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Three individuals of Bdelloid rotifer (J1, J2 and J3) were isolated from a MBR system in Nagasaki University and one individual of rotifer (J4) in the original seed sludge collected from a wastewater treatment plant for the MBR was isolated. The four rotifer species were able to proliferate in toxic Microcystis cell suspension. The partial sequence of 18S rRNA gene of each isolated rotifer was determined using In-fusion cloning and searched by BLAST. The gene of the four rotifers J1, J2, J3 and J4 showed the same sequence, then the consensus sequence was in the branch of Bdelloid rotifers in the phylogenetic tree. Furthermore, a specific Bdelloid forward primer 55F and reverse primer 395R for real-time PCR was designed based on the consensus sequence for quantitative researches on the Bdelloid rotifers population. We succeeded to quantify the population of a Bdelloid rotifer cultured in toxic Microcystis cell suspension using the new designed primer pairs.

Toxic cyanobacterial blooms frequently develop in eutrophic freshwater bodies worldwide. Microcystis species produce microcystins (MCs) as a cyanotoxin. Certain bacteria that harbor the mlr gene cluster, especially mlrA, are capable of degrading MCs. However, MC-degrading bacteria may possess or lack mlr genes (mlr+ and mlr− genotypes, respectively). In this study, we investigated the genotype that predominantly contributes to biodegradation and cyanobacterial predator community structure with change in total MC concentration in an aquatic environment. The 2 genotypes coexisted but mlr+ predominated, as indicated by the negative correlation between mlrA gene copy abundance and total MC concentration. At the highest MC concentrations, predation pressure by Phyllopoda, Copepoda, and Monogononta (rotifers) was reduced; thus, MCs may be toxic to cyanobacterial predators. The results suggest that cooperation between MC-degrading bacteria and predators may reduce Microcystis abundance and MC concentration.