Functional robustness and gene pools of a wastewater nitrification reactor: comparison of dispersed and intact biofilms when stressed by low oxygen and low pH
2008
Mao, Yuejian | Bakken, Lars R. | Zhao, Liping | Frostegård, Åsa
The functional robustness of biofilms in a wastewater nitrification reactor, and the gene pools therein, were investigated. Nitrosomonas and Nitrosospira spp. were present in similar amounts (cloning-sequencing of ammonia-oxidizing bacteria 16S rRNA gene), and their estimated abundance (1.1 x 10⁹ cells g⁻¹ carrier material, based on amoA gene real-time PCR) was sufficient to explain the observed nitrification rates. The biofilm also had a diverse community of heterotrophic denitrifying bacteria (cloning-sequencing of nirK). Anammox 16S rRNA genes were detected, but not archaeal amoA. Dispersed biofilms (DB) and intact biofilms (IB) were incubated in gas-tight reactors at different pH levels (4.5 and 5.5 vs. 6.5) while monitoring O₂ depletion and concentrations of NO, N₂O and N₂ in the headspace. Nitrification was severely reduced by suboptimal O₂ concentrations (10-100 μM) and low pH (IB was more acid tolerant than DB), but the N₂O/NO₃⁻ product ratio of nitrification remained low (<10⁻³). The NO₂⁻ concentrations during nitrification were generally 10 times higher in DB than in IB. Transient NO and N₂O accumulation at the onset of denitrification was 10-10³ times higher in DB than in IB (depending on the pH). The contrasting performance of DB and IB suggests that the biofilm structure, with anoxic/micro-oxic zones, helps to stabilize functions during anoxic spells and low pH.
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