Eukaryotic and prokaryotic microbiome profiles of incoming seawater treated with electrolytic mixed oxidants used in a land-based fish farm

Abstract The ability to control pathogens in fish farm influent water is becoming ever more important, and electrolytic mixed oxidants (EMO) treatment is known as one of the most useful methods for disinfecting large amounts of water. However, the dearth of information about the susceptibility of individual microorganisms makes it difficult to judge the optimal EMO conditions for different types of microorganisms. Therefore, in this study, we performed microbial counts and microbiome analyses of influent seawater samples with and without EMO treatment to determine the microbicidal effects of EMO treatment. Approximately 10 L of normal and EMO-treated seawater were concentrated into a volume of ~1.5 mL, and viable bacterial count and semi-quantification of eukaryotes were performed. In addition, prokaryotic and eukaryotic microbiome were profiled by sequencing the V3-V4 and V4 regions of the 16S and 18S rRNA genes, respectively. The number of culturable prokaryotes decreased from 4.19 × 105 CFU L−1 to 2.5 × 103 CFU L−1, representing a greater than 100-fold reduction in association with EMO treatment. Likewise, EMO-treated seawater showed a relatively low concentration of eukaryotes. The results of microbiome analyses showed that α-diversity decreased in both prokaryotes and eukaryotes after treatment, but the change in prokaryotes was more significant. In particular, the number of Bacteroidia, which was the second most common phylum before EMO treatment, diminished greatly, and the relative concentration of the micro-aerophilic Campylobacteria phylum increased in its place. Among eukaryotes, plant and phytoplankton cells were more susceptible to EMO treatment than zooplankton. This finding may be due to the presence of phytoplankton, which can demonstrate hypersensitive responses to reactive oxygen species via chloroplast factors and which may be particularly susceptible to other types of electrolytic by-products such as electrolytic chlorine. The results of this study clearly show which types of prokaryotic and eukaryotic organisms in seawater off the coast of Jeju Island are most susceptible and resistant to EMO treatment.