Sunscreen mucilage: a photoprotective adaptation found in terrestrial green algae (Zygnematophyceae)

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.

A phylogenomically informed five-order system for the closest relatives of land plants

“…nov. In our phylogeny, the branch in question comprises three distinct groups of organisms: Mougeotiopsis calospora (one strain known), the genus Serritaenia (several strains known 25 ), and strain SAG 12.97, a unicellular zygnematophyte that is often referred to as ''Mesotaenium endlicherianum''. Currently, there is only one existent ordinal name that is based on the mentioned taxon names, namely Mesotaeniales Fritsch.…”

A phylogenomically informed five-order system for the closest relatives of land plants

“…nov. currently comprises the name-giving genus Serritaenia (unicells with a plate-like chloroplast and a mostly aerophytic life style; [19]), the genome-sequenced strain SAG 12.97 (often referred to as "Mesotaenium endlicherianum" [20]; unicells with half-pipe-like chloroplasts and an aquatic life style) and Mougeotiopsis calospora, strain MZCH580. Although these species differ markedly in growth form (unicells vs. filaments), their chloroplasts are all characterized by indented or undulated margins [19,20] that are otherwise rare in zygnematophytes. Yet, Mougeotiopsis calospora is the only known zygnematophyte that lacks pyrenoids.…”

“…In our phylogeny, the branch in question comprises three distinct groups of organisms: Mougeotiopsis calospora (one strain known), the genus Serritaenia (several strains known; [19]), and strain SAG 12.97, a unicellular zygnematophyte that is often referred to as “ Mesotaenium endlicherianum ”. Currently, there is only one existent ordinal name that is based on the mentioned taxon names, namely Mesotaeniales Fritsch.…”

Temperature- and light stress adaptations in Zygnematophyceae: The challenges of a semi-terrestrial lifestyle

“…The purple morphotype was better protected against high irradiation than the green morphotype, indicating an additional photoprotective function of the pigment ( Aigner et al, 2013 ; Herburger et al, 2016 ). Pigments with UV-screening ability can also be stored in extracellular mucilage, as shown in the newly described zygnematophycean genus Serritaenia ( Busch and Hess, 2022 ). This specific strategy has not so far been found in other Zygnematophyceae and differs from the accumulation of reddish, water-soluble pigments in the vacuoles, as found, e.g., in Z. ericetorum ( Aigner et al, 2013 ; Herburger et al, 2016 ).…”