Colonization of Land by Conjugating Green Algae

Conjugating green algae (Zygnematophyceae) are considered to be closest algal relatives to land plants (Embryophyta). Several members of this group typically grow in an aeroterrestrial environment that allowed colonization of land. In Polar or high Alpine regions, some filamentous Zygnematophyceae often dominate and typically produce relatively large amounts of biomass. Therefore, they are ideal model organisms for studying stress tolerance mechanisms connected with transition to land. The central goal of the project is to understand strategies that allowed plants to colonize land. It is known that Zygnematophyceae have the ability to form particularly tolerant permanent stages, akinetes, derived from vegetative cells. Within this project naturally occurring stress scenarios (desiccation, freezing, UV irradiation) will be simulated under simplified but monitorable laboratory conditions. Members of the genera Zygnema, Zygnemopis and Zyogonium will serve as research objects. The Czech project partners will characterize strains suitable for the key stress scenarios, using methods of multi gene phylogeny and perform desiccation and freezing stress experiments. The Austrian project partners will perform UV stress experiments and investigate the biochemical composition by a metabolomics approach. Moreover, cell walls will be investigated by immunological methods, with a particular focus on polymers known to act as plasticizers of the cell walls, which is important for desiccation tolerance. For the first time the spectral properties of Zygnematophyceae will be investigated at the subcellular level by hyperspectral imaging, a technique that allows to map the spectral properties in the range from 250 -950 nm. This will further contribute to our understanding of UV tolerance and integrate the biochemical with the structural findings. The complimentary expertise of the Czech and Austrian project partners will gain an added value resulting from the bilateral research project. The gained results will bring new insight into survival strategies of Zygnematophyceae and will be analyzed in an evolutionary and global change context.

The bilateral project investigated conjugating green algae (Zygnematophyceae) currently believed to be closest algal relatives to land plants (Embryophyta). These filamentous algae grow at aeroterrestrial environments that allowed colonization of land. Zygnema sp. and near relatives from polar and high Alpine habitats were investigated to study their phylogenetic variability, their stress tolerance to various naturally occurring stress scenarios (desiccation, freezing, UV irradiation). The variability of Zygnema sp. from Svalbard was studied after a joint expedition of the Austrian and the Czech research partners. 12 different arctic genotpyes could be described according to molecular and morphological data. Zygospores resulting from sexual reproduction were rare, but vegetative filaments formed permeant stages, termed akinetes. These akinetes were tested for their metabolite composition, and a shift in fatty acids could be found during this transition process. Particularly oleic and linoleic acid were enriched in the aktines, whereas metabolites related to primary metabolic processes were reduced. When subjected to desiccation stress, transcriptomic changes were described, where a much higher transcript regulation was observed in young cultures when compared to akinetes, suggestion that an acclimation process was responsible for this change. When exposed to experimental UV irradiation in a sun-simulation system, particularly the young cells showed a flexible reaction to this stress. While young cells could not recover from freezing down to - 8 C, akinetes tolerated freezing down to - 26 C and could recover their physiological activity afterwards. In a combined transcriptomic and metabolite profiling of Zygnema sp. field samples, it was found that the upper layers of a mat, exposed to the environmental factors had a much higher transcript regulation than the protected lower layers of a mat. Stress related metabolites were up-accumulated in the upper layer, enabling their survival under exposed conditions. The complementary expertise of the Austrian and the Czech research partners allowed to study many different aspects of Zygnematophycean cell physiology and structure. Taken together, our observations lead to the conclusion that Zygnematophyceae are well prepared to withstand harsh environmental conditions that allowed them to conquer land and ultimately gave rise to all land plants.