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. Cryptomonascurvata 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.
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