Inactivation of Heterosigma Akashiwo in Marine Water by UV-Activated Periodate: Efficacy and Mechanism

The combination of ultraviolet (UV) and periodate (PI) is a promising advanced oxidation process. We investigated the inactivation of a typical harmful algae species, Heterosigma akashiwo by UV/PI treatment and explored the inactivation mechanism at the cellular and molecular levels. The inactivation efficacy of H. akashiwo by UV/PI and UV treatment alone was 9.21-ln and 1.23-ln within 37.5 min, respectively. Hydroxyl radicals (•OH) and singlet oxygen (1O2) contributed to H. akashiwo inactivation in the UV/PI system. Obvious destruction of the cell structure observed by transmission electron microscopy and the increases in extracellular DNA levels indicated membrane damage by the reactive species. The activity changes in the antioxidant enzymes and the decreases in ATP contents of H. akashiwo after UV/PI treatment were both more severe than that after UV treatment. The results indicated that the antioxidant defense system and the mitochondria of H. akashiwo cells were disrupted. The photosynthesis was also affected as both of the chlorophyll-a content and the maximum dark-adapted photochemical efficiency (Fv/Fm) of H. akashiwo decreased obviously. The transcriptomics analysis proved that UV/PI treatment caused abnormal energy metabolism and cell function. Moreover, the down regulation of gene expression enriched in the phagocytic pathway indicated that the cell was severely damaged and inactivated under oxidative stress.