Effect of chlortetracycline on the growth and intracellular components of Spirulina platensis and its biodegradation pathway

Highlights: • Half maximal effective concentration for Spirulina platensis was 8.76 mg/L at 96 h. • Risk quotient value indicated CTC had high risk to S. platensis in wastewater. • Inhibition rate of CTC on S. platensis was more than that of CTC + oxytetracycline. • High CTC improved yield of dissolved extracellular organic matters in S. platensis. • Hydroxylation and side-chain breakdown were the main biodegradation pathways of CTC. Chlortetracycline (CTC) usually presents in livestock wastewater with oxytetracycline (OTC), causing damage to human health and ecosystems. It's urgent to seek low-cost and ecofriendly technology for antibiotics removal. In this study, effects of CTC and CTC + OTC (CTC:OTC= 1:1, g/g) on Spirulina platensis have been investigated. EC₅₀ value of CTC for S. platensis was 8.76 mg/L at 96 h and risk quotient value in wastewater was 15.85. Inhibition of CTC on S. platensis gradually enhanced with increase of CTC, but CTC + OTC below 1.0 mg/L didn't harm the growth of microalgae. Chlorophyll-a (Chl-a) and carotenoid content showed a parabolic trend with extension of time at CTC group. Chl-a synthesis gradually decreased with increase of CTC + OTC stress. High concentrations of CTC and CTC + OTC showed obvious inhibition on phycocyanin production. Polyunsaturated fatty acids (PUFA) and saturated fatty acids (SFA) contents peaked at 1.0 mg/L CTC, corresponding to the minimum of superoxide dismutase (SOD) activity in S. platensis. SFA and PUFA contents decreased when CTC + OTC content was above 2.0 mg/L. CTC and CTC + OTC (both over 2.0 mg/L) stimulated production of dissolved extracellular organic matters in S. platensis. Removal efficiency of CTC by S. platensis was about 98.63–99.95% and its biodegradation pathways were hydroxylation and side-chain breakdown.