Low cell density: the unifying principle for unicell development inScenedesmus(Chlorophyceae)

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Abstract
Scenedesmus strain F3 is identified as S. armatus (Chod.) Chod. Morphological expression of the strain was dependent on the age of the culture. Low cell densities stimulated the development of the unicell stage. As cell densities increased, spine-bearing coenobia became more frequent. Spineless coenobia were formed as the cultures approached the carrying capacities of the media. Two types of spine-bearing coenobia were classified. Type I coenobia were observed in young cultures. Two different forms were identified. Type Ia coenobia possessed elongate cells with terminal cells shorter than median cells. This coenobium was most common in media with elevated phosphorus levels. Type Ib coenobia were most common in media with elevated levels of ammonium. The elongate cells of this latter type were generally equal in length, and were often arranged sub-alternately in series. Spine lengths of Types Ia and Ib coenobia exceeded cell lengths. Unicells were formed only by Type I coenobia. The cells of Type II coenobia were broadly ovate, equal in length and wider than those of Type I coenobia. Spines were shorter than Type I coenobial spines. Populations comprising 100% unicells occurred in daily transfer cultures at 1000 cells ml-1. Unicell production was retarded at higher cell densities. Consistently higher daily growth rates occurred after unicell populations became established. Unicells were formed in batch cultures at initial cell densities of 100 and 1000 cells ml-1 in medium 7 and Bristol's medium, respectively. A seasonal cycle incorporating the role of the unicell is presented.