Gyrogonites and oospores, complementary viewpoints to improve the study of the charophytes (Charales)

Charophytes (Charales) are multicellular algae growing in fresh- to hypersaline waters and recorded as fossils by their oospores (formed by resistant organic walls) and gyrogonites (formed by intracellular calcification of the spiral cells surrounding the oospore). Preservation of oospores is mostly restricted to Late Pleistocene–Holocene sediments, whereas calcareous gyrogonites provide the fossil record of charophytes since the Late Silurian (425 Ma) and constitute the only link between living and fossil charophytes. Oospores and gyrogonites represent successive developmental stages occurring after fertilisation of the oogonia; however, gyrogonites do not develop in the extant genera Nitella and Tolypella section Tolypella of Tribe Nitelleae, and in most species of the subgenus Charopsis. For Tribe Chareae and Sphaerochara (Tolypella section Rothia), gyrogonites display more useful characters than the oospores, allowing easy identification of the extant genera Nitellopsis, Lychnothamnus, Lamprothamnium, Sphaerochara and Chara subgenus Chara. Determination to species level, and in particular the study of plastic species, needs a statistical approach. Extant oospores/gyrogonites can be used as modern analogues to infer palaeohydrological conditions and record past biodiversity. The ecological requirements of the living species provide important information for the restoration/conservation of wetlands and their management, and contribute to the reconstruction of Quaternary palaeoenvironments. The aim of the present paper is to highlight the importance of the gyrogonite, especially when studying extant charophytes, the application of both gyrogonites and oospores to palaeoenvironmental reconstructions, and to provide an accurate terminology and determination criteria for their study.