Charophyceae (Charales)

Abstract

The charophytes, or stoneworts, are a group of green algae with six extant genera in one family, distributed worldwide in freshwater ponds and lakes. They are among the green algal groups most closely related to land plants and exhibit a complex thallus, with multinucleate internodal cells joined at nodes comprising smaller, uninucleate cells giving rise to whorled branchlets. Two genera (Chara, Nitella) contain most of the described species, with a third (Tolypella) containing several dozen taxa. The remaining genera have one or a few species. Reproduction is oogamous, with sperm and eggs produced in separate multicellular structures. The thallus is haploid; the zygote is the only diploid cell in the life cycle, and meiosis is followed by the development of a resistant spore. Thalli and spores are often encrusted with calcium carbonate. Such spores are abundant in the fossil record of the Charales, which extends to the Upper Silurian, and many genera and families have become extinct. These algae provide important ecosystem services, for example, as colonizing species, as biological agents for producing water clarity, or as the base of the food web. Charophytes are important for the study of evolution of embryophyte development, growth meristems, and cell biophysics. As one of the green algal groups most closely related to land plants, the rich charophyte fossil record may reveal clues regarding the earliest algae that invaded the land.