Despite its phylogenetic importance, Thalassiosira lindstroemii has received little attention to date. In this study, the morphology of T. lindstroemii was examined in detail from upper Pliocene sediments of the subpolar North Atlantic. The valve has a valve face with linear areolation and scattered fultoportulae, a valve face/mantle junction with a single rimoportula and spines developed from triangular bases, and a mantle area with marginal fultoportulae and ribs. Both the valvocopula and second copula possess a characteristic ring of poroid areolae arranged along the advalvar margin. There is no doubt that T. lindstroemii is closely related to T. eccentrica, a major Thalassiosira species, because they possess many common morphological characteristics. Distinct differences between the two species include only the valve face areolation and the position of poroid areolae in the valvocopula. Although additional research is needed to reveal the precise phylogenetic relationship, the close morphological similarities suggest that T. lindstroemii is an ancestral (or possibly descendant) species of T. eccentrica.

A discoidal, heteropolar diatom with bilateral valve symmetry is morphologically close to Lauderia excentrica and I propose a new genus, Disymmetria to accommodate them both and restore Lauderia to Hasle’s concept of the genus. The new genus is characterized by a pseudoloculate outer rim with anastomosing costae, valve face divided transversely into a bounded basal dome and a depression with radiating striae separated by costae. In Disymmetria reticulata sp. nov., the valve surface is strongly undulate and the basal dome is also pseudoloculate with numerous fultoportulae, in contrast to D. excentrica, in which the valve face is weakly undulate and only the periphery is pseudoloculate. The new genus also leads to an emended definition of Lauderiaceae to focus on the shared character of valve structure.

In searching for freshwater diatoms, five poorly known centric diatom species in South Korea were discovered. They were collected from different sources in the type and size of water bodies. Aulacoseira scalaris was collected from a small reservoir with few pollutants, Aulacoseira subborealis and Cyclostephanos delicatus from highly eutrophic streams, and Spicaticribra kingstonii from both a small reservoir and a large lake. Edtheriotia shanxiensis has been found in a river system that flows through limestone areas and has relatively low pollution. They are nanoplanktons with a diameter of less than 20 µm, and because there are many neighboring taxa with similar valve morphology, they were closely examined and compared with previously reported specimens under light and electron microscopy. In particular, C. delicatus showed a wide range of morphological variations because the interstrial ribs are largely bifurcated on the mid-valve. In addition, morphological comparisons between S. kingstonii and its related three species are intensively reviewed.

Information on the flora of ice algae is essential for understanding the biological processes related to sea ice melting (e.g., seeding phytoplankton blooms) in polar oceans. Here, we report on the biomass and species composition of ice algae collected off Cape Darnley (Southern Ocean) in February 2019. Sea-ice chlorophyll a concentration ranged from 3.4 to 127.9 µg l^-1. The diatom Fragilariopsis curta (Van Heurck) Hustedt was predominant in samples with ice-algal blooms (>110 µg chlorophyll a l^-1), accounting for 79.7–87.7% of total cells and 73.2–84.7% of total algal carbon. Fragilariopsis cylindrus (Grunow ex Cleve) Helmcke & Krieger was predominant in the remaining samples with low chlorophyll a concentration (<4.0 µg l^-1). The ice-algal bloom samples showed a low particulate organic carbon: nitrogen ratio (6.9–7.5) and a high total algal carbon relative to particulate organic carbon (0.67–1.46). In contrast to the high F. curta cell abundance in sea ice, phytoplankton in surface seawater was dominated by F. cylidrus (45.7% of total cells). The long periods after substantial sea ice melting likely helped F. cylindrus, which is better adopted to the open-water environment, to become more predominant over F. curta in seawater. Our results suggest that F. curta may be an important species for ice-algal production and as the assemblages released into seawater as well as F. cylindrus in this region.

Cleaning and mounting diatom valves and frustules are necessary to observe their fine morphological structures under a light microscope. However, the presence of salt in the sample (seawater) and/or small numbers of diatom cells make the process laborious and time-consuming. To solve this problem, we propose to use a modified filter-transfer-freeze (FTF) technique, first proposed by Hewes & Holm-Hansen (1983), to prepare permanent slides. The protocol involves 1) concentrating diatom cells by filtering, 2) cleaning and rinsing diatom valves and frustules on the filter, and 3) recovering the cleaned material to a cover slip via the FTF technique. In terms of the number of species, diversity index, and the ratios of intact valves, no significant difference was observed between the slides prepared via the FTF and conventional centrifugation methods. Our results suggest that the modified FTF technique is a promising approach for the qualitative analysis of diatoms because it facilitates the rapid preparation of permanent slides.

After 2020, we discovered a Cymbella species from rivers in Western Japan that does not match any native species in Japan. The diatom was identified as Cymbella compactiformis B.Liu & D.M.Williams. It was found in Amano River and Inukami River flowing into Lake Biwa, Shiga Prefecture, and in Yakiyama River in the Onga River System, Fukuoka Prefecture. The populations sampled from the three rivers mostly matched the original description in valve length, breadth, stria and areola density, but their valve ends were less capitate than those of the type population. Cymbella distalebiseriata–liyangensis species complex, which may have been recently introduced from China, is also found in all three study sites. Recently Cymbella species regardless of native and alien have bloomed in Japanese freshwaters and have caused damage to fisheries. Therefore, the spread of C. compactiformis and its impact on river ecosystems should be monitored carefully.