Biofilm and microbiome response of attached growth nitrification systems across incremental decreases to low temperatures

The study investigates low temperature attached growth rates and nitrifying biofilm, biomass, and microbiome responses down to 1 °C. Two moving bed biofilm reactors (MBBR) were acclimatized from 10 °C to 1 °C by incremental decreases in temperatures. A non-linear decline in nitrification with decreasing temperatures was recorded with a kinetic threshold temperature delineated between 4 °C and 2 °C, at which a significant decline in efficiency of 22.47% occurred. An increase in biofilm thickness of 25.47% occurs between 10 °C and 4 °C, followed by lesser increases of 6.55% and 9.60% at 2 °C and 1 °C. Biofilm density decreases between 10 °C and 1 °C. An increase in embedded viable cells of 24.01% occurs between 10 °C and 4 °C, followed by lesser non-statistically relevant increases at 2 °C and 1 °C. No statistically significant change in the number of estimated species is observed; however, β-diversity analysis indicates greater microbial community shifts above the kinetic threshold followed by lesser shifts between 2 °C and 1 °C. Nitrosomanadacea and Nitrosopira are the ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) families in the biofilm with relative abundances of 3.43 ± 0.51% and 0.94 ± 0.44% respectively at 10 °C. A change in NOB relative abundance occurred between the temperatures of 10 °C and 1 °C, decreasing to 0.19 ± 0.10% above the kinetic threshold. The estimation of nitrifiers per carrier in the bioreactors, based on biofilm thickness, biomass viability and relative abundances shows an increase of 30.2% in AOB/carrier above the kinetic threshold temperature (> 4 °C) followed by lesser increases down to 1 °C.