Towards an integrated understanding of charophyte biology and paleobiology

In this special issue, we present an overview of the presentations given at the 7th Meeting of the International Research Group on charophytes held in Astana in 2016. Charophytes, understood as the members of extant Charales and their direct ancestors (fossil orders Sycidiales and Moellerinales), represent a significant plant group that provides an iconic research system for many diverse fields, such as evolution, ecology, electrophysiology, phytoremediation and management of wetlands. Charophytes have been proposed as one of the closest ancestors of land plants. Their fossil record goes back to 425 Ma (million years ago), but the rise of the oldest living charophyte genera is dated from ca 155-125 Ma. An example of vegetations already dominated by early Characeae has been recently found in China, dating back to that time. The definition of charophyte species has been a matter of debate, since the morphology of these plants shows a great plasticity. The elucidation of charophyte taxonomy is of major importance for the definition of endangered species. New light is shed by the use of genetic coding for the distinction of problematic species within the Chara genus. Climate change is one of the major concerns in the present scenario of disturbance of charophyte habitats. Specific adaptations, including changes in thallus morphology, parthenogenesis and enhanced production of oospores, develop when charophyte habitats become unstable, desiccate or increase their salinity. Brackish charophytes are also a subject of special attention, since they include species highly specialized in particular niches and highly vulnerable to extreme salinities reached through habitat desiccation. The chemistry of water controls the distribution of species, even at the biogeographic scale. Experiments, related to the transport of ions across the charophyte cell membrane, provide an understanding of how charophytes adapt physiologically to those changes.