The curious case of cyanobacteria: a tale of light and darkness

Toxic algal bloom-forming cyanobacteria are a persistent problem globally for many aquatic environments. Their occurrence is attributed to eutrophication and rising temperatures due to climate change. The result of these blooms is often loss in biodiversity, economic impacts on tourism and fisheries, and risks to human and animal health. Of particular concern is the poorly understood interplay between viruses and toxic species that form blooms because viruses may exacerbate their harmful effects. Concurrently, cyanobacteria are also a source of bioactive compounds other than toxins, which makes them good candidates for drug discovery. We show that virus infection of the cyanobacterium Microcystis aeruginosa, results in as high as a 40-fold increase in the toxin microcystin two days post virus infection (dpi), and predict that microcystin levels may remain high in a body of water up to 7 dpi, long after water discoloration and cell lysis. This implicates viruses as major contributors to toxin release from cyanobacteria and emphasizes the importance of taking them into account in predictive models and in the assessment of water safety. We also show that bioactive compounds of M. aeruginosa inhibit and delay infection of single stranded DNA and single stranded RNA viruses. This highlights the potential of cyanobacteria as an excellent source for the discovery of novel antiviral compounds, and the ease with which screening for cyanobacterial antivirals can be achieved.