Abstract
The alga Chlamydomonas nivalis thrives in polar snow fields and on high-altitude mountain tops, and contributes significantly on primary production in the polar regions, however, the mechanisms underlying this adaptation to low temperatures are unknown. Here, we compared the growth, photosynthetic activity, membrane lipid peroxidation, and antioxidant activity of C. nivalis with those of the model alga C. reinhardtii, under grow temperature and low temperatures. C. nivalis maintained its photosynthetic activity in these conditions by reducing the light-harvesting ability of photosystem II and enhancing the cyclic electron transfer around photosystem I, both of which limited damage to the photosystem from excess light energy and resulted in ATP production, supporting cellular growth and other physiological processes. Furthermore, the increased cyclic electron transfer rate, carotenoid content, and antioxidant enzyme activities jointly regulated the reactive oxygen species levels in C. nivalis, enabling recovery from excess excitation energy and reduced photooxidative damage to the cell. Therefore, we propose a model in which adaptive mechanisms related to photosynthetic regulation promote the survival and even blooming of C. nivalis under polar environment, suggesting that C. nivalis can provide organic carbon sources as an important primary producer for other surrounding life in the polar regions for maintaining ecosystem.
Highlights
The eukaryotic and prokaryotic single-celled organisms known as microalgae are important inhabitants of ocean, freshwater, and even terrestrial ecosystems (Msanne et al, 2012; Li et al, 2016)
C. reinhardtii cultured at 4◦C didn’t die for a long time, it consistently failed to grow (Figure 1C). This indicates that C. nivalis adapted to the low temperatures, but C. reinhardtii was more seriously affected by the cold
For determining the change of cyclic electron transfer (CET) more intuitively, the electron transfer rate of CET was measured using oxygen electrode by providing electron donors and receptors, and the results clearly showed that the CET of C. nivalis increased at low temperatures (Figure 3C). 77 K fluorescence emission spectra of C. nivalis at 22◦C showed a major peak at 722 nm (F722), which corresponded to photosystem I (PSI), and a smaller peak at 687 nm (F687), which originated mainly from photosystem II (PSII) (Figure 6)
Summary
The eukaryotic and prokaryotic single-celled organisms known as microalgae are important inhabitants of ocean, freshwater, and even terrestrial ecosystems (Msanne et al, 2012; Li et al, 2016). Microalgae are ideal model organisms due to their fast growth, strong adaptability to extreme environments, and high oil contents. Most of the algae in polar snow algal community belong to Chlorophyceae, Chlamydomonadales or Volvocales, some. C. nivalis, belonging to the genera Chlamydomonas (Chlorophyta) and closely relating to the model algae Chlamydomonas reinhardtii, is a typical snow alga and a leading system for investigating cold adaptation (Cvetkovska et al, 2016). C. nivalis produces substantial biomass even under extreme conditions, such as low temperature, high light, low pH, nutrient deficiency, freeze-thaw cycles, and UV irradiation, and serves as a vital food source for other cold-adapted organisms, such as ice worms, collembola, and bacteria (Thomas and Duval, 1995; Ursula et al, 1996; Painter et al, 2001)
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