Effect of silver nanoparticles on planktonic and biofilm cell growth

Nitrifying bacteria and microalgae are important microorganisms in open pond wastewater treatment systems. Nitrification involving the sequential oxidation of ammonia to nitrite and nitrate, mainly due to autotrophic nitrifying bacteria, is essential to biological nitrogen removal in wastewater and global nitrogen cycling. When a continuous flow autotrophic bioreactor, initially designed for nitrifying bacterial growth was contaminated by microalgae, we monitored both the microalgal and nitrifying bacterial activity by measuring specific oxygen production rate (SOPR) for microalgae and specific oxygen uptake rate (SOUR) for nitrifying bacteria to better understand algalbacterial interactions. The growth of microalgae inhibited the maximum nitrification rate by a factor of 4 although the ammonium nitrogen fed to the reactor was almost completely removed. Terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that the community structures of nitrifying bacteria remained unchanged, containing the dominant Nitrosospira, Nitrospira and Nitrobacter species. PCR amplification coupled with cloning and sequencing analysis resulted in identifying Chlorella emersonii and an uncultured cyanobacterium as the dominant species in the autotrophic bioreactor. Notwithstanding the fast growth rate of the microalgae and the algal toxicity to nitrifiers, algae were more easily lost in effluent than nitrifying bacteria 6 Environmental Science and Technology, 2009, in review. 161 because of their poor settling characteristics. Both microorganisms grew together in the bioreactor with constant individual biomass fractions because of the uncoupled solids retention times for algae and nitrifiers. The results indicate that compared to conventional wastewater treatment systems, even longer minimal SRTs (e.g., by a factor of 4) should be considered in phototrophic bioreactors for complete nitrification and nitrogen removal.