Frequent antibiotic exposure stabilized the associated bacterial community while altering physiological and biochemical characteristics of the coccolithophore Chrysotila roscoffensis

The application of microalgae-based techniques in antibiotic-contaminated wastewater treatment frequently exposes the mass-cultured microalgae to antibiotics. The effects of frequent antibiotic exposure on the physiological processes and biochemical metabolites of microalgae remain to be determined. In addition, the response of the associated bacteria is unclear. A strain of coccolithophore, Chrysotila roscoffensis, which was frequently treated with antibiotics for several research purposes, was applied in the present study. The final biomass from a batch culture was sampled to detect how another fresh antibiotic treatment affected growth, photosynthetic performance, pigment and phytohormone production, and nutrient absorption. Changes in the associated bacteria in the cultures were comparatively studied. Compared with the control of the non-treated group, the growth rate, maximum biomass maintenance and photosynthetic performance were significantly improved in the antibiotic-treated group. The microalgae with antibiotics were less stressed according to the data from chlorophyll fluorescence parameters. No significant differences were observed in the contents of pigments such as chlorophyll a, diatoxanthin, zeaxanthin, prasinoxanthin, and fucoxanthin. However, the violaxanthin content decreased significantly by 61.36 %, and the diadinoxanthin content increased significantly by 28.02 %, showing that xanthophyll cycling for photoprotection, both violaxanthin cycle and diadinoxanthin cycle, was probably changed by the antibiotic treatments. The contents of phytohormones in the microalgae, such as indole acetic acid, salicylic acid, jasmonic acid, strigolactone and isopentenyl adenine, were not different, except for the concentration of 6-benzylaminopurine (decreased significantly by 43.51 %) and abscisic acid (undetectable in the antibiotic-treated group while the concentration in the non-treated group was 18.20 ± 2.15 ng/1010cells). Absorption of nutrients PO43−-P and SiO32−-Si was relatively inhibited, while NO3−-N absorption was promoted. Phosphorus limitation was probably alleviated in the antibiotic-treated group. No significant differences in the bacterial biomass and community were observed between the antibiotic-treated and untreated group. However, final associated bacterial communities in both groups differed significantly from the initial bacterial community. This phenomenon showed that although repeated antibiotic treatment did not change the associated bacterial community, the growth of C. roscoffensis exerted a considerable impact on the community. The present study provided new insights into the application of a microalgae-based strategy in wastewater treatment for the removal of antibiotic contaminants.