A new diatom species, Thalassiosira hexagona sp. nov., is described from lowermost Pleistocene sediments in the subpolar North Atlantic. The new species is morphologically characterized by linear striae, a ring of six evenly spaced subcentral fultoportulae, a single ring of marginal fultoportulae, spines fused with external tubes of the marginal fultoportulae, and a shallow mantle area with ribs. The new species is readily identified under scanning electron microscope observation because both the hexagonal ring of subcentral fultoportulae and the spines fused with external tubes of marginal fultoportulae are species-specific characteristics. However, the new species may have been misidentified as other centric diatom species with linear striae in previous studies because the species-specific characteristics are difficult to identify under light microscope observation. Among the known species, T. anguste-lineata is the morphologically most similar species to T. hexagona because both species exhibit linear striae and a ring of subcentral fultoportulae. However, all species with a ring of subcentral fultoportulae, except T. hexagona, display intraspecific changes in the valve face striation pattern and the number of subcentral fultoportulae. The changes in the shape of the ring of subcentral fultoportulae and the changes in the valve face striation may be related to each other. Thalassiosira hexagona also resembles species in the genus Planktoniella in the presence of two rimoportulae and mantle ribs. A recent molecular phylogenetic study suggested that T. anguste-lineata, the species most similar to T. hexagona, belongs to a clade that is sister to Planktoniella species. Thus, in future it may be necessary to transfer T. hexagona and T. anguste-lineata to another genus.

We report recent occurrences of Fragilaria longifusiformis ssp. eurofusiformis Lange-Bertalot & S. Ulrich from Japanese lakes. This diatom was firstly found from Uokiri Dam Reservoir on the Yahata River, Hiroshima in 2011, followed by Lake Kasumigaura, Ibaraki in 2014 and Lake Biwa, Shiga in 2021. It is considered to be introduced into Japan around 2010. We displayed morphological characters of Japanese specimens and distinguishing points from other confusing species.

Fragilaria saxoplanctonica Lange-Bertalot & Ulrich from Lake Kasumigaura, is reported as an official new Japanese species with photographs. The species has been misidentified as Synedra filiformis Grunow, Fragilaria gracilis Østrup or Fragilaria tenera (W. Smith) Lange-Bertalot. In Kasumigaura, it appears during the spring bloom from March to July. As a planktonic species, there are no similar species and identification is easy, however since many attached diatom species in genus Fragilaria, are morphologically similar, care should be taken in identifying them in samples including periphyton.

The ubiquitous diatom Skeletonema costatum (sensu lato) has been subdivided into several species based on genetic analyses with rDNA and fine morphological analyses in 2005 and 2007. In this report, most of the identification methods were the same as above, but a new method using mtDNA was used for some isolates. 1057 Skeletonema vegetative strains and 493 germinated strains were isolated from 11 coastal waters in Japan ranging from subtropical to subarctic zones from 2007 to 2015. The aim of this study is to clarify the ecological characteristics of every Skeletonema species.

The Skeletonema marinoi-dohrnii complex was the most abundant species. It was distributed broadly throughout the year, except in the subtropics, formed blooms even at low temperatures; and germinated over a wide range of temperatures. The next abundant species was S. japonicum. It is adapted to temperate, cool waters, where vegetative cells appeared at temperatures <28°C. It never germinated at 30°C. Skeletonema costatum (sensu stricto), S. menzelii, and S. psedocostatum are adapted to warm temperatures in a year. Three species—S. ardens, S. grevillei, and S. tropicum—are adapted to conditions during late summer and autumn at temperate sites. Skeletonema grevillei and S. menzelii were observed at subtropical sites. Skeletonema potamos occurred in brackish waters near fresh waters; S. costatum (s.s.) was able to grow even at salinities near zero; and the remaining seven species occurred in brackish and marine waters. Based on their distributions, S. costatum, and the other seven species were classified as a brackish water species, and marine species respectively. Skeletonema species except for S. grevillei and S. potamos were observed to form blooms and S. grevillei was not confirmed to germinate in Japanese coastal waters. Each Skeletonema species was associated with a different niche characterized by water temperature and salinity.

Freshwater lacustrine diatom fossils are reported from samples of the Dorobu Formation around Dorobu, eastern Kaminoyama City, Yamagata Prefecture, Japan. The Dorobu Formation is composed of caldera lake deposits (about 250 m in thickness) of the Kaminoyama Caldera, consisting of tuff in the lower part, alternating beds of sandstone, mudstone and tuff in the middle part, and unstratified tuff breccia in the upper part. Freshwater diatoms occur in thinly laminated mudstone in the middle part of the formation. The authors have examined six samples and identified a total of 64 taxa belonging to centric diatoms (5 genera, 7 taxa), araphid pennate diatoms (9 genera, 13 taxa), monoraphid pennate diatoms (6 genera, 10 taxa) and biraphid pennate taxa (19 genera, 34 taxa), including unidentified taxa. The diatom assemblages of this formations are characterized by an abundance of Cyclotella comta (up to 94%), Tertiariopsis cf. costata (up to 86%) with few taxa such as Cyclotella antiqua and Pliocaenicus omarensis. Cyclotella comta from the Dorobu Formation may be the oldest record of the taxon in Japan. This paper is the first reliable report in Japan, describing the occurrence of C. antiqua, which is a rare diatom species limited in subarctic lakes of low pH and low conductivity. The abundant taxa and some accompanying ones are shown in LM and SEM photographs. The age of the Dorobu Formation has been previously considered to be late Miocene to Pliocene based on plant fossil assemblages. However, the diatom fossils and the K-Ar ages of the volcanic products unconformably covering the Dorobu Formation suggest that the age of this formation may be Pliocene to earliest Pleistocene.

We report the occurrence of Cymbella janischii in Ado River flowing into the Lake Biwa in 2022; this is the first report of this diatom from Kinki Area, Japan. Cymbella janischii has been known as an endemic species in the Pacific Northwest of North America. In Japan, however, it has become known as an invasive species. It is likely that it was introduced from the native locality into Kyushu in 2006 or shortly before, and has rapidly spread throughout Japanese rivers. In Ado River, it formed massive colonies on rocks by means of mucilage stalks secreted from one end of the cells, but the colony scattered only on the river bed. The cell had a dorsiventral outline, with an intricately shaped plastid. Fluorescence microscopy on living cells stained with SYBR Green and fluorescence-labeled lectin revealed that the position of the nucleus was appressed to the ventral side, and polysaccharide covered the entire frustule as well as the mucilage stalks. We also confirmed the identity of the species with the sequence of the 18S ribosomal RNA gene.

We report the age and composition of Neogene freshwater diatom assemblages found in the lower part of the Satomi Formation and the Okukamisato lignite bearing Formation in the Kamisato area in the southern part of Tsubetsu Town, eastern Hokkaido. The lower part of the Satomi Formation contains fossil marine diatoms correlative with the upper part of the NPD7A zone (6.8–6.5 Ma in age) and the NPD7Ba subzone (6.5–5.6 Ma in age) of the North Pacific diatom biostratigraphic zonation. This marine diatom biochronology indicates that the age of the lower part of the Satomi Formation is estimated to be 6.8–5.6 Ma, which corresponds to the Messinian Age (7.25–5.33 Ma) in the Miocene Period. The Okukamisato lignite bearing Formation shows similar lithology to that of the plant-bearing mudstone in the lower part of the Satomi Formation and is likely to be included in the Satomi Formation. The freshwater diatom assemblages found in the plant-bearing mudstone in the lower Satomi Formation exposed along the Sen-untsubetsu River show a highly diverse composition consisting of 43 genera such as Pinnularia (12 taxa), Eunotia (11 taxa), Gomphonema (7 taxa) and so on. This is consistent with the paleoenvironmental reconstruction based on plant fossils, “wetland around river in coastal lowland”. Diverse freshwater diatom assemblages from the Okukamisato lignite bearing Formation along the Tsubetsu River are composed of Eunotia (14 taxa), Pinnnularia (13 taxa), Gomphonema (11 taxa) and others. They are also presumed to be similar to those of the lower part of the Satomi Formation with more influence of running water. Fossil freshwater diatom assemblages during the Messinian Age have rarely been reported in Japan, and therefore the diatom assemblages in the Satomi Formation are significant in clarifying the history of diatom assemblages in Japan.