat did not rise when exposed to H2O2; whereas, its encoding protein KAT enhanced the antioxidant actively of anammox bacteria by H2O2 decomposition like the oxidoreductase enzyme catalase. The sod and sor gene were upregulated with NO treatment, and SOD and SOR can combine with NO and decrease its concentration efficiently. These confirmed the important role of kat, sod, and sor as ROS/RNS scavengers in anammox bacteria, with which anammox bacteria protect themselves when they are exposed to the stressful environment. These verified functional enzymes provide directions for the future regulation of anammox systems, which helps to mitigate the inhibitory effect of the stressful environment on anammox bacteria.