Growth inhibition and beyond: hormetic and environmentally relevant effects of pharmaceuticals on cosmopolitan cyanobacteria.

Pharmaceutical residues are increasingly detected in aquatic environments as a result of continuous anthropogenic inputs. These compounds represent emerging contaminants capable of affecting primary producers, including cyanobacteria, which play key ecological roles in freshwater systems and are often responsible for harmful algal blooms. This review critically examines experimental studies evaluating the effects of pharmaceuticals on Synechococcus sp. and Microcystis aeruginosa - two cosmopolitan and widely investigated cyanobacteria. The available evidence indicates that pharmaceutical exposure can significantly influence cyanobacterial physiology in a species- and concentration-dependent manner. High concentrations of antibiotics typically inhibit cyanobacterial growth, whereas subinhibitory concentrations can induce hormesis, characterized by stimulated proliferation at low doses and inhibition at higher doses. Repeated exposure may additionally promote the development of antibiotic tolerance and resistance. Importantly, the bloom-forming species M. aeruginosa may respond to pharmaceutical-induced stress by increasing the production and release of microcystins even when growth inhibition is limited. Environmentally relevant concentrations of certain antibiotics, particularly fluoroquinolones, may therefore simultaneously stimulate cyanobacterial growth and toxin production, potentially altering microbial community structure and favouring bloom development. Because cyanobacteria in natural systems are exposed to complex contaminant mixtures, environmental risk assessment remains challenging. Further investigation of mixture toxicity is required to better evaluate the ecological implications of pharmaceutical pollution in aquatic ecosystems. Hormetic responses to pharmaceuticals require further investigation because they may trigger ecological effects whose long-term consequences remain uncertain.