Zygnematophyta

Abstract

The Zygnematophyta are among the most diverse green algae, with a variety of thallus types (filaments, unicells, colonies), cell wall structure (one to several layers, with varying degrees of ornamentation), and approximately 4,000 described species. The group lacks flagella at all stages of the life cycle. Several types of asexual spores are produced. Sexual reproduction, when present, involves conjugation or the union of two haploid vegetative protoplasts (individ- ual cells of filaments or unicells) to form a zygospore, which undergoes meiosis to produce a new haploid thallus upon germination. Almost exclusively freshwater, these algae are common in ponds, lakes, and streams, in surface mats, or as phytoplankton or benthic growths. Many, but not all, are found in oligotrophic to mesotrophic waters of moderate to low pH, although the diversity of habitats occupied spans a wide range and may be quite specific for individual species. The fossil record extends at least to the Carboniferous. Recent analyses have placed this group as the sister taxon to land plants, despite the dramatic differences in morphology, life cycles, and reproduction. The group includes the well-known Spirogyra and numerous beautiful unicellular forms known as desmids, many of which have elaborate external ornamentations (e.g., spines, granules, large lobes). The conjugating green algae are important as ecological indicator species and for the ecological services they provide.