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Thalassiosira allenii is reported for the first time from marine phytoplankton of the Russian far East. Five strains representing three morphological variants of T. allenii, from three locations in the Sea of Japan, were investigated. The D8–D10 domain of LSU rDNA and rbcL of T. allenii were sequenced for the first time. Three morphological forms of T. allenii showed completely identical rbcL sequences and distinctly different nuclear rDNA sequences. The genetic differences between morphotypes were determined by the number of allelic variants of LSU rDNA, one of which was common to all studied strains. Two different types of the D1–D3 domain and three different types of the D8–D10 divergent domain were detected in the cultured strains of T. allenii. The three morphotypes are described in detail and compared. One allele of D1–D3 and one allele of D8–D10 were identified in the strains of T. allenii from two stations in Ussuri Bay. Two and three alleles of the D8–D10 domain were identified in the strains with an ambiguous morphotype and in T. allenii var. striata, respectively, from one station in Amur Bay. Subdivision of T. allenii into morphotypes based on morphology matched that based on molecular-genetic analyses using D8–D10 LSU rDNA. Caution is recommended in using LSU rDNA sequences and rbcL divergence as the principal criterion for delimiting and metabarcoding T. allenii varieties/morphotypes. The lack of genetic differentiation in rbcL, and the presence of clear intra-specific genetic differentiation among morphotypes in two regions of LSU rDNA suggest a link between morphological differences and the nuclear genome. The D8–D10 domain of LSU rDNA appears more suitable for discriminating morphotypes within T. allenii. The rbcL is an effective marker for recognizing T. allenii at species level, but not at infraspecific level. [ABSTRACT FROM AUTHOR]

Thalassiosira species significantly contribute to phytoplankton communities, forming algal blooms in temperate and polar regions, and are broadly used as model organisms for nanotechnology and as important food sources in the aquaculture industry. Despite being widespread in the world's oceans, data on abundance and seasonal patterns of individual species of Thalassiosira are sparse. In the present study, we have identified Thalassiosira tenera from the northwestern Sea of Japan, using light and scanning electron microscopy (SEM), and molecular genetic analysis of 28S rDNA sequence. Based on field and cultured material, morphological variability of T. tenera was studied; a detailed diagnostic description of the species is provided, and its morphometry is compared with that of the morphologically similar T. exigua. Growth peculiarities of the species were investigated in laboratory culture; variations in frustule morphology occur at different growth stages. The degree of frustule silicification is assumed to depend on the stage of cell development. By SEM observations of field samples, the seasonal dynamics of the species was examined and data on its abundance are provided for the first time. Findings indicate that T. tenera is present year-round in the phytoplankton assemblage in the northwestern Sea of Japan. The species dominates the community from November to February, accounting for more than 60% of the total phytoplankton abundance. The highest numbers, 7,281 cells l−1, were recorded under the sea ice in January, and the lowest numbers, less than 100 cells l−1, occurred in April. In addition, the study gives information on the species ecology. [ABSTRACT FROM AUTHOR]