The unicellular red alga Porphyridium cruentum is a primitive member of the Rhodophyta, order of Porphyridiales. It can be found in sea water and in humid soils. The cells, which can be solitary or massed together in colonies, are spherical, their diameter ranging from 4 to 9pm. The wall-less cells contain a single large chloroplast and are surrounded by a sheath consisting of a water soluble sulphated polysaccharide (Ramus, 1972; see also Chapter 12). For the taxonomy, morphology and growth conditions of P. cruentum, see Vonshak (1987).