Nitzschia Hassall (1845) A.H. Hassall (1845).
A history of British freshwater algae. S. Highley, N. Baillière, Edinburgh, Paris, Leipzig. p. 435, nom. cons.
Cells solitary or, more rarely, forming stellate or chain-like colonies, or living in mucilage tubes. Frustules isopolar. Cells and valves seen in valve or girdle view, depending.on species
Valves structurally asymmetrical, because the raphe system is almost always placed off-centre. The valve outline may appear asymmetrical and dorsiventral, but is usually symmetrical if seen in
the correct orientation (i.e. with the valve margins in a plane perpendicular to the line of sight). But as usually seen, valves are symmetrical or asymmetrical about the longitudinal axis. They
are usually highly elongate, may or may not be constricted or nicked centrally, and have rounded, rostrate or capitate poles. Transverse striae sometimes visible but often delicate, very fine or
unresolvable in LM. Raphe system fibulate, usually appearing to run along one edge of the valve but subcentral in a few species; except in a few cases (e.g. N. filiformis, N. obtusa), the raphe
maintains the same position on the valve over its whole length. The raphe itself is almost always invisible and its presence can be inferred only indirectly, by the presence of fibulae, which are
rib-like, square or rectangular structures. The fibulae are usually small and discrete, but in a few species, they are extended across part or all of the valve. There may or may not be central
raphe endings and this is an important taxonomic character; where present, they are usually detectable in LM as a slight thickening of the wall of the subraphe canal at the centre. The raphe
systems of the two valves may lie on the same side ('hantzschioid symmetry') or on opposite sides ('nitzschioid symmetry'). Generally two chloroplasts per cell, one towards each pole (very
occasionally >2 chloroplasts, in some marine species). Each chloroplast is usually simple and is appressed to one or other valve or to the girdle. The central gap between the chloroplasts is
where the nucleus lies.
A nomenclaturally and taxonomically difficult genus, as a result of the delicate structure and similar morphology of many species, and because of a complex series of changes in species circumscription since c. 1975. Some genera have recently been split fromNitzschia (e.g. Tryblionella, Psammodictyon), because they appear to be no more closely related to each other and to Nitzschia sensu stricto than some traditionally separate genera, e.g. Bacillaria, Denticula, Cylindrotheca, Pseudo-nitzschia. Further splitting off of genera can be anticipated. Nitzschia is conserved against the earlier name Sigmatella.
M.G. Kelly, H. Bennion, E.J. Cox, B. Goldsmith, J. Jamieson, S. Juggins D.G. Mann & R.J. Telford (2005). Common freshwater diatoms of Britain and Ireland: an interactive key. Environment Agency, Bristol.
Nitzschia acula Hantzsch in Rabenhorst
Nitzschia dissipata (Kützing) Grunow
Nitzschia linearis W. Smith
Nitzschia microcephala Grunow in Cleve & Möller
Nitzschia nana Grunow in Van Heurck
Nitzschia sigmoidea (Nitzsch) W. Smith