Morphology and comparative ultrastructure of Pseudotetracystis terrestris Arneson and Helicodictyon planctonicum (Whitford) Whitford and Schumacher

Abstract

Pseudotetracystia terrestrls Arneson, a green edaphic alga isolated from dry West Texas soil and Helicodictyon planctonicum (Whitford) Whitford and Schumacher, a green fresh-water planktonic alga, are both described and compared using electron microscopy. Both taxa reproduce by wall-less (naked) bi-flagellate motile cells which produce groups or colonies of cells. These colonies are produced from a single cell by vegetative cell division (desmoschisis) where the daughter cell walls are formed within the parent cell wall; this type of cell division where packets or colonies are formed by desmoschisis is characteristic of the order Chlorosarcinales. P. terrestris is currently assigned to this order. It is recommended that H. planctonicum be transferred from the filamentous Ulotrichales to the colonial ChlorosareInales based on the observed mode of cell division. Both P. terrestris and H. planctonicum are ultrastructurally similar to other green algae. Typical chloroplast, pyrenoid, nuclear and mitochondrial structure is evident. Dictyosomes with associated vesicles are evident in the motile cells of P. terrestris and less evident in the mature vegetative cells. The reverse is true for H. planctonicum where the dictyosomes, vesicles and myelin configurations are very abundant in the mature vegetative cells. Nine plus two flagellar structure and eyespot configuration typical of the green algae are evident in the motile cells of these taxa.