Ultrastructure of Lobocharacium coloradoense, gen. et sp nov (Chlorophyta,Charciciosiphonaceae), an unusual coenocyte from Colorado

Light and electron microscopic descriptions are provided for Lobocharacium coloradoense, gen, et sp, nov., a unicellular coenocytic green alg-a isolat ed from a power plant's retaining pond in northcentral Colorado, Vegetative cells range from 120-230 mu m in length and 80-120 mu m in diameter in cul ture, The large vegetative cells are attached to substrates by small discoi d attachment pads. The cells are multinucleate and consist of distinct cyto plasmic lobes, with each lobe containing a chloroplast and a basal nucleus. Chloroplasts are somewhat cone-shaped in profile and stellate or lobed whe n viewed from the surface, and each has a central, basal pyrenoid, Hundreds of these cytoplasmic lobes occur within a cell, and thin cytoplasmic bridg es interconnect the lobes, When a vegetative cell matures, each of the cyto plasmic lobes cleaves to form numerous fusiform zoospores or spherical isog ametes. The biflagellate isogametes range in size from 4-10 mu m, they lack a cell wall, they have a cup-shaped chloroplast with a pyrenoid and stigma , and they have a nucleus close to the basal bodies. Isogametes are incapab le of forming vegetative cells. Zoospores are biflagellate and fusiform, me asuring 8-12 mu m in length and 4-6 mu m in diameter. Each zoospore has a c ell wall, a single parietal chloroplast with a prominent pyrenoid in the ce nter of the chloroplast, and a long oval stigma, Gamete and zoospore releas e involves a dissolution of the entire vegetative wall, Released zoospores usually settle and cluster near the vegetative cell from which they were pr oduced, attach to the substrate with their flagella, and, shortly after los ing their flagella, extrude mucilage through the flagellar pores in the wal l to form a small discoid attachment pad, The incipient vegetative cell is fusiform and uninucleate, but it becomes more rounded and multinucleate as enlargement occurs, Most vegetative cells in culture become dormant, and th e chloroplast becomes orange in color, Some cells form single aplanospores that can withstand desiccation, but occasionally numerous ap lanospores may also be formed later in the larger vegetative cells.