Invasion of fish parasite Prosorhynchoides ozakii (Trematoda: Bucephalidae) into Lake Kasumigaura and surrounding rivers of eastern Japan.

In 2019 to 2021, the golden mussel Limnoperna fortunei and several freshwater fishes were sampled from 22 sites of the Tone River system including Lake Kasumigaura, Honshu, Japan, to examine the invasion of bucephalid trematodes. The parasite species identification was performed by morphological observation and DNA barcoding based on the sequences of nuclear 28S rDNA and mitochondrial cytochrome c oxidase subunit 1 (cox1). A total of 1719 mussels were collected from 10 sites, and trematode-infected mussels were detected from 8 sites with prevalences between 0.3 and 42.9%. The sporocysts and cercariae were identified as Prosorhynchoides ozakii, a newly introduced species in the river system. A total of 700 fish individuals belonging to 24 species were collected from 15 sites. Two species of catfishes (Silurus asotus and Ictalurus punctatus) harbored mature or immature adults of Pr. ozakii in the intestine with prevalences between 8.3 and 20% including both host species. The metacercariae of Pr. ozakii were found from the fins and epidermis of 13 fish species from 10 sites (prevalence 4.8-100%). Fishes were heavily infected with metacercariae in fins, which were surrounded by the infiltration of hemocytes and rodlet cells. A population genetic analysis of Pr. ozakii did not show an obvious bottleneck, suggesting the possibility that the parasite was intentionally and repeatedly introduced into the river system.

Radiocesium accumulation in Lake Kasumigaura by riverine input and migration following the Fukushima Dai-ichi nuclear power plant accident.

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 (137Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the 137Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The 137Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating 137Cs migration. However, the rate of decrease subsequently slowed due to the 137Cs supply from the river. The 137Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate 137Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in 137Cs concentrations will be smaller. The 137Cs concentration in the Koise River will continue to decrease; thus, the difference in the 137Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of 137Cs in the entire lake were 3.72 × 1012 Bq in December 2012 and 4.18 × 1012 Bq in August 2016. The accumulated amount of 137Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate 137Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of 137Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes.

Genomic Characteristics of the Toxic Bloom-Forming Cyanobacterium Microcystis aeruginosa NIES-102.

Microcystis aeruginosa, a bloom-forming cyanobacterium distributed mainly in freshwater environments, can be divided into at least 12 groups (A-K and X) based on multi-locus phylogenetic analyses. In this study, we characterized the genome of microcystin-producing M. aeruginosa NIES-102, assigned to group A, isolated from Lake Kasumigaura, Japan. The complete genome sequence of M. aeruginosa NIES-102 comprised a 5.87-Mbp circular chromosome containing 5,330 coding sequences. The genome was the largest among all sequenced genomes for the species. In a comparison with the genome of M. aeruginosa NIES-843, which belongs to the same group, the microcystin biosynthetic gene cluster and CRISPR-Cas locus were highly similar. A synteny analysis revealed small-scale rearrangements between the two genomes. Genes encoding transposases were more abundant in these two genomes than in other Microcystis genomes. Our results improve our understanding of structural genomic changes and adaptation to a changing environment in the species.

Vertical/spatial movement and accumulation of 137Cs in a shallow lake in the initial phase after the Fukushima Daiichi nuclear power plant accident.

Movement of 137Cs in the bottom sediment of a shallow lake was investigated by collecting columnar sediment cores from the entire area once a year, and also from three representative points every 3 months over 3 years from the 137Cs fallout in March 2011. The 137Cs inventory in the sediment increased remarkably near the mouths of urban rivers; however, the distribution of the 137Cs inventory did not change remarkably in most areas, and horizontally slow downstream movement of 137Cs was observed.