Correlation of seasonal nitrification failure and ammonia-oxidizing community dynamics in a wastewater treatment plant treating water from a saline thermal spa

In this work we evaluated the effect of a perturbation on nitrification performance in a wastewater treatment plant (WWTP) treating urban and saline thermal bath wastewater, which regularly occurred during summer months. We wanted to find out if this related to changes in the ammonia oxidizing communities. The bacterial and ammonia-oxidizing bacterial (AOB) community from three different basins of the WWTP were evaluated using PCR-DGGE and cloning and sequencing of 16S rRNA gene fragments, over a six month survey. Both eubacterial and AOB communities underwent continuous change over time, with a particularly prominent shift between the third and fourth month of monitoring for eubacteria and the fourth month and fifth month for AOB. At the same time, reduction of nitrification performance was observed in the WWTP. The AOB community in the activated sludge was dominated by clones with high 16S rRNA sequence identity to halophilic-halotolerant organisms from the Nitrosomonas oligotrohpa and Nitrosomonas marina clusters. A significant correlation (R=0.97) was detected between the structure of the AOB community and environmental parameters, indicating that the AOB community structure changed in line with the environmental changes that took place over the sampling period. This study reports a clear association of microbial community dynamics (strongly correlated to salinity, temperature, and dissolved oxygen) to nitrification performance. Particularly, ammonia-oxidizing bacteria are severely affected by drastic changes in operational conditions, with direct consequences on WWTP performance.