The diatoms of the early Miocene Masaragawa Formation distributed around Sado Island, Niigata, Japan were investigated. The formation contains diatomaceous mudstone yielding an abundance of Aulacoseira taxa, such as Aulacoseira praegranulata var. praeislandica (Jousé) Moisseeva and A. praegranulata var. praeangustissima (Jousé) Moisseeva. Aulacoseira praegranulata var. praeislandica has three forms of coarsely areolated, densely areolated and thickly formed valves joined in the same chain. Three forms of Aulacoseira praegranulata var. praeislandica and accompanying taxa are shown with LM and SEM.

Fossil diatom assemblages and stratigraphic features revealed the environmental history and an event deposit at a coastal lowland in Tosashimizu City, facing the Nankai Trough. The paleoenvironment of the study site changed during the last 600 years from a sandy tidal flat, a standing-water environment with submergent plants, to a wetland. A sandy event deposit identified by bare eyes and computed tomography (CT) images was interpreted to have formed under the standing-water environment. The event deposit (Sand B) was diagnosed as a tsunami deposit based on sedimentary structures, mixed assemblages of freshwater, brackish–marine diatoms, and the environmental change before and after the event. The depositional age of the Sand B was constrained by radiocarbon ages as 550–495 cal yr BP (1411–1453 CE; Common Era). This constrained event age indicated that the Sand B might have been formed by an unknown tsunami associated with an earthquake along the Nankai Trough, although either 1361 Shohei or 1498 Meio earthquake were the other possible origins of the Sand B regarding the error range of radiocarbon dating.

Thalassionema bacillare, which is a temperate and warm-water species in modern oceans, was recently revealed to have been one of the major diatom species in the subpolar North Atlantic around the Pliocene–Pleistocene boundary. In this study, the morphology of T. bacillare was examined through detailed observation of fossils collected from upper Pliocene to lower Pleistocene sediments in the subpolar North Atlantic. Although many morphological characteristics of the fossil valves examined in this study are consistent with results from previous studies of modern specimens of T. bacillare, there are some new important findings. For example, the fossil valves often have apical pores and lack spines on the tapered apex, though this is likely related to the weak silicification of the examined fossil valves. In addition, the minimum valve length is much shorter than previously reported, and external openings of rimoportulae differ slightly in shape between both poles. Fossils of T. bacillare are potential proxies for reconstructing paleoceanographic conditions because of the species’ limited modern distribution.

We investigated the diatom flora attached to the inner walls of indoor medaka breeding tanks. Such diatoms are troublesome for the breeder, because they ruin the appearance of the tank and take time to clean. A total of 62 species belonging to 25 genera were identified in the 50 tanks surveyed. In each of the tanks, 0–17 diatom species belonging to 0–11 genera were found. Most of the diatoms were circumneutral to alkaliphilous species, but some acidophilous species were also found in some tanks. Gomphonema was the genus with the highest number of species found, with 11. Cocconeis lineata was the most frequent species, as it appeared in 42% of the surveyed tanks. This study will provide basic information on the diatoms that appear in aquariums.

After 2020, we discovered distinct Cymbella species from three different rivers in Western Japan; the Amano River flowing into Lake Biwa, the Chikusa River, and the Onga River. They are characterized by having valves with biseriate striae near the apices and uniseriate striae around the center. The population from the Amano River is identified as Cymbella distalebiseriata B.Liu & D.M.Williams, while that from the Chikusa River is more suitable to identify as Cymbella liyangensis W.Zhang, I.Jüttner & E.J.Cox. Those from the Onga River are similar to C. distalebiseriata in the valve shape, but morphometrically more similar to C. liyangensis. Because these two species are very similar in valve outline and stria density except for the difference of areola density, we regarded them as Cymbella distalebiseriata-liyangensis species complex. We would assume that they have recently invaded Japan since, to our knowledge, they have not been reported in Japanese diatom flora.

Neogene marine deposits in Japan sometimes contain considerable amounts of lacustrine planktonic diatom fossils transported from inland lakes as silt-sized particles through rivers. This study describes morphology and stratigraphic range of such allochthonous lacustrine centric diatoms preserved in the late Miocene and early Pliocene marine sediments of the Onnagawa, Funakawa and Tentokuji formations distributed in the Dewa Mountains, Akita Prefecture, Northeast Japan. Ten lacustrine centric diatom species belonging to seven genera, excluding Aulacoseira, were identified from these formations. Age of the first occurrence (FO) of each taxon is estimated by a sediment accumulation rate curve constructed based on marine planktonic diatom biochronology. Five significant events of lacustrine diatoms on generic level were recognized: the FO of Mesodictyon (8.3–8.5 Ma), the FO of Cyclotella (5.9−6.1 Ma), the FO of Tertiariopsis (4.5–4.9 Ma), the FO of Stephanodiscus (4.4–4.7 Ma) and the FO of Lindavia (3.5−3.9 Ma). Of these events, the FOs of Mesodictyon, Cyclotella and Lindavia are almost synchronous between the Dewa Mountain and Niigata area, suggesting a possible biostratigraphic utility of these biohorizons. Stratigraphic occurrence pattern of lacustrine diatoms preserved in marine sediments may reflect turnover of lacustrine diatoms in Japanese Islands through geologic time.

A new Licmophora with an apical window and additional perforations in the septum was discovered in a sample from Majuro Atoll, Marshall Islands. Valves are so narrow that they almost always present in girdle view, when they appear triangular, the mantle comprising most of the valve surface. A eucentric stage was used to tilt specimens up to 80° to see the shape in valve view and to see rimoportulae. Valves were 92–140 µm long, 8 µm wide, with three expanded areas, the broadest in the middle. The areolae were elongated in the apical axis but separated by wide virgae and vimines, unlike the narrow and closely spaced areolae in Licmophora cf. abbreviata, but with essentially similar vela. The morphology of Licmophora complanata is most like Licmophora peragallioides and Licmophora clevei in having a moderately long and wide rostrum with a moderately large apical window in the septum.

We investigated the diatom flora of Fujiganaru Moor, an intermediate moor in Okayama Prefecture, Western Japan. It is a Sphagnum moor formed at the bottom of a valley at 220–240 m above sea level and is surrounded by a slightly sloped ridge. A field survey and sampling were taken place on 14 March 2016. The water was slightly acidic (pH 5.8–6.5) with low electrical conductivity (2.4–3.2 mS m⁻¹). From 15 samples, 111 diatom taxa belonging to 32 genera were found, although 16 of them have not been identified at the species level. They are all listed and illustrated here. In terms of taxa richness, the dominant genus was Pinnularia, represented by 24 taxa, followed by Eunotia, with 17 taxa.

Colonies of Aulacoseira ambigua (Grunow) Simonsen is known to take two forms, linear and helical ones. Whether this difference is regulated by gene or environment, however, has not been known. We observed that helical colonies of A. ambigua became loose during cultures of isolated strains. Although helical colonies of this species observed in Lake Biwa are various in the helix pitch, such loosely wavy colony has not been observed yet. Our finding implies that the helical and linear colonies of this species can be environmental variations, but further research is needed to confirm this because the complete conversion from helical to linear colonies, and vice versa, have not yet been observed.

A large amount of pumice was produced by the eruption of the Fukutoku-Oka-no-Ba submarine volcano in the Ogasawara Islands in August 2021. The pumice formed pumice rafts that drifted westward on the Pacific Ocean and washed ashore along the coast of Okinawa Island, more than 1000 km away, in October 2021. In this study, we observed surface scrapings of pumice clasts that had drifted onto Okinawa Island by light microscopy and surface of the pumice clast by scanning electron microscopy, and revealed the presence of attached diatoms. Sixteen diatom taxa were observed on the pumice clasts. The genus Mastogloia was the most abundant, with 10 taxa, and accounted for more than 80% of the diatom assemblage on all pumice clasts observed. The second most abundant taxon was Rhopalodia pacifica, accounting for 5–10% of the diatom assemblages. Our results suggest that drifting pumice may contribute to the dispersal of some attached marine diatoms.