A unique Aulacoseira species with an elliptical valve face was found in Early Miocene sediment of the Shichiku Formation, Fukushima Prefecture, Japan. The valve face of the separation valve displays areolae covering the whole surface while the valve face of the linking valve has fewer areolae, mainly in the marginal area. Also, the mantle of the separation valve is curved while that of the linking valve is nearly straight, characteristics not observed before in any Aulacoseira species with an elliptical valve face. This paper describes the new taxon, Aulacoseira iwakiensis, and compares it to four similar taxa: Aulacoseira elliptica Tsoy, A. hachiyaensis H. Tanaka, A. ovata Usoltseva & Tsoy and Melosira distans var. ovata Iwahashi.
A new diatom species, Pinnularia sakurajimensis O.Ishii & Ji.Tanaka, is described from brackish waters in the coastal area of Japan. Valve shape is linear with broadly rounded ends. Raphe fissure is filiform with straight central endings and bent terminal fissures to the secondary side. Axial area is linear. Central area forms a narrow fascia, and sweeping depressions exist at both sides of central raphe endings in the central area. Striae are almost parallel. Each stria consists of an alveolus with a rounded aperture on the inside of the valve. In LM, an array of the apertures is observed as a longitudinal line across the striae. Cingulum is composed of a valvocopula (an opened band with central protrusions), a short segmental band which lines the open end of the valvocopula and a pleura (a closed ring). This species has similarities to the figure by Peragallo & Peragallo (1897–1908) of Pinnularia cruciformis (Donkin) Cleve in valve length and stria densities, but the latter differs by possessing broader longitudinal lines on the striae.
We examined fossil diatoms in the middle Miocene Oidawara Formation distributed in Mizunami City, Gifu Prefecture, central Japan, to determine the precise sedimentation age. Well preserved fossil marine diatoms with high diversity have been found in the diatomaceous mudstone of the Oidawara Formation. The diatom assemblage of the formation can be assigned to the short interval between diatom biohorizons D41 (the last occurrence of Denticulopsis praelauta, 15.8 Ma) and the first occurrence of Actinocyclus ingens f. nodus (15.7 Ma)in the diatom zone NPD 4A (Denticulopsis lauta Zone) of the Neogene North Pacific diatom zonation. As a result, the sedimentation age of the Oidawara Formation was determined to be between 15.8 and 15.7 Ma. Reversed magnetic polarity reported from the Oidawara Formation corresponds to chron C5Br.
The lower Miocene marine diatom biostratigraphy has been established for the Yunagaya Group distributed in the Joban area, Fukushima and Ibaraki Prefectures, eastern Japan. The Yunagaya Group is composed of the Kunugidaira, Goyasu, Mizunoya, Kamenoo, Honya and Misawa Formations in ascending order. Diatoms are contained in the mudstone of the Mizunoya, Kamenoo, Honya and Misawa Formations. The Mizunoya, Kamenoo, Honya and lower Misawa Formations correlate with the Crucidenticula sawamurae Zone (NPD 2B)and the upper Misawa Formation is assigned to the Crucidenticula kanayae Zone (NPD 3A). The first occurrence of Mediaria splendida s.l., a useful biohorizon dated at 17.6 Ma, is found near the base of the Honya Formation. On the basis of diatom biochronology, the ages of the base and top of the Yunagaya Group are estimated at 18.2 Ma and 16.9 Ma, respectively. The late early Miocene marine and terrestrial climate changes reconstructed by well-preserved plant, molluscan and various microfossil assemblages are synchronous with changes in oxygen isotope records, suggesting that the late early Miocene climate fluctuations in the Joban area were controlled by global climatic change.
Pleistocene marine clay layer Ma11 in the Osaka sedimentary basin is correlated with marine oxygen isotope stage (MIS) 7. We analyzed fossil diatom assemblages and sedimentary sulfur in core sediments including the Ma11. Pseudopodosira kosugii, which is known as a useful indicator of past sea levels during the Holocene, was present in the uppermost part of the marine facies, which formed as a result of the marine transgression during MIS 7. Therefore, the abundance peaks of this species can also be used to reconstruct sea-level variations in sediments of pre-Holocene age.
Investigation of nine selected samples from a group of sixteen taken from five different outcrops of the Yoshino Formation (Middle Pleistocene), Kumamoto Prefecture, Japan, revealed six taxa in four genera of fossil freshwater centric diatoms with a relative frequency of 81–99%. Each of the nine samples was dominated by a specific centric diatom. The four dominant species were Cyclotella rhomboideo-elliptica var. rounda Y.Z.Qi & J.R.Yang, Cyclostephanos cf. numataensis H.Tanaka & Nagumo, Cyclotella cf. lacunarum Hust. and Discostella stelligera (Cleve & Grunow) Houk & Klee var. stelligera with C. rhomboideo-elliptica Skuja var. rhomboideo-elliptica as the sub-dominant species in one sample. Taxonomic information on these diatoms is reported here, with light and scanning electron microscope photographs shown for the five reported taxa.
Macroscopic fibrous mats formed by diatoms have been found extensively in the upper reaches of the Chikugo River, Kyushu, Japan. They are mainly aggregations of Cymbella janischii, which colonizes surfaces using mucilage stalks (secreted by the cells) that branch into tree-like shapes. In addition, Gomphoneis minuta attaches to these colonies using its own mucilage stalk. These species have not been recorded in Japan, and were probably introduced from North America. Careful monitoring of the bloom and distribution of these alien species is necessary to avoid becoming “nuisance” diatoms such as Didymosphenia geminata, which is expanding its habitat in North America, Europe and New Zealand.
Didymosphenia geminata (Lyngb.) Mart. Schmidt was found in the original sketch collection by the late Dr. Masayuki Watanabe. It was illustrated using a sample taken from Lake Panketo on the 13th August, 1966 (TNS-AL-50254). This old record suggests that D. geminata is native to Japan. In addition, its rarity and restricted distribution to the cool waters in alpine areas indicate that it is not a ‘new ecotype’ as described by Whitton et al. (2009).
Diatom assemblages and water quality were investigated at six sites in the hot spring of Oyasukyo gorge. The pH was alkaline and the water temperature was higher than 30°C except at one site. In total, 38 genera and 82 taxa were recorded; the most predominant being Achnanthidium exiguum, which is frequently found in alkaline hot springs. The other taxa of relatively high abundance are as follows: Pinnularia joculata, Diadesmis confervacea, Ulnaria oxyrhynchus, Sellaphora seminulum, Navicula rostellata, and Nitzschia brevissima.
Diatom assemblages and water quality were investigated at a site in the Kosaka River into which drainage flows from a non-ferrous metal refinery. At the site electrical conductivity was 1,529 μS/cm and the concentrations of inorganic nitrogen, copper, and zinc were 5.11 mg/L, 14 μg/L, and 130 μg/L, respectively. Only five diatom taxa were recorded; dominant and subdominant species were Achnanthidium minutissimum (88%) and Cyclostephanos fritzii (11%), respectively. A. minutissimum has a special copper-resistance mechanism and is an indicator species of heavy metal-polluted rivers, while the heavy metal tolerance of the other four taxa has never been reported.
A diatom assemblage in a drainage waterway (LD-G1) of the Hachirogata polder in Akita Prefecture was investigated. The waterway receives spring water with high concentrations of phosphate, and salinity load from the soils of paddy fields. The assemblage comprised 18 genera and 27 taxa, dominated by Navicula gregaria (48%), and followed by Navicula slesvicensis (22%), Navicula salinarum (12%), Nitzschia tubicola (7%) and Navicula lanceolata (7%). The most abundant taxa are eutraphentic or indifferent to saprobity, and circumneutral to alkaliphilous. Moreover, N. salinarum and N. tubicola usually grow in brackish water. Our results suggest that the high concentrations of phosphate, nitrogen and salinity in the LD-G1 water have an influence on the diatom composition.
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