To mitigate cyanobacterial blooms, the naphthoquinone derivative, NQ 2-0, which has selective algicidal activity against cyanobacteria, has been developed. However, due to a lack of information on its algicidal mechanisms, there are significant gaps in our understanding of how this substance is capable of selectively killing cyanobacteria. Here, we investigated the selective algicidal mechanisms of NQ 2-0 using target (Microcystis aeruginosa) and non-target (Cyclotella sp. and Selenastrum capricornutum) species. NQ 2-0 showed selective algicidal activity against only M. aeruginosa, and this activity was strongly light-dependent. This NQ compound has selectively reduced the oxygen evolution rate and photosystem II (PSII) efficiency of M. aeruginosa throughout blocking electron transfer from the photosynthetic electron transport system, and significantly (p ≤ 0.05) increased levels of reactive oxygen species (ROS), resulting in membrane damage through lipid peroxidation. In ultrastructural observations, thylakoid membranes were disintegrated within 12 h after NQ 2-0 treatment, and cytoplasmic vacuolation and disintegrated cellular membrane were observed at 24 h. These findings suggest that increased ROS levels following NQ 2-0 treatment may induce cell death. Interestingly, compared to non-target eukaryotic cells, M. aeruginosa showed relatively late antioxidant response to reduce the increased ROS level, this may enhance algicidal activity against this cyanobacterium.

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The purpose of this investigation was to assess the possible use of algicidal bacteria in conjunction with an immobilization technique for efficient termination of natural blooms of Stephanodiscus concomitant with minimization of adverse effects caused by a single application of bacteria. The performance of Pseudomonas fluorescens cells immobilized on cellulose sponges (CIS) was compared with that of freely suspended cells (FSC) of the organism at low water temperature (WT) of\10°C in co-cultures and natural microcosms. In the co-cultures, CIS resulted in more effective lysis of Stephanodiscus, irrespective of similar bacterivore (Spumella sp.) density, and significantly reduced the concentrations of nitrate and soluble reactive phosphorus (SRP), but not that of silicate, compared with FSC. In the microcosms containing natural freshwater, CIS reduced the densities of Stephanodiscus spp. and Aulacoseira spp. but had no effect on other phytoplankton. Compared with FSC, removal of nutrients by the CIS prevented secondary blooms caused by other phytoplankton. Our results indicate that the CIS affords effective protection of P. fluorescens from low WT and heterotrophs, and restrained regeneration of both SRP and nitrate. Thus, it was demonstrated that the CIS may be an attractive alternative to FSC for control of natural blooms.

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