Morphological, physiological characteristics and molecular phylogeny of Rhizoclonium riparium (Cladophorales, Chlorophyta) from freshwater and marine environments

Rhizoclonium riparium (Roth) Harvey is a filamentous green alga without branches. Here we collected the freshwater alga from a rice field (0.1 PSU, pH 8.0) at Koganei-shi, Tokyo, and the marine alga from a tide pool (50 PSU, pH 8.0) at Aburatsubo-bay, Kanagawa Prefecture. The size of the marine algal cells was smaller than that of the freshwater algal cells: in the marine and freshwater algae the average widths of cells were 21.2 mu m and 25.5 mu m, and the average lengths of cells were 25.8 mu m and 48.6 mu m, respectively. Rhizoclonium species are widely distributed in different salinity environments from freshwater, brackish to seawater. The freshwater alga grew well in seawater media from 0.1 to 40 PSU. Upper limit of the salinity for growth was 50 PSU in the freshwater alga and 90 PSU in the marine alga. On photosynthetic rates of oxygen evolution in saline media, the optimum and upper limit of salinity were 60 PSU and 100 PSU in the freshwater alga, and 30 PSU and 120 PSU in the marine alga, respectively. The analysis of 18S rDNA sequences indicated that 1778 base pairs were all identical in sequences of the two algae. To investigate whether these algae are two ecotypes of the same species, analyses of other DNA sequences will be needed. The freshwater alga showed salinity tolerance, suggesting that Rhizoclonium riparium may have gradually spread from marine to freshwater habitats.