Understanding the environmental roles of herbicides on cyanobacteria, cyanotoxins, and cyanoHABs

Agrochemicals such as herbicides change the physical and chemical conditions of aquatic ecosystems and alter the community structure and dynamics of phytoplankton. Cyanobacteria are photosynthetic organisms found at the base of aquatic food chains. When cyanobacteria form blooms and produce toxins, they harm humans and the environment. Herbicides contaminate the aquatic environment when they are leached and transported via surface runoff from farms and industries. In this review, we show that these compounds have different mechanisms of action, but at high concentrations, they cause oxidative stress, interfere with the normal functioning of enzymes, and change the metabolic profile of microalgae and cyanobacteria. This paper demonstrates that at environmentally relevant concentrations, some herbicides facilitate the formation of cyanobacterial harmful algal blooms (cyanoHABs). The formation of blooms is driven by the tolerance of cyanobacteria to herbicides, where some of these compounds are degraded and converted into non-toxic forms. The degradation by-products are also used as a source of nutrients to support cyanobacterial growth. This adaptation sometimes leads to higher concentrations of bioactive compounds such as cyanotoxins in the aquatic environment. The increased levels of cyanotoxins and herbicides in water bodies can trigger a cascading toxicological effect on non-targeted organisms and the aquatic food chain. Despite the evidence confirming herbicides influence the growth of cyanobacteria and alter the structure of the phytoplankton community toward the formation of cyanoHABs, there is a lot that remains to be done to fully understand their impact on these organisms.