Targeted Nonlethal Disruption Of Ultraviolet Photoprotection In Microcystis Aeruginosa

Global warming and nutrient pollution stimulate rapid surface growth of cyanobacteria, forming algal blooms that kill organisms through oxygen depletion and lethal toxins. Mycosporine-like amino acids (MAAs) are species-specific cellular sunscreens for cyanobacteria, enabling absorption of ultraviolet radiation to allow surface clumping. Shinorine is a MAA synthesized by Microcystis aeruginosa, a hardy, wide-ranging cyanobacterium. Glyphosate (N-(phosphonomethyl)glycine), a phosphonate herbicide, inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase), a pathway molecule for shinorine formation. The hypothesis was that application of doses of glyphosate to M. aeruginosa at expected sublethal levels would result in measurable changes in growth and shinorine concentrations under optimal environmental conditions.  

M. aeruginosa B2667 was cultured at 28C under mild UV light.  Samples were filtered, frozen, and extracted for HPLC analysis to identify MAAs.  Two treatments with glyphosate at expected nonlethal concentrations (100 μM and 10 μM) and a control (0 μM) in three replications were tested. The MAA analysis indicated a decline in shinorine associated with the treatments, with a larger effect at 10 μM. The treated cultures exhibited no change in growth rates, tracked by daily fluorometer readings, but a greater range in cell size distribution than the control. Cellular concentrations of shinorine were significantly lower in the gylphosate-treated cultures than in the untreated control.  These results indicated that cell uniformity and UV protection - factors that enable cyanobacteria to form harmful algal blooms (HAB) – may be affected by low levels of glyphosate without altering the life cycle of the organism.  The current research is a step toward chemical control systems that manipulate cellular processes and shows great promise for developing effective, highly organism-specific HAB control.