Juglone and Other Biogenic Quinones Differentially Inhibit Cyanobacterial Growth and Could Be Used to Help Maintain Monospecificity of Microalgae Cultures

Raceway ponds would allow the sustainable production of algal biomass because of their lower cost. However, for successful cultivation, the target organism needs to prevail despite unavoidable contamination by environmental strains. The development of efficient methods to control cyanobacterial proliferation is thus highly desirable. With the aim to identify new cyanobactericidal substances, a set of natural compounds was screened for the ability to inhibit the growth of a model cyanobacterial strain, Synechococcus elongatus PCC 6301. Three compounds, namely hydroquinone, juglone and plumbagin, were found to be active in the 10-6 to 10-4 M range. Activity was confirmed on a panel of 10 other cyanobacteria that showed different sensitivity, with concentrations causing 50% growth inhibition varying up to 2 orders of magnitude. Co-cultivation experiments showed that the growth of Microcystis aeruginosa PCC 7941 was almost completely suppressed at quinone concentrations at which that of Tolypothrix PCC 7601 was substantially unaffected. Juglone and plumbagin in the micromolar range also exerted toxic effects on eukaryotic microalgae, bacteria and yeast, whereas the growth of higher plants was affected only at higher concentrations. In the case of juglone, activity was lost with time after being dissolved, allowing water discharge/recycling. The results point at the aromatic 1.4-quinone/diol ring as a lead moiety for the development of chemicals to help maintaining monospecificity of microalgae cultures.