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SUMMARY Species of the green algal genus Mychonastes have been recognized as ubiquitous ultrasmall components of freshwater communities worldwide, mainly based on molecular data. However, detailed taxonomic identification of Japanese members of Mychonastes has not yet been conducted. We taxonomically identified species of 13 Mychonastes strains in the Microbial Culture Collection at the National Institute for Environmental Studies originating from Japan based on molecular data and comparative morphology. The strains were classified into 10 species, including four new species, Mychonastes ariiae Nozaki sp. nov. (NIES‐2900), Mychonastes takamurae Nozaki sp. nov. (NIES‐2334, 2336), Mychonastes kasaiae Nozaki sp. nov. (NIES‐3743) and Mychonastes gigas Nozaki sp. nov. (NIES‐2340). Ultrastructural features of the cell walls and chloroplasts of these four species were consistent with the generic concept of Mychonastes. The four new species are distinguishable from other species of Mychonastes based on differences in nuclear ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequences and morphological features including the mucilaginous matrix surrounding the cells, mode of rupture of the mother cell wall during daughter cell release, formation of large colonies in liquid culture and colony morphology on agar plates. The evolution of Mychonastes species is discussed based on phylogenetic inferences obtained using a combined dataset for four chloroplast genes (psaB, rbcL, psbC and atpB) and rDNA ITS sequences. [ABSTRACT FROM AUTHOR]

Stentor polymorphus ( Müller) Ehrenberg ( Heterotrichea, Ciliophora) is a trumpet-form ciliate with endosymbiotic green algae (photobionts). Previous reports have indicated that this photobiont has a form typical of Chlorella Beijerinck ( Trebouxiophyceae), with one chloroplast and a distinct pyrenoid. We collected S. polymorphus from an artificial pond on the shore of Lake Biwa in Japan. Microscopic examination demonstrated that there was no pyrenoid in the photobiont, and thus we analyzed the cytological, genetic, and biochemical characteristics of these algal cells. Cultured photobionts were smaller than those in hospite. Phylogenetic analyses placed the photobiont within the genus Mychonastes Simpson and Van Valkenburg, a chlorophycean alga occurring in fresh/brackish waters that lacks pyrenoids. Mychonastes provided little photosynthate to its host; the quantity of carbohydrate amounted to only 2% of that provided to Paramecium bursaria ( Ehrenberg) Focker by its photobiont. [ABSTRACT FROM AUTHOR]