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Abstract
Nitzschia sigmoidea is the type species of the diatom genus Nitzschia. Recent evidence implies that the genus is polyphyletic. As a reference for future studies of the genus, we here provide a thorough morphological examination of the silica frustule, including new details of copulae and poroids, as well as compiling previously published information on the cellular ultrastructure. Observed specimens of Nitzschia sigmoidea are linear in frustule view and sigmoid in girdle view with a length of 346–359 μm, and a width of 9–13 μm. Both nitzschioid and hantzschioid cells are found. The raphe is highly eccentric, elevated on a keel, without a central nodule. The raphe ends externally in a hook-shaped terminal fissure, internally in a helictoglossa forming an excrescence. Tubular fibulae are slim and irregularly distributed with a density of 6–9 in 10 μm. A conopeum runs the entire length of the cell. The striae are uniseriate with 4–5 poroids in 1 μm, each with a hymen with a hexagonal pattern of perforations. Two types of copulae are found, both with two rows of poroids, 3.2–3.4 poroids in 1 μm. One type of copula is fimbriate with evenly distributed fimbriae, while the other type has a veil.
Initially, growth of the diatom was restricted by lack of carbon in the freshwater medium, after addition of bicarbonate it grew well. HPLC analysis showed no domoic acid production by N. sigmoidea.