Ecophysiology of the conjugating green algae (Zygnematophyceae) with emphasis on semiterrestrial habitats and specialized metabolic compounds.

The conjugating green algae (Zygnematophyceae) are regarded as sister group to land plants. Despite their morphological simplicity, recent genomic and other "omics" breakthroughs revealed a molecular toolkit for terrestrial survival that was established before the rise of embryophytes. This mini-review synthesizes advances by focusing on extreme habitats like glaciers, acidic peat bogs and aeroterrestrial surfaces. We highlight how environmental pressures have driven adaptations, including: (a) Genomic and cellular plasticity: substantial genome size variation and horizontal gene transfer events from soil bacteria that enhance metabolic flexibility. (b) Resistant life stages: the formation of zygospores and pre-akinetes with complex, multi-layered cell walls. (c) Specialized metabolites: the accumulation of unusual phenolics in vacuoles, cell walls or their excretion into the surroundings. These compounds, sometimes complexed with iron, function as photoprotectants against excessive UV and visible radiation but may have further roles as well. We address the lack of cultures to study zygospore formation and germination for life cycle studies, and the general under-sampling of "orthodox" habitats, where particularly members of the Serritaeniales and Zygnematales prevail. Resolving the phylogeny and chemical characterization of these algae is essential to understanding the ancestral stress-response mechanisms that enabled the colonisation of land.