Cysts of the Snow Alga Chloromonas krienitzii (Chlorophyceae) Show Increased Tolerance to Ultraviolet Radiation and Elevated Visible Light

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.

Clarification of Most Relevant Concepts Related to the Microalgae Production Sector

“…Generally, microalgae, including cyanobacteria, are aquatic organisms, growing in fresh-, brackish-and saltwater, up to very high salinities. Some species can grow on rocks and other surfaces intermittently exposed to even very little moisture, and many microalgae species can survive for long periods, even hundreds of years, as resting stages or cysts, in extreme conditions, in soils, sediments, dry environments and low temperatures [11,12]. Seaweeds live strictly in seawater, and they can survive in littoral areas only if exposed to daily tides.…”

Differential Responses to UV-A Stress Recorded in Carotenogenic Microalgae Haematococcus rubicundus, Bracteacoccus aggregatus, and Deasonia sp.

“…Protection of a photosynthetic cell against UV damage includes enzymatic systems for DNA reparation, chemical reaction of glutathione and ascorbate toward ROS neutralization, non-photochemical quenching of the excited Chl states, optic shielding by sunscreens such as phenolics, mycosporine-like amino acids (MAAs) and carotenoids [ 2 , 4 , 5 , 6 , 7 , 15 , 16 , 17 ]. Particularly, photolyases are activated by radiation in the range of 370–450 nm [ 2 ].…”

“…Most of data on the effect of UV on PSA were obtained from the experiments with model organisms, such us Pisum sativum , Spinacia oleracea , Arabidopsis thaliana , or Chlorella spp. [ 2 , 3 , 9 , 10 , 15 , 19 , 21 ]. There are three main experimental strategies to study UV effect on phototrophs: outdoors culturing with natural light, outdoor culturing with addition of UV and culturing in artificial controlled conditions [ 8 ].…”

“…There are three main experimental strategies to study UV effect on phototrophs: outdoors culturing with natural light, outdoor culturing with addition of UV and culturing in artificial controlled conditions [ 8 ]. Latest include experiments on whole plants as well as plant organs, cell cultures and isolated plastids [ 2 , 8 , 9 , 10 , 11 , 15 ]. Some works were carried out on the cultures of unicellular algae.…”

“…Changes in their pigment composition were evaluated by common approaches of absorbance spectroscopy in the visible range [ 2 , 6 , 10 , 11 , 12 , 14 , 19 , 22 , 23 , 24 ]. Valuable information on primary photochemistry was obtained by the analysis of chlorophyll fluorescence kinetics, O 2 emission and CO 2 assimilation rates [ 1 , 9 , 10 , 11 , 13 , 14 , 15 , 19 , 21 , 22 , 23 , 24 , 25 ]. Some data were obtained with the help of studies of thermoluminescence [ 9 ] and low-temperature fluorescence spectra [ 10 ], electron paramagnetic resonance spectroscopy [ 14 ], D1 protein immunoblotting [ 14 ], analysis of expression of genes of UV response [ 21 , 25 ], etc.…”