New Strains of the Deep Branching Streptophyte Streptofilum: Phylogenetic Position, Cell Biological and Ecophysiological Traits, and Description of Streptofilum arcticum sp. nov.

Streptofilum capillatum was recently described and immediately caught scientific attention, because it forms a phylogenetically deep branch in the streptophytes and is characterised by a unique cell coverage composed of piliform scales. Its phylogenetic position and taxonomic rank are still controversial discussed. In the present study, we isolated further strains of Streptofilum from biocrusts in sand dunes and Arctic tundra soil. Molecular and morphological characterisation including transmission electron microscopy confirmed that both new strains belong to Streptofilum. The Arctic strain is described as a new species, Streptofilum arcticum sp. nov., based on molecular differences, a specific sarcinoid morphology and unique ultrastructure with massive cell coverage composed of pili-shaped scales. A comprehensive characterisation of the ecophysiological traits of both new Streptofilum isolates and the original one revealed a broad temperature tolerance, a rapid recovery of photosynthetic performance after desiccation, an efficient photosynthesis at low light and a tolerance to high-light conditions. In addition, Streptofilum could cope with UV irradiation, but only S. capillatum grew under UV exposure. All Streptofilum strains are well-adapted to water-deprived terrestrial habitats such as biocrusts. From this study it can be concluded that already early-branching streptophytes were able to tolerate terrestrial conditions.