Antibiotics promoted the recovery of Microcystis aeruginosa after UV-B radiation at cellular and proteomic levels.

Abstract

Elevated UV-B radiation due to ozone layer depletion may prevent the growth of bloom-forming cyanobacteria in aquatic environments, while antibiotic contaminants may cause effects opposite to that of UV-B due to hormesis. This study investigated the influence of a quaternary antibiotic mixture on Microcystis aeruginosa after UV-B radiation through a 15-day exposure test. UV-B radiation extended the lag phase of M. aeruginosa at doses of 600 and 900mJ/cm2, and significantly (p<0.05) reduced the growth rate and the Fv/Fm value at doses of 300-900mJ/cm2. Although UV-B radiation significantly (p<0.05) stimulated the microcystin production ability in each cyanobacterial cell, the total microcystin concentration still significantly (p<0.05) decreased due to the reduction of cell density. Mixed antibiotics and UV-B regulated the proteomic expression profile of M. aeruginosa in different manners. UV-B radiation upregulated 19 proteins and downregulated 49 proteins in M. aeruginosa, while mixed antibiotics upregulated 45 proteins and downregulated 25 proteins in UV-B treated cells. Mixed antibiotics significantly (p<0.05) stimulated growth and photosynthesis, increased cell density and microcystin concentration, and reduced oxidative stress in UV-B treated cells through the upregulation of proteins involved in photosynthesis, biosynthesis, cell division, oxidation-reduction, gene expression and microcystin synthesis. This study verified the hypothesis that antibiotics accelerated the recovery of M. aeruginosa from UV-B induced damage. A safe threshold of 20ng/L was suggested for mixed antibiotics (5ng/L for each antibiotic), in order to eliminate the stimulatory effects of antibiotics on bloom-forming cyanobacteria.