Phosphonate degradation by Spirulina strains: Cyanobacterial biofilters for the removal of anticorrosive polyphosphonates from wastewater

The ability of Spirulina spp. to metabolize the recalcitrant xenobiotic Dequest 2054® [hexamethylenediamine-N,N,N′,N′-tetrakis(methylphosphonic acid)], a CaSO4 inhibitor used for boiler treatment and reverse osmosis desalination, was investigated. The compound served as sole source of phosphorus, but not of nitrogen, for cyanobacterial growth. In vivo utilization was followed by 31P NMR analysis. The disappearance of the polyphosphonate proceeded only with actively dividing cells, and no release of inorganic phosphate was evident. However, no difference was found between P-starved and P-fed cultures. Maximal utilization reached 1.0 ± 0.2 mmol l−1, corresponding to 0.56 ± 0.11 mmol g−1 dry biomass, thus residual amounts were still present in the exhausted medium when the compound was supplied at higher initial concentrations. At low substrate levels metabolism rates were lower, suggesting that a concentration-driven uptake may represent a limiting step during the biodegradation process. The compound was not retained by biocolumns made with immobilized cyanobacterial cells, either alive or dead. A lab-scale pilot plant, consisting of a series of sequentially connected vessels containing an actively proliferating algal culture, was built and tested for wastewater treatment. Results showed 50% removal of the polyphosphonate added to an initial concentration of 2.5 mM. Although further optimization will be required, data strengthen the possibility of using cyanobacterial strains for bioremediation purposes.