Adaptation of Novosphingobium aromaticivorans for growing with aromatic compounds adsorbed to granular activated carbon as a potential cause for the stable occurrence of Sphingomonadaceae in fourth clarification stages of wastewater treatment plants

Activated carbon filters are increasingly being installed in wastewater treatment plants (WWTPs) to remove organic micropollutants from wastewater effluent through a physico-chemical adsorption process. Microbial biodegradation also contributes to the removal of micropollutants in those filters but the microbiomes and the underlying processes are mainly unexplored. Particularly, it is unknown whether active microbial desorption of micropollutants takes place.

In this study, the prokaryotic microbiome of an activated carbon filter newly installed as the fourth clarification stage of a municipal WWTP was analyzed over 12 months. Amplicon sequencing of genes for 16S rRNA revealed a large diversity of bacteria. Families with relative abundancies above 1% stabilized after 2-6 months and included potential micropollutant degraders Hyphomonadaceae and Sphingomonadaceae. Among the latter, members of the genus Novosphingobium made up around 45% and were closely related to the type strain N. aromaticivorans DSM 12444, which is known to degrade synthetic chemicals. Members of the genus Pseudomonas were not detected in the microbiome but enrichment cultures with filter material yielded Pseudomonas sp. strains Teo15 and Salic. Laboratory experiments with strains DSM12444 and the pseudomonads showed that strain DSM12444 reached a significantly higher final optical densities with benzoic and salicylic acid that was pre-adsorbed to granular activated carbon (GAC) than strains Teo15 and Salic, respectively. Extraction of residual benzoic and salicylic acid from activated carbon granules indicated that more than 95 % of the aromatic compounds had been depleted by strain DSM12444 compared to about 70 % by strains Teo15 and Salic. Transcriptomic analysis of strain DSM1244 during growth with benzoic acid showed differential gene expression in the presence and absence of GAC. In particular, a different pathway for degrading aromatic compounds and two genes for hemerythrin-domain containing proteins, which can serve as redox sensors, were upregulated in the presence of GAC.

This study indicates that strain DSM12444 can specifically adapt to utilize aromatic compounds adsorbed to GAC which might be a selective advantage colonizing activated carbon filters. This property calls also attention to the potential use of this organism for bioaugmentation of fourth clarification stages of WWTPs.