Affinity-based alleviation of dissolved organic matter (DOM) on tetracycline toxicity to photosynthesis of green algae Chlorella vulgaris: roles of hydrophilic and hydrophobic DOM.

The environmental fate and toxic effects of antibiotics such as tetracycline (TC) could be influenced by the ubiquitous dissolved organic matter (DOM). However, DOM from different origins has different hydrophilic and hydrophobic properties. It is still unknown the effects of hydrophilic and hydrophobic DOM on the toxic effect of TC. In this study, DOM with hydrophilicity and hydrophobicity was separated and used to investigate their roles in affecting TC toxicity to the photosynthesis of green algae Chlorella vulgaris. Results showed that 10 mg L-1 TC inhibited the efficiency of photosystem II (PSII) of C. vulgaris using light by hindering electron transfer from QA- to QB/QB-, and the O2 release rate of C. vulgaris decreased by a third after 12-h treatment of 10 mg L-1 TC, while both hydrophilic and hydrophobic DOM (20 mg L-1 TOC) alleviated TC toxicity to the photosynthesis of C. vulgaris. In the presence of hydrophilic or hydrophobic DOM, stable complex of TC-hydrophilic DOM or TC-hydrophobic DOM was formed immediately, due to the good affinity of both DOM for TC. Fourier transform infrared spectroscopy result showed that both hydrophilic and hydrophobic DOM could reduce C=O in TC to C-O, and isothermal titration calorimetry result suggested that reactions of both DOM with TC were exothermic (△H < 0) and spontaneous (△G < 0). Thereinto, the reaction constant (Ka) of TC reacting with hydrophobic DOM (Ka=9.70) was higher than that with hydrophilic DOM (Ka=8.93), indicating hydrophobic DOM with more chemical binding sites and accessible fractions for TC. The present study suggests that DOM, especially the hydrophobic DOM, is an important consideration in the environmental impact assessment of antibiotics.