Studies on Endosymbiotic Algae in Green Paramecium from Japan

Abstract

Endosymbiotic algae of Japanese Paramecium bursaria were isolated and their axenicity was finally confirmed by DGGE method. Japanese symbiotic strains (F36-ZK and others) and American one, NC64A from ATCC, could use ammonia and organic nitrogen but not nitrate or nitrite as a sole nitrogen source. The measurement of nitrate (NR) reductase activity of NC64A and F36-ZK revealed the difference between these symbionts, i.e., weak NR activity was detected in NC64A cell extract but not in F36-ZK. Since Japanese symbionts preferred several amino acids for their growth, amino acid uptake were studied. F36-ZK was able to incorporate all amino acids through three amino acid transport systems, which considered to be driven by proton motive force. SSU rDNA analysis indicated four genetically discrete symbiotic algal groups depending on the strains of P. bursaria, of which three phylogenetically belonged to the Chlorellaceae, but one appeared at a different lineage in the Trebouxiophyceae. Species level analyses (ITS2 and other protein-coding genes) exposed genetic dissimilarity and polyphyletic relations of the three chlorellacean symbionts, which suggests all four types were independently captured in the evolutionary history of P. bursaria. The host cell-free extract enhanced symbiotic algal carbon fixation about 3-fold, however, release of photosynthate hardly changed. On the other hand, the release of photosynthate was obviously increased with acidic condition. Thus, the host seems to regulate the photosynthesis and the release of photosynthate of the symbiont in perialgal vacuole via a specific compound and pH in the vacuole, respectively.