Constructed wetlands (CWs) are successfully applied for treatment of waters contaminated with aromatic compounds. In these systems, plants provide oxygen and root exudates to the rhizosphere and thereby stimulate microbial degradation processes. Root exudation of oxygen and organic compounds depends on photosynthetic activity, and thus may show day-night fluctuations. While diurnal changes in CW effluent composition have been observed, information on respective fluctuations of bacterial activity are scarce.

We investigated microbial processes in a CW model system treating toluene-contaminated water and which showed diurnal oscillations of oxygen concentrations using metaproteomics. Quantitative real-time PCR was applied to assess diurnal expression patterns of genes involved in aerobic and anaerobic toluene degradation.

We observed stable aerobic toluene turnover by Burkholderiales during day and night. Polyhydroxyalkanoate synthesis was upregulated in these bacteria during day, suggesting that they additionally feed on organic root exudates, while re-utilizing the stored carbon compounds during night via the glyoxylate cycle. Although mRNA copies encoding for the anaerobic enzyme benzylsuccinate synthase (bssA) were relatively abundant and increased slightly at night, the corresponding protein could not be detected in the CW model system.

Our study provides insights into diurnal patterns of microbial processes occurring in the rhizosphere of an aquatic ecosystem.