Stichochrysis immobilis is a diatom, not a chrysophyte

Stichochrysis immobilis E. Pringsheim was described as a new genus and species of the Chrysophyceae (Pringsheim 1955) on the basis of a pure culture grown on agar. There is now con­ vincing evidence that the organism in culture was in fact a clone of the diatom Cyclotella meneghiniana Kiitzing. The clone upon which Stichochrysis immobilis is based was isolated by Dr Jack Myers from plates of agar enriched with Chu # 1 0 medium (Chu 1942), a freshwater medium lacking added vitamins. The sample from which the isolation was made was collected in February 1949 from a waste-water clar­ ification pond receiving discharge from the city of Alamo, Texas. The water, for which no chemical data were given, was golden-brown in colour because of a dense algal population. The cells of this clone at first appeared square, but later became more rectangular. An axenic culture on agar was furnished to Dr Pringsheim in June 1949. The organism was described (Pringsheim 1955) as consisting of cylindrical cells, mostly twice as long as broad, 7-10 J.Lm in length and 5 J.Lm in diameter, forming chains under certain circumstances. It was considered to represent an undescribed genus and was assigned to the Chrysophyceae on the basis of colour and storage products as deduced by Pringsheim from light microscope examination. Pringsheim's culture from Myers, considered to be the type clone, was deposited with CCAP (Cambridge, UK) as clone number 956/1. Some difficulties with the taxonomy of the genus developed fairly quickly. In 1957, one of us (RRLG) isolated (from agar) several strains of small euryhaline mi­ croalgae from brackish ponds and coastal waters near Milford, Connecticut (USA). These algae grew in chains and were first thought to be clones of Stichochrysis. Drawings of the 'Milford clones' were remarkably like Pringsheim's figures (Pringsheim 1955, figs I-I to 1-8) except that cells or chains were often narrower. However, suspicion was aroused by observations of rows of small 'protuberances' on chains of liquid-grown cells supplied with high levels of silicon. That nominally unicellular