| Among raphid diatoms, the Nitzschia is a long-established and taxonomically difficult genus, including more than 900 species described worldwide. The most important feature of this genus is the presence of keel supported by the fibulae. Since Hassall erected this genus in 1845, some studies have been carried out for morphology and taxonomy of this genus. Although Nitzschia taxa are commonly recorded in most assemblages from fresh to brackish waters, the determination and delimitation of many taxa still remain difficult and unclear because of the lack of comprehensive descriptions of nomenclatural types and the overlap of diagnostic criteria, which leads to confusion in taxonomy of this group. In Japan, there exist some reports about the description of the genus Nitzschia in brackish water areas; nevertheless, most of these studies focused on the flora of the diatom, and there is no detail description on the microstructures and taxonomic comparison among different species within the genus. Therefore, in this study, we aim to present the morphological observations of Nitzschia taxa from brackish water areas in detail, and try to discuss their taxonomic position within the genus.Samples were collected from brackish water areas in Japan. All materials were fixed by 1% glutaraldehyde to preserve live cells. Vegetative cells were observed directly by the light microscopy (LM). In order to obtain clean frustules, live cells were cleaned by two methods: bleaching and boiling. Morphological observation of frustules was carried out with LM, scanning electronic microscopy (SEM), and transmission electronic microscopy (TEM).A total of 12 Nitzschia taxa were observed in present study. All species could be divided into 4 groups after full consideration of all observation results (LM, SEM and TEM). Group 1 only contains N. lanceolata due to its following features: (1) frustules straight in both valve view and girdle view; (2) keel and raphe quite eccentric. Group 2 includes two species: N. vermicularis and Nitzschia. sp. 1 considering the sigmoid frustules, closely spaced interstriae (transapical costa) covering areolae, and the presence of central area. N. fasciculata and N. sigma were divided into Group 3 because of the markedly sigmoid frustules and continuous raphe from pole to pole. Group 4 comprises N. scalpelliformis, N. clausii, N. aremonica, Nitzschia. sp. 2, Nitzschia. sp. 3,Nitzschia. sp. 4 and Nitzschia. sp. 5 with common features such as (1) frustules present sigmoid-linear, slightly to strongly concave at the median portion; (2) central nodule markedly present with two raphe branches bent to distal margin in the same direction;(3) along the either side of the raphe, a single row of puncta arrays and a single row of alveolus followed; (4) keel eccentric, inflected at the median portion.Nitzschia species are characterized by various valve structures, such as the keel morphology, the arrangement of areolae and the presence of central nodule. Sigmoid species within the genus Nitzschia are still taxonomically most difficult groups. In this study, all species except N. lanceolata have sigmoid frustules and are separated into 3 groups, which correspond to Sigmoideae, Sigmata and Obtusae groups in Grunow's system (1880). Unlike Sigmoideae group, the conopeum was not present in Group 2 and therefore, the interstriae morphology was used as taxonomy criteria to cluster two species together. But using this feature for taxonomy had received little attention in previous studies. On the other hand, presence or absence of the central nodule has great weight in separation of Nitzschia species, as species in Group 4 have the same central nodule in present study. |
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