Influence of hydrogen peroxide on heterotrophic communities in ammonia-oxidizing enrichment cultures

Ammonia-oxidizing bacteria (AOB) are responsible for the first step of nitrification and are thereby important players in the global nitrogen cycle. Microorganisms in aquatic environments are frequently exposed to oxidative stress from metabolic byproducts and photochemical processes. Heterotrophic bacteria aid AOB under oxidative stress; however, the impacts of oxidative stress on the heterotrophic communities in co-culture with the autotrophic microorganisms have not been investigated in detail. In this study, we exposed 2 AOB enrichment cultures to oxidative stress via hydrogen peroxide (H2O2) and evaluated the effects of H2O2 on the activity of the AOB and the heterotrophic communities. Cultures received different frequencies of exposure to 10 or 100 µM H2O2 as well as time to recover after exposure to H2O2. Ammonia-oxidizing activity was unaffected by 10 µM H2O2; however, an increase in lag phase was observed in the presence of 100 µM H2O2. The heterotrophic community structure changed with repeated exposure to 100 µM H2O2. Microorganisms such as Xanthobacter sp. and Variovorax sp. increased in abundance in the presence of 100 µM H2O2, and Methyloversatilis sp. stayed stable under all conditions. It is likely that these microorganisms were resistant to the H2O2 stress and supported the AOB. Ammonia-oxidizing activity recovered from repeated exposure to H2O2 stress immediately after removal of the H2O2 stress. In contrast, the heterotrophic community needed repeated exposure to non-stress conditions to change back towards pre-stress conditions, indicating that different heterotrophic communities can provide the same support to the ammonia oxidizers.