Transcriptomic mechanisms for the promotion of cyanobacterial growth against eukaryotic microalgae by a ternary...

This study evaluated the responses of a mixed culture of two cyanobacterial species (Microcystis aeruginosa and Synechocystis sp.) and two eukaryotic microalgal species (Raphidocelis subcapitata and Tetradesmus obliquus) to a mixture of three frequently detected antibiotics (tetracycline, ciprofloxacin and sulfamethoxazole) at environmentally relevant exposure doses of 60-300 ng/L. Mixed antibiotics selectively stimulated (p < 0.05) the growth and photosynthetic activity as well as generated transcriptomic responses in cyanobacteria without disrupting co-existing eukaryotic microalgae. Mixed antibiotics stimulated the growth of M. aeruginosa through the regulation of genes related to ribosome, photosynthesis, redox homeostasis, quorum sensing and nutrient metabolism. The proportion of M. aeruginosa among the four phytoplankton species in the mixed-culture system was increased from 33% to 38-44% under antibiotic exposure, which promoted the dominance of M. aeruginosa. Up-regulation of carbon catabolism-related genes contributed to the increased growth of Synechocystis sp. under antibiotic exposure. Since the antibiotic-stimulated growth rate of Synechocystis sp. was still lower than that of M. aeruginosa, the proportion of Synechocystis sp. in the mixed-culture system remained stable. Synechocystis sp. was less adaptive to antibiotic exposure than M. aeruginosa, due to a lower number of up-regulated ribosomal genes and photosynthesis-related genes. Antibiotic exposure reduced the proportions of two eukaryotic microalgal species in the mixed-culture system through a selective promotion of cyanobacterial competitiveness against eukaryotic microalgae, which may facilitate the formation of cyanobacteria bloom.