Summary
Chlorella Beijerinck (Chlorellaceae, Trebouxiophyceae) strains from the collection of the National Institute for Environmental Studies (NIES) were characterized using gene sequence data. The misidentification of a number of strains was rectified. Chlorella vulgaris Beijerinck NIES‐2173 was reclassified as C. sorokiniana Shihira et Krauss. Chlorella sp. NIES‐2171 was described as a new species in the genus Micractinium Fresinius, M. inermum Hoshina et Fujiwara. Chlorella sorokiniana NIES‐2167 and Chlorella sp. NIES‐2330 were found to be phylogenetically related to Didymogenes Schmidle. We propose these two strains be transferred to the genus Didymogenes and given new names: D. sphaerica Hoshina et Fujiwara and D. soliella Hoshina et Fujiwara. Taxonomic decisions were primarily based on small subunit‐internal transcribed spacer ribosomal DNA phylogeny for genus assignment and ITS2 sequence‐structure to determine species autonomy. Our findings suggest that this strategy is the most effective way to use the species concept among autosporic coccoids.
Summary
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.
The green ciliate, Paramecium bursaria, has evolved a mutualistic relationship with endosymbiotic green algae (photobionts). Under culture conditions, photobionts are usually unified (to be single species) within each P. bursaria strain. In most cases, the algal partners are restricted to either Chlorella variabilis or Micractinium reisseri (Chlorellaceae, Trebouxiophyceae). Both species are characterized by particular physiology and atypical group I intron insertions, although they are morphologically indistinguishable from each other or from other Chlorella-related species. Both algae are exclusive species that are viable only within P. bursaria cells, and therefore their symbiotic relationship can be considered persistent. In a few cases, the other algal species have been reported as P. bursaria photobionts. Namely, P. bursaria have occasionally replaced their photobiont partner. This paper introduces some P. bursaria strains that maintain more than one species of algae for a long period. This situation prompts speculations about flexibility of host-photobiont relationships, how P. bursaria replaced these photobionts, and the infection theory of the group I introns.
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