THE EXTRACELLULAR CALCIFICATION OF ZOOSPORES OF PHACOTUS-LENTICULARIS(CHLOROPHYTA, CHLAMYDOMONADALES)

In nature the zoospores of Phacolus lenticularis usually have a conspi cuous, bivalved lorica. It consists of two clearly distinguishable lay ers: an outer calcite layer formed of rings of staggered calcite cryst als with electron-opaque rod-like components between the crystals, and an underlying electron-opaque layer. Underneath the lorica a mucilagi nous sheath is present. The ultrastructural development of these extra cellular layers was studied by scanning electron microscopy and transm ission electron microscopy of flat-embedded specimens from the natural habitat, Lake Feldberger Haussee (Baltic Lake District, Germany). Dur ing vegetative propagation, the two mother shells separate but remain attached to the mucilaginous zoosporangium which is formed between the shells. Usually two longitudinal cell divisions result in four naked zoospores in the sporangium. These begin to form new extracellular mat erial, consisting initially of an electron-opaque layer that becomes d etached by the formation of a mucilaginous inner sheath. During this s tage the dictyosomes are highly active and the cells contain vesicles with rod-like structures resembling the electron-opaque material betwe en the crystals of the adult lorica. Zoospores are motile in the spora ngium and the outermost extracellular layer gradually calcifies with s mall crystals developing on the electron-opaque layer in a regular man ner and separated by equal distances (spotted sphere stage). The growi ng crystals develop in a pattern resembling a tiled roof and merge to form the final, massive calcite layer of the lorica. The zoosporangium ruptures between the former mother shells and the zoospores are relea sed. Several possible nucleation and calcification mechanisms are disc ussed.