Monitoring of Cyanobacterial Levels in Irrigation Retention Ponds Reveals Reduced Potential for Mycrocistin Toxin Production in Municipal Reclaimed Wastewater.

Increasing demands on limited water resources have made wastewater reclamation for municipal irrigation an attractive option for extending water supplies throughout the world. However, there remain health concerns about the potential risks of human contact with reclaimed water used for irrigating public areas. Largely unexplored is the potential for formation of toxin-producing algal blooms in reclaimed water retention ponds. This issue is of particular public health concern as the neurotoxic microcystins produced by a subset of cyanobacteria have been the cause of human and animal sickness, and even death, in recent years. Over one year, we utilized real-time PCR to quantify 16S rDNA markers for: (1) total cyanobacteria (CYAN); (2) Microcystis spp. (MICR), a cyanobacterial group commonly associated with toxin production; and (3) microcystin toxin synthesase genes of Microcystis aeruginosa (mycD). Monitoring was performed in a reclaimed water retention pond (RWP) and a nearby pond filled with groundwater (GWP) in Maricopa, Arizona. Over 12 months, CYAN markers in the RWP (averaging 1.3 x 10⁸ 100 mL^-1) were higher than the GWP (6.6 x 10⁷ 100 mL^-1). During the same period, however, MICR markers were higher in the GWP (1.2 x 10⁷ 100 mL^-1) than in the RWP (6.0 x 10⁶), while mcyD synthesase gene markers showed the same trend (3.0 x 10⁵ and 3.4 x 10⁴ markers 100 mL^-1 in the GWP and RWP, respectively). While mcyD markers were equal in the two ponds during the summer, toxin markers averaged 7.9 x 10³ 100 mL^-1 in the GWP but were below detection limits in the RWP in winter. We hypothesize that higher levels of heavy metals in the reclaimed water may be inhibiting development of toxin-producing cyanobacteria in the reclaimed water pond. Analyses of RWP and GWP samples for heavy metal concentrations and other chemical parameters are currently underway to test this hypothesis. Identification of factors regulating microtoxin biosynthesis could aid in development of water management strategies to deter toxin production in natural and engineered water bodies.