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The diatoms are an ecologically important group of algae that have been extensively studied by ecologists and taxonomists. However, the large-scale patterns of diatom distribution and the factors under- lying this distribution are largely unknown. The aims of this study were to identify the large-scale spatial patterns of benthic diatom assemblages in Korean streams and rivers, and to assess the importance of numer- ous environmental factors on diatom distribution. We classified 720 study sites based on diatom flora. Benthic diatoms, water chemistry, altitude, and riparian land cover and use were characterized by multivariate analyses, Monte Carlo permutation tests, and indicator species analysis. In total, we identified 531 diatom taxa. Diatom assemblages were mostly dominated by species of the genera Achnanthes, Navicula, Nitzschia, Cocconeis, Fragilaria (Synedra included), Cymbella, Gomphonema, and Melosira. Cluster analysis partitioned all 720 sites into eight groups based on diatom species composition. Canonical correspondence analysis in- dicated that altitude, land cover and use, current velocity, electrical conductivity, and nutrient levels explained a significant amount of the variation in the composition of assemblages of benthic diatoms. At the national scale, a downstream ecological gradient was apparent, from fast-flowing, mostly oligotrophic highland streams to slow-flowing, mostly eutrophic lowland rivers. Our data suggest that spatial factors explain some of the variation in diatom distribution. The present investigation of the spatial patterns of benthic diatoms, the ecological determinants of diatom occurrence, and the identification of diatom indicator species contributes to development of a program for assessing the biological integrity of lotic ecosystems in Korea.

We have made a synopsis of the morphology, ultrastructure, taxonomy, distribution and genetics of two widely distributed but little known species because this is important for the general understanding of species delineation in green algae. The taxa Desmodesmus perforatus and D. tropicus have in common central gaps between coenobial cells, but otherwise according to the type descriptions and type illustrations, they are morphologically well defined and easy to distinguish. However, in screening the literature, the situation becomes confusing and a clear delineation seems to be impossible. To solve the question of whether the two species are in fact different, or extremes of a single morphologically variable taxon, we isolated strains from different localities worldwide as well as strains of the related taxon D. maximus. The strains were studied under both optical and electron microscopes (EM) and the ITS-2r DNA of available strains was compared. With the results of the studied strains, and a comp...

Symbiotic nitrogen fixation requires the internalization of rhizobia into root cells of legumes and subsequent formation of symbiosomes, rhizobia-containing organelles inside the plant cells. A small GTP-binding protein from soybean, sRab7, was shown to be essential for nodule development. When yeast ypt7 (Rab7) null mutant was transformed with the srab7 gene, highly fragmented vacuoles seen in the mutant were replaced with a prominent central vacuole, as in the wild-type cells. These results confirm that sRab7 is an authentic homolog of Ypt7p. When nodules obtained at different stages of development were hybridized with srab7 cDNA, 7-day-old nodules showed the most intense hybridization signal. Hybridization was only detected in the infection zone of forming nodules, implying that sRab7 may be involved in endocytosis of rhizobia. SCaM-1 and SCaM-4, soybean calmodulin genes, were also expressed in the same region at this stage of nodulation as shown by in situ hybridization. RT-PCR analysis confirmed the expression of these genes. These results indicate that both the srab7 and SCaM genes were expressed at an early stage of nodulation. Since membrane fusion of vesicles has been shown to be dependent on calmodulin and Ypt7p, the similarity of the expression patterns between the two genes in soybean nodules may indicate that sRab7 and SCaM are essential for vesicular fusion during rhizobial endocytosis and symbiosome formation.

The eight-celled coenobia of Pediastrum privum differ from all other Pediastrum species by having an asymmetrical central cell with one or two protuberances, more or less pointing out from the coenobial plane.