No abstract

No abstract

Terrestrial microalgae evolved a variety of photoprotective strategies enabling a life on land. This includes the production of sunscreen compounds, which shield cells from excess radiation. Here, we report a new genus of conjugating green algae, Serritaenia gen. nov., whose members produce extracellular mucilage with a striking pigmentation. This phenomenon is very unusual for eukaryotic algae and poses cell biological and functional questions. So far, extracellular sunscreen pigments are exclusively known from cyanobacteria, while eukaryotic algae typically contain intracellular sunscreens. We demonstrate that pigmented mucilage in Serritaenia spp. can be induced by experimental exposure to UVB in an intensity-dependent manner, and that it strongly absorbs deleterious wavebands. Microscopic details of UVR-treated cells suggest that the directional secretion of pigmented mucilage is responsible for the defined and well-oriented pigment layers observed in natural material. Even though the chemical nature of the pigment remains to be elucidated, several pieces of evidence suggest that the 'sunscreen mucilage' of Serritaenia represents an elaborate photoprotective adaptation, unprecedented in eukaryotic algae.

No abstract

The evolution of streptophytes had a profound impact on life on Earth. They brought forth those photosynthetic eukaryotes that today dominate the macroscopic flora: the land plants (Embryophyta) [1]. There is convincing evidence that the unicellular/filamentous Zygnematophyceae--and not the morphologically more elaborate Coleochaetophyceae or Charophyceae--are the closest algal relatives of land plants [2, 3, 4, 5, 6]. Despite the species richness (>4,000), wide distribution, and key evolutionary position of the zygnematophytes, their internal phylogeny remains largely unresolved [7, 8]. There are also putative zygnematophytes with interesting body plan modifications (e.g., filamentous growth) whose phylogenetic affiliations remain unknown. Here, we studied a filamentous green alga (strain MZCH580) from an Austrian peat bog with central or parietal chloroplasts that lack discernible pyrenoids. It represents Mougeotiopsis calospora PALLA, an enigmatic alga that was described more than 120 years ago [9], but never subjected to molecular analyses. We generated transcriptomic data of M. calospora strain MZCH580, and conducted comprehensive phylogenomic analyses (326 nuclear loci) for 46 taxonomically diverse zygnematophytes. Strain MZCH580 falls in a deep-branching zygnematophycean clade together with some unicellular species, and thus represents a formerly unknown zygnematophycean lineage with filamentous growth. Our well-supported phylogenomic tree lets us propose a new five-order system for the Zygnematophyceae, and provides evidence for at least five independent origins of true filamentous growth in the closest algal relatives of land plants. This phylogeny provides a robust and comprehensive framework for performing comparative analyses and inferring the evolution of cellular traits and body plans in the closest relatives of land plants.