Abstract
Melting mountainous snowfields are populated by extremophilic microorganisms. An alga causing orange snow above timberline in the High Tatra Mountains (Poland) was characterised using multiple methods examining its ultrastructure, genetics, life cycle, photosynthesis and ecophysiology. Based on light and electron microscopy and ITS2 rDNA, the species was identified as Chloromonas krienitzii (Chlorophyceae). Recently, the taxon was described from Japan. However, cellular adaptations to its harsh environment and details about the life cycle were so far unknown. In this study, the snow surface population consisted of egg-shaped cysts containing large numbers of lipid bodies filled presumably with the secondary carotenoid astaxanthin. The outer, spiked cell wall was shed during cell maturation. Before this developmental step, the cysts resembled a different snow alga, Chloromonas brevispina. The remaining, long-lasting smooth cell wall showed a striking UV-induced blue autofluorescence, indicating the presence of short wavelengths absorbing, protective compounds, potentially sporopollenin containing polyphenolic components. Applying a chlorophyll fluorescence assay on intact cells, a significant UV-A and UV-B screening capability of about 30 and 50%, respectively, was measured. Moreover, intracellular secondary carotenoids were responsible for a reduction of blue-green light absorbed by chloroplasts by about 50%. These results revealed the high capacity of cysts to reduce the impact of harmful UV and high visible irradiation to the chloroplast and nucleus when exposed at alpine snow surfaces during melting. Consistently, the observed photosynthetic performance of photosystem II (evaluated by fluorometry) showed no decline up to 2100 μmol photons m–2 s–1. Cysts accumulated high contents of polyunsaturated fatty acids (about 60% of fatty acids), which are advantageous at low temperatures. In the course of this study, C. krienitzii was found also in Slovakia, Italy, Greece and the United States, indicating a widespread distribution in the Northern Hemisphere.
Highlights
Long-lasting, melting snow is an extremophilic habitat for specialised phototrophic eukaryotes (Hoham and Remias, 2020)
In the High Tatras, the Sarntal Alps and the Northern Pindus mountain range, spots of orange snow were found in June and July at elevation from 1862 to 2240 m a.s.l., all of them situated above timberline (Table 1)
The surface was dominated by “black snow” typical for this region (Kol, 1966). This blackish snow was removed before harvest, impurities were still present in the sample collected in subsurface layers (Figure 1)
Summary
Long-lasting, melting snow is an extremophilic habitat for specialised phototrophic eukaryotes (Hoham and Remias, 2020). Snow algae cause the macroscopic phenomenon of coloured snow. Such blooms frequently occur in combination with abundant intracellular secondary carotenoids, altering the snow colour from green to red. This has an impact on snowmelt due to a significant albedo reduction, as shown in Polar (Onuma et al, 2016; Gray et al, 2020; Khan et al, 2020) and Alpine regions (Di Mauro et al, 2020). There are not many works about snow dwelling Chloromonas that cause colours other than red (Hoham et al, 2006; Remias et al, 2010; Procházková et al, 2019b)
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