Aulacoseira Guide
Credit: Sarah Spaulding
  1. Valve mantle deep and ornamented, cells usually seen in girdle view
  2. Ringleiste may be present
  3. Frustules form colonies joined by spines
  4. Collum may be present

The centric frustules of Aulacoseira are linked to one another by spines to form filaments. Cells are typically seen in girdle view, because of the deep valve mantle. The mantle is highly ornamented. Cells often form colonies and, depending on the species, may be joined by linking spines. The shape of the linking and separation spines and relationship between spines and striae are important characters that distinguish species within Aulacoseira. Rimoportulae are present on the valve mantle and are often associated with the ringleiste, an internally thickened ring of silica. This feature, however, is often difficult to view under light microscopy. The external expression of the rimoportulae may be as simple pores, while internally the rimoportulae may form complex structures. Some species possess a hyaline area, or unoramented area called a collum at the edge of the valve mantle. The copulae, or girdle bands, are split rings with ligulae.

The valve structure and thickness of Aulacoseira reflects the concentration of silica in surrounding waters which impacts cell growth rate. As a result, frustules vary in morphology within the same species, even within the same filament.

Cells may form resting cells, in which the cytoplasm condenses around the nucleus. Such cells have been found to survive in sediments up to hundreds of years. Resting spores are metabolically inactive cells that are formed in some species. Resting spores have thickened frustules, morphologically distinct from vegetative cells. Such spores are able to survive in sediments. Living cells contain multiple discoid chloroplasts.

Aulacoseira is one of the most common freshwater diatom taxa, especially abundant in plankton of lakes and large rivers. One of the most successful species of freshwater centric diatoms, it is found in great abundance in fossil diatomites. The genus was resurrected to separate freshwater Aulacoseira from Melosira, a primarily marine genus.