Characterization and algicidal activity of bacteria from the phycosphere of the harmful alga Karenia mikimotoi.

Harmful algal blooms (HABs) are detrimental to aquatic ecosystems; thus, economical and practical HAB control methods are needed. We analyzed a microbial community closely related to the alga Karenia mikimotoi, which has HABs that can be toxic to aquatic environments. We studied the relationship between algicidal bacteria and the microbial community of K. mikimotoi culture using culture-dependent and culture-independent methods. Bacterial strains Marinobacter sp. (O-7) and Pseudomonas sp. (D-2) were isolated from a K. mikimotoi seawater culture containing a mixed microbial community and determined to have algicidal activity. Both strains produced alga-lysing substances that were toxic to K. mikimotoi. The algicidal extracellular substances produced by D-2 were stable at temperatures ranging from - 80 to 120 °C but sensitive to strong acidic/alkaline conditions. The substances produced by O-7 were inactivated at high temperatures and strong alkaline conditions. Extracellular substances produced by O-7 and D-2 caused K. mikimotoi and Prorocentrum donghaiense cell lysis, but no changes or inhibitory effects occurred in two other chlorophyta groups. O-7 and D-2 exhibited significantly greater algicidal activity during the logarithmic growth phase of K. mikimotoi growth compared to the stationary phase. Culture-independent analysis of the microbial community in association with K. mikimotoi was made using Illumina MiSeq sequencing. Phylogenetic analysis showed that Proteobacteria was the dominant bacterial population in the phycosphere of K. mikimotoi, containing Marinobacter sp. and Pseudomonas sp.. The Marinobacter was abundant and accounted for 5.3% of the total. Our results indicate that certain bacterial species from K. mikimotoi culture might be effective for removal of HABs.