Environmental-level antibiotics disrupt Microcystis stoichiometry: An overlooked risk in the context of cyanobacterial harmful algal blooms

At trace levels (nanograms to micrograms per liter, ng L^-1 – μg L^-1), antibiotics exert stimulatory effects on cyanobacteria, potentially posing a threat to aquatic ecosystems. Here, environmentally relevant concentrations of antibiotics were firstly observed to disrupt the stoichiometry of cyanobacterium Microcystis (i.e., the quota and allocation patterns of carbon (C), nitrogen (N), and phosphorus (P)). In scenarios where the doses of antibiotics ranged from 50 to 200 ng L^-1, cyanobacteria displayed a complex response – a reduction of their intracellular P pools by 22% to 24% and an adjustment of elemental allocation patterns, such as by a 51% increase in the ratio of C:P. Furthermore, at these doses, antibiotics significantly impacted the physiology of cyanobacteria, resulting in increased production of extracellular polymeric substances (EPS), a substantial rise in maximum biomass (169% to 219% increase), and an enlargement of colony size (22% to 37% increase). Transcriptomic analyses revealed that these changes were linked to the downregulation of ABC transporters and the upregulation of photosynthetic and ribosomal processes. This research offers novel insights into the hormesis effects on cyanobacteria and bloom formation, highlighting the necessity of using a multidimensional evaluation framework that integrates comprehensive bloom factors (biomass, colony size, and EPS) and elemental profiles (C, N, and P quotas and ratios) for accurate assessment.