Cell lysis and detoxification of cyanotoxins using a novel combination of microbubble generation and plasma microreactor technology for ozonation.

Abstract

There has been a steady rise in the global incidences of algal blooms and, worryingly, there is increasing evidence that changes in the global climate are leading to a shift toward cyanobacterial blooms. Many cyanobacterial genera are harmful, producing several potent toxins including microcystins, for which there are over 90 described analogues. There are a wide range of negative effects associated with these toxins, including gastroenteritis, cytotoxicity, hepatotoxicity and neurotoxicity. Although a variety of oxidation-based treatment methods have been described, ozonation and advanced oxidation are acknowledged to be the most effective, as they readily oxidise microcystins into non-toxic degradation products. However, scaling up most ozonation technologies poses various challenges including high costs and sub-optimum efficiencies; it is therefore necessary to develop a low-cost and scalable ozonation technology. In this study, we designed a low-temperature plasma dielectric barrier discharge (DBD) reactor with an incorporated fluidic oscillator for microbubble delivery of ozone. Both technologies have the potential to drastically reduce the costs of ozonation at scale. Mass spectrometry analysis revealed very rapid ( < 2 minutes) destruction of two pure microcystins (MC-LR and MC-RR), together with removal of by-products even at low flow rate (1 L per minute), where bubble size was 0.56-0.6 mm and the ozone concentration within the liquid was 20 ppm. Toxicity levels were calculated through protein phosphatase inhibition assays, indicating a loss of toxicity as well as confirming that the by-products were also non-toxic. Finally, treatment of whole Microcystis aeruginosa cells showed that even at these very low ozone levels, cells can be killed and toxins (MC-LR and Desmethyl MC-LR) removed. Little change was observed in the first 20 minutes of treatment, followed by a rapid increase in extracellular toxins indicating cell lysis, with the most significant release at the higher 3 L per minute flow rate compared to 1 L per minute. This lab-scale investigation demonstrates the potential of the novel plasma micro reactor, with applications for in situ treatment of harmful algal blooms and cyanotoxins.