Growth inhibition of the toxic cyanobacterium Cylindrospermopsis raciborskii by extremely low-frequency electromagnetic fields

This study investigates the effects of extremely low-frequency electromagnetic fields (ELF-EMFs) on the growth and antioxidant defence enzymes of the toxic cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju. To determine resonance frequency of growth inhibition of C. raciborskii, cells were subjected to ELF square amplitude modulated waves (QAMW) with a range of frequencies (0.1, 0.3, 0.5, 0.7, 0.9 Hz) at single intensity of 100 V m–1 for 30 minutes. The results revealed that the highest growth inhibition of Cylindrospermopsis occurred upon exposure to 0.7 Hz QAMW for 30 min. ELF-EMF-exposed cultures exhibited a marked decrease in cell number, chlorophyll-a content and activity of antioxidant enzymes compared to control cultures, and this effect increased with the prolongation of exposure time. Moreover, ELF-EMF induced morphological changes in Cylindrospermopsis cells upon exposure to 0.7 Hz QAMW for 120 min, including shrinking and disintegration of cytoplasmic contents, and thickening of the cell wall. Changes in dielectric properties, as a measure of interaction of cellular constituents (e.g., plasma membrane, cell wall and cytoplasm), with electromagnetic fields were also observed for treated cells. Our results provide a new possibility for using ELF-EMFs to eliminate toxic cyanobacteria from drinking and recreational water sources.

Cooccurrence of Broad- and Narrow-Host-Range Viruses Infecting the Bloom-Forming Toxic Cyanobacterium Microcystis aeruginosa

ABSTRACT Viruses play important roles in regulating the abundance and composition of bacterial populations in aquatic ecosystems. The bloom-forming toxic cyanobacterium Microcystis aeruginosa is predicted to interact with diverse cyanoviruses, resulting in Microcystis population diversification. However, current knowledge of the genomes from these viruses and their infection programs is limited to those of Microcystis virus Ma-LMM01. Here, we performed a time series sampling at a small pond in Japan during a Microcystis bloom and then investigated the genomic information and transcriptional dynamics of Microcystis-interacting viruses using metagenomic and metatranscriptomic approaches. We identified 15 viral genomic fragments classified into three groups, groups I (including Ma-LMM01), II (high abundance and transcriptional activity), and III (new lineages). According to the phylogenetic distribution of Microcystis strains possessing spacers against each viral group, the group II-original viruses interacted with all three phylogenetically distinct Microcystis population types (phylotypes), whereas the groups I and III-original viruses interacted with only one or two phylotypes, indicating the cooccurrence of broad- (group II) and narrow (groups I and III)-host-range viruses in the bloom. These viral fragments showed the highest transcriptional levels during daytime regardless of their genomic differences. Interestingly, M. aeruginosa expressed antiviral defense genes in the environment, unlike what was seen with an Ma-LMM01 infection in a previous culture experiment. Given that broad-host-range viruses often induce antiviral responses within alternative hosts, our findings suggest that such antiviral responses might inhibit viral multiplication, mainly that of broad-host-range viruses like those in group II. IMPORTANCE The bloom-forming toxic cyanobacterium Microcystis aeruginosa is thought to have diversified its population through the interactions between host and viruses in antiviral defense systems. However, current knowledge of viral genomes and infection programs is limited to those of Microcystis virus Ma-LMM01, which was a narrow host range in which it can escape from the highly abundant host defense systems. Our metagenomic approaches unveiled the cooccurrence of narrow- and broad-host-range Microcystis viruses, which included fifteen viral genomic fragments from Microcystis blooms that were classified into three groups. Interestingly, Microcystis antiviral defense genes were expressed against viral infection in the environment, unlike what was seen in a culture experiment with Ma-LMM01. Given that viruses with a broad host range often induce antiviral responses within alternative hosts, our findings suggest that antiviral responses inhibit viral reproduction, especially that of broad-range viruses like those in group II. This paper augments our understanding of the interactions between M. aeruginosa and its viruses and fills an important knowledge gap.