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

Abstract

Light and electron microscopic descriptions are provided for Lobocharacium coloradoense, gen. et sp. nov., a unicellular coenocytic green alga isolated from a power plant's retaining pond in north‐central Colorado. Vegetative cells range from 120–230 μm in length and 80–120 μm in diameter in culture. The large vegetative cells are attached to substrates by small discoid attachment pads. The cells are multinucleate and consist of distinct cytoplasmic lobes, with each lobe containing a chloroplast and a basal nucleus. Chloroplasts are somewhat cone‐shaped in profile and stellate or lobed when viewed from the surface, and each has a central, basal pyrenoid. Hundreds of these cytoplasmic lobes occur within a cell, and thin cytoplasmic bridges interconnect the lobes. When a vegetative cell matures, each of the cytoplasmic lobes cleaves to form numerous fusiform zoospores or spherical isogametes. The biflagellate isogametes range in size from 4–10 μm, 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 incapable of forming vegetative cells. Zoospores are biflagellate and fusiform, measuring 8–12 μm in length and 4–6 μm in diameter. Each zoospore has a cell wall, a single parietal chloroplast with a prominent pyrenoid in the center of the chloroplast, and a long oval stigma. Gamete and zoospore release involves a dissolution of the entire vegetative wall. Released zoospores usually settle and cluster near the vegetative cell from which they were produced, attach to the substrate with their flagella, and, shortly after losing their flagella, extrude mucilage through the flagellar pores in the wall 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 the chloroplast becomes orange in color. Some cells form single aplanospores that can withstand desiccation, but occasionally numerous aplanospores may also be formed later in the larger vegetative cells.