Abstract
SummaryWithin streptophyte green algae Zygnematophyceae are the sister group to the land plants that inherited several traits conferring stress protection. Zygnema sp., a mat‐forming alga thriving in extreme habitats, was collected from a field site in Svalbard, where the bottom layers are protected by the top layers. The two layers were investigated by a metatranscriptomic approach and GC–MS‐based metabolite profiling. In the top layer, 6569 genes were significantly upregulated and 149 were downregulated. Upregulated genes coded for components of the photosynthetic apparatus, chlorophyll synthesis, early light‐inducible proteins, cell wall and carbohydrate metabolism, including starch‐degrading enzymes. An increase in maltose in the top layer and degraded starch grains at the ultrastructural levels corroborated these findings. Genes involved in amino acid, redox metabolism and DNA repair were upregulated. A total of 29 differentially accumulated metabolites (out of 173 identified ones) confirmed higher metabolic turnover in the top layer. For several of these metabolites, differential accumulation matched the transcriptional changes of enzymes involved in associated pathways. In summary, the findings support the hypothesis that in a Zygnema mat the top layer shields the bottom layers from abiotic stress factors such as excessive irradiation.
Highlights
Algae of the genus Zygnema are commonly found in Polar ecosystems, forming extensive mats in shallow pools, meltwater streams or on moist soil surfaces (Kim et al, 2008; Zidarova, 2008; Holzinger et al, 2009; Pichrtová et al, 2018)
We present a combined approach of metatranscriptomics and metabolite profiling in environmental samples of Zygnema sp. collected from the Arctic Svalbard to understand the spatial organization of Zygnema mats in a meltwater streamlet in which the upper layers are already exposed to higher photosynthetically active radiation (PAR) and ultraviolet radiation (UVR)
The starch grains of the cells collected from the bottom layer had a typical appearance with electron dense and electron translucent areas (Fig. 2B), whereas the starch grains surrounding the pyrenoids from the top layer of the mat had an irregular shape and the central contents appeared fibrillosus, which can be regarded as a degradation product (Fig. 2C)
Summary
Algae of the genus Zygnema are commonly found in Polar ecosystems, forming extensive mats in shallow pools, meltwater streams or on moist soil surfaces (Kim et al, 2008; Zidarova, 2008; Holzinger et al, 2009; Pichrtová et al, 2018) These habitats are characterized by extreme environmental conditions, including high seasonality. To form different specialized cell types such as parthenospores, akinetes and so-called pre-akinetes, which are modified vegetative cells, supports survival under unfavourable conditions (McLean and Pessoney, 1971; Stancheva et al, 2012; Herburger et al, 2015) This is especially important in polar environments, as the formation of diploid zygospores is an extremely rare event in Zygnematophyceae from Arctic regions (Elster et al, 1997; Pichrtová et al, 2018). The top layers of a Zygnema mat do not exhibit substantial bleaching when dried out, and some cells die, most of the population is subsequently converted into vegetative pre-akinetes (Holzinger et al, 2009)
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