Grazing impact on the cyanobacterium Microcystis aeruginosa by the heterotrophic flagellate Collodictyon triciliatum in an experimental pond

We estimated the grazing impact of the heterotrophic flagellate Collodictyon triciliatum on the harmful, bloom-forming cyanobacterium Microcystis aeruginosa in an experimental pond during a Microcystis bloom from summer to winter in 2010. For these experiments, we calculated the grazing rates from the digestion rate of C. triciliatum and its food vacuole contents. During the study period, M. aeruginosa exhibited one bloom event with a maximum density of 1.1 × 10[5] cells ml[−1]. The cell density of C. triciliatum fluctuated from below the detection limit to 291 cells ml[−1]. The number of M. aeruginosa cells ingested by C. triciliatum food vacuoles ranged between 0.4 and 10.8 cells flagellate[−1], and the digestion rate of C. triciliatum at 25 °C was 0.73 % cell contents min[−1]. The grazing rate of C. triciliatum on the M. aeruginosa prey was 0.2–6.9 cells flagellate[−1] h[−1], and its grazing impact was 0.0–25.3 % standing stock day[−1]. The functional response of C. triciliatum to the M. aeruginosa prey followed the Michaelis–Menten model of significance (r[2] = 0.873, p < 0.001) in our experimental systems, in which the prey concentration varied from 1.0 × 10[4] to 2.1 × 10[6] cells ml[−1]. The maximum grazing rate was 6.2 prey cells grazer[−1] h[−1], and the half-saturation constant was 1.2 × 10[5] cells ml[−1]. We present evidence that C. triciliatum grazing explained the remarkable decrease in M. aeruginosa cell density in the pond. The present study is the first demonstration of the high potential of protistan grazing on M. aeruginosa to reduce cyanobacterial blooms

Detection and identification of potentially toxic cyanobacteria: Ubiquitous distribution of Microcystis aeruginosa and Cuspidothrix issatschenkoi in Japanese lakes

We studied the frequency and composition of potentially toxic cyanobacteria in 30 samples from 25 Japanese lakes using automated rRNA intergenic spacer analysis (ARISA) and eight primer sets for nodularin, microcystin, cylindrospermopsin, anatoxin-a, and saxitoxin synthetase genes. Potential microcystin- and anatoxin-a-producers were detected in 25 and 7 samples, respectively. Cylindrospermopsin- or saxitoxin-producers were not detected. PCR and clone library analyses indicated that Microcystis was the sole microcystin-producing genus. Moreover, potential microcystin-producing Microcystis strains were detected in 25 of 26 samples which included Microcystis ARISA fragments, suggesting that toxic Microcystis is ubiquitous. Potential anatoxin-a-producers detected in the samples were estimated to be Cuspidothrix issatschenkoi (synonym: Aphanizomenon issatschenkoi) from clone library analyses of the anatoxin-a biosynthesis gene cluster and 16S–23S rDNA intergenic spacer region. Anatoxin-a-producing strains of C. issatschenkoi are known from two lakes in New Zealand and Germany. In contrast, C. issatschenkoi was the second most common toxic cyanobacteria in our study. This is the first report of potential anatoxin-a-producing C. issatschenkoi strains in Asia

Genotypic composition and the relationship between genotypic composition and geographical proximity of the cyanobacterium Microcystis aeruginosa in western Japan

Microcystis aeruginosa is one of the bloom-forming harmful algae in freshwater ecosystems. We genetically characterized Microcystis populations during bloom-forming periods in various reservoirs, lakes, and ponds in Japan during 2009. Using phylogenetic analysis, we evaluated the relationship between current genotype expansions and geographic location within western Japan and intraspecific variation. Microcystis aeruginosa colonies were isolated at 15 sites and were analyzed by sequencing the 16S-23S internal transcribed spacer (ITS) region of the ribosomal operon, and the potential to produce toxins was assessed by PCR-based detection of the microcystin synthetase gene mcyG. In total, 171 colonies were separated into 41 genotypes. The highest genotypic composition was detected in the south basin of Lake Biwa and the lowest in Lagoon Iba. Cluster analysis indicated no obvious association between genotypic composition and geographic distance. Thus, clear genetic differentiation accompanied by geographic origins was not found in western Japan. The resulting neighbor-joining tree revealed 3 clusters, 2 of which contained strains that showed both nonamplification and amplification of the mcyG gene

Body size--dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta

We examined whether gender role in the simultaneous hermaphrodite freshwater snail, Physa acuta, is determined by relative body size in a manner predicted by the size-advantage model. We observed the body-size combinations of pairs in the laboratory by using field-collected populations. Smaller individuals tended to play the "male" role (sperm donor), and larger snails the "female" (sperm recipient). Next, we analyzed the mating behaviors involved in gender-role decision in snail pairs of three different body-size combinations, using "large" and "small" snails. Smaller snails were more likely to approach the partner as a male in different-size combination (large/small), whereas frequent initial approaches as a male and rejection behavior as a female were observed in the large/large combination. Third, we examined the body size preference when a snail can freely choose the partner from two other individuals of different body sizes (large/large/small or large/small/small). Small individuals had a significant tendency to act as the male and positively selected large snails as the female partner in both triple combinations. However, the large individual acted as both the male and the female with nearly equal frequency. In the size-differing pairings, copulations occurred after fewer male approaches and fewer rejections than in pairings involving two large snails, suggesting that body size difference is one of the behavioral solutions in gender conflict. Clear gender-role switching associated with body size was not seen. Smaller snails thus have a tendency to play the male role more frequently but adopt both gender roles when their body size is sufficiently large. Copyright 2004.freshwater snails; gender role; Physa acuta; simultaneous hermaphrodites; size advantage model

Identification of species and genotypic compositions of Cryptomonas (Cryptophyceae) populations in the eutrophic Lake Hira, Japan, using single-cell PCR

Single-cell PCR and gene sequencing were conducted to evaluate species and genotypic compositions of Cryptomonas in the eutrophic Lake Hira, Japan. We determined the sequences of nuclear internal transcribed spacer 2 region from single Cryptomonas cells with a high success rate (83.3–97.9 %), excluding one case (56.3 %). A total of 325 sequences were obtained over eight sampling days from May 28, 2012, to October 3, 2012, and phylogenetic analysis indicated that all sequences were divided into six groups. Four groups were clustered together with known sequences of C. curvata, C. marssonii, C. pyrenoidifera or C. tetrapyrenoidosa, although the sequences of the other two groups did not show high similarity to known Cryptomonas species. Cryptomonas curvata dominated during the study period (45–98 %), and unidentified Cryptomonas species (group 2) became dominant at high water temperatures. The genotypic composition of C. curvata also varied temporarily, suggesting that the genotypic composition of Cryptomonas was susceptible to environmental changes. These results indicated that single-cell PCR can be used to analyze the species composition and ecology of Cryptomonas