Minidiscus guide
  1. Cells very small (<5 µm)
  2. Single rimoportula near valve center
  3. At least one central or subcentral fultoportulae
  4. Internal rimoportula opening is a sessile, ellipsoid, labiate ring

Minidiscus is a common planktonic, centric, diatom genus in estuarine and oceanic waters. It is rarely reported, however, as cells are very small (mostly less than 5 µm, but up to 10 µm in diameter) and difficult, often impossible, to identify by light microscopy. Live cells are observed solitarily, in mucilaginous aggregations, or in small chains with cells linked by mucilage threads extruded from central fultoportulae.

Minidiscus cells are cylindrical, with flat to lenticular valve faces and loculate areolae of varied organization. Minidiscus, along with Shionodiscus, Livingstonia, and Mediolabrus are thalassiosiroid (characterized by the presence of fultoportulae) genera separated from the genus Thalassiosira by the presence of a rimoportula on the valve face away from the valve margin. A ring of marginal fultoportulae may - or may not - be present. However, at least one fultoportula is located near the valve center. Fultoportulae are open to the exterior by a very short tube, surrounded by a ring of unornamented silica. Internally, fultoportulae are surrounded by 2-3 satellite pores. The rimoportula is opens externally by a small pore, and internally by an ellipsoid, sessile (unstalked) labiate ring.

Minidiscus is readily distinguished from Shionodiscus by the latter’s larger valve diameter and by features of the rimoportula (stalked, fan-shaped internal rimoportula in Shionodiscus) (Park et al. 2017). Livingstonia and Mediolabrus are similar, small-celled genera. The external openings of the marginal fultoportulae of Livingstonia are ornamented with hood shaped processes and are much more elaborately ornamented than those that may be present in Minidiscus. Mediolabrus can be distinguished from Minidiscus by the location of the valve face fultoportulae. Those of Mediolabrus are located midway between the valve center and valve face/mantle junction (Li et al. 2020).

Eleven species names within the genus are currently recognized (Algaebase, M.D. Guiry in Guiry and Guiry, 2021).