Increasing tetracycline concentrations on the performance and communities of mixed microalgae-bacteria photo-bioreactors

This study investigated the impact of varying concentrations of tetracycline on the performance of mixed microalgae-bacteria photo-bioreactors. Photo-bioreactors were assessed for their ability to remove carbon dioxide (CO₂) from the biogas of anaerobic membrane bioreactor (anMBR), and nutrients from the anaerobic effluent. The varying concentrations of tetracycline had no impact on the removal of CO₂ from biogas. 29% v/v of CO₂ was completely removed to generate > 20% v/v of oxygen (O₂) in all reactors. Removal of nutrients and biomass was not affected at low concentrations of tetracycline (≤ 150 μg/L), but 20 mg/L of tetracycline lowered the biomass generation and removal efficiencies of phosphate. Conversely, high chlorophyll a and b content was observed at 20 mg/L of tetracycline. High tetracycline level had no impact on the diversity of 18S rRNA gene-based microalgal communities but adversely affected the 16S rRNA gene-based microbial communities. Specifically, both Proteobacteria and Bacteroidetes phyla decreased in relative abundance but not phylum Chloroplast. Additionally, both nitrogen-fixing (e.g. Flavobacterium, unclassified Burkholderiales and unclassified Rhizobiaceae) and denitrifying groups (e.g. Hydrogenophaga spp.) were significantly reduced in relative abundance at high tetracycline concentration. Phosphate-accumulating microorganisms, Acinetobacter spp. and Pseudomonas spp. were similarly reduced upon exposure to high tetracycline concentration. Unclassified Comamonadaceae, however, increased in relative abundance, which correlated with an increase in the abundance of tetracycline resistance genes associated with efflux pump mechanism. Overall, the findings demonstrate that antibiotic concentrations in municipal wastewaters will not significantly affect the removal of nutrients by the mixed microalgae-bacteria photo-bioreactors. However, utilizing such photo-bioreactors as a polishing step for anMBRs that treat wastewaters with high tetracycline concentration may not be effective as evidenced from the lower nutrient removal and occurrence of antibiotic resistance genes.