| Antibiotics exist widely in environment because of their extensive use, but have received comparatively little attention as pollutants. Unlike many other pollutants, they have a direct biological action on microbes and interactions between antibiotics and microorganisms have attracted lots of interests. It has been reported that subinhibitory concentrations (SICs) of antibiotics, which have been detected in various environments, impact the physiology of microorganisms. In the presence of antibiotics, microbes evolved various responses, such as different colonization, motility, stress response or biofilm formation. Nowadays, most research focused on the pathogens and model microorganisms, but few studies investigated the ecological effects of antibiotics at SICs in environments. Cyanobacteria are typical photosynthetic microorganisms with grand ecological importance. On one hand, cyanobacteria are the main participants in local and global carbon cycle; and on the other hand, they dominate in the algal blooms in eutrophic ecosystems. It would be scientifically and practically important to understand how cyanobacteria response to antibiotics at SICs in environments.The main contents and results are as follows:(1) Two typical antibiotics, kanamycin and tobramycin, were selected and their subsequent effects on the aggregation of cyanobacterium Synechococcus elongates were investigated. The results showed that when 0.1 μg/mL antibiotics were added along with cultivation, the aggregation efficiency increased by 30%～40%, and the enhancement could also be identified with shorter contacting time at appropriate antibiotics. Meanwhile, the antibiotics would trigger variation in surface properties, especially zeta potentials and hydrophobicity, as well as morphology. These variations led to an enhanced aggregation. The role of extracellular polymeric substances (EPS) were as well discussed and the results indicated a dominant role of extracellular proteins (PN). Antibiotics triggered various secretion of PN, which had an effect on bacterial surface properties, thus inducing aggregation. We then changed the pH and ions of the culture to further simulate the natural aquatic system and found that at low pH or concentrations of ions, aggregation of S. elongates had a closer relationship with antibiotics at SICs.(2) Overgrowth of Microcystis aeruginosa is responsible for the algal bloom in aqueous ecosystems. In this study, we investigated the effect of antibiotics at different SICs on the aggregation of M. aeruginosa and got a 14.9% increase of aggregation at 0.02 μg/mL kanamycin, or 25.4% increase at 0.01 μg/mL tobramycin. Research on the surface properties after induction showed tiny modifications. When we shortened the contacting time, changes of aggregation efficiency and surface properties were even slighter. This evidenced more about the role of surface properties on aggregation and different response were showed among different cyanobacteria when they met antibiotics at SICs.(3) As aggregation plays a vital role in biofilm formation, considering the results in chapter 2, we then investigated the biofilm formation of S. elongates when they encountered kanamycin at SICs. Ecological role was also studied via transcriptional analysis. It was obvious that 0.05～0.1 μg/mL antibiotic promoted the biofilm formation, and studies of EPS secretion showed the important role of PN. Besides, the modified morphology also accounted for the enhanced biofilm formation. Through transcriptional analysis, ribulose-1.5 biphosphate carboxylase-oxygenase (RuBisCo) and carbonic anhydrases (CA) showed different expression with the existence of kanamycin at SICs. Combined with the changes of biofilm formation and enzyme expression, cyanobacteria-mediated calcification would be enhanced, which could cause an increase of carbon dioxide (CO2), thus having a global ecological effect. |
PDF Full Text Request