Chlamydomonas nivalis
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| Chlamydomonas nivalis | |
|---|---|
 | |
Scientific classification  | |
| Kingdom: | Plantae |
| Division: | Chlorophyta |
| Class: | Chlorophyceae |
| Order: | Chlamydomonadales |
| Family: | Chlamydomonadaceae |
| Genus: | Chlamydomonas |
| Species: | C. nivalis |
| Binomial name | |
| Chlamydomonas nivalis | |
| Synonyms[1][2][3] | |
Chlamydomonas nivalis, commonly known as snow algae, also referred to as Chloromonas typhlos,[5][4] is a unicellular red-coloured photosynthetic green alga. It is found in the snowfields of the alps and polar regions all over the world and on all seven continents.[7] There are four known genera causing red snow: Sanguina, Chlainomonas, Chlamydomonas, and Chloromonas.[8] Chlamydomonas nivalis is one of the primary algae responsible for causing the phenomenon of watermelon snow (also blood snow, raspberry snow[9]), where patches of snow appear red or pink. The first account of microbial communities that form red snow was made by Aristotle[10]. Researchers have been active in studying this organism for over 100 years.
Although C. nivalis is closely related to Chlamydomonas reinhardtii, the environmental conditions each species inhabits are very different with C.nivalis inhabiting the meltwater in-between the spaces of snow and ice crystals in the snow pack.[11] The habitat of C. nivalis subjects the cells to environmental extremes including limited nutrients, low temperatures, and intense sunlight. In comparison with the mesophilic C. reinhardtii, C. nivalis has special mechanisms that allow it to be cryotolerant and survive on rock surfaces as well as in soil, meltwater, and snow. Secondary carotenoids, a thick cell wall, and particles on the cell wall are some characteristics that protect the cyst from light, drought, and radiation stress. Although the seasonal mobile to dormant life cycle of C. nivalis is complex, it also helps the algae exploit its niche and survive unfavourable conditions. As a result, C. nivalis is one of the best known and studied snow algae. When taking account of the photoprotective effect of its secondary carotenoid, astaxanthin, among the other adaptive mechanisms to its extreme habitat, it can be understood how C. nivalis became so dominant in microbial snow algae communities. Green motile offspring are produced in the spring and throughout the summer. They develop into red dormant cysts, the stage where this organism spends most of its life cycle, as the winter season begins and remain a cyst until the spring.
This alga is an interesting organism for researchers in various fields to study due to its possible role in lowering global albedo, ability to survive in extreme environments, and production of commercially relevant compounds. Additionally, its life cycle is still being studied today in an effort to better understand this organism and amend previous classification errors.
The name Chlamydomonas nivalis is of compound Greek and Latin origin. Chlamydomonas is ultimately derived from the Ancient Greek χλαμύς (khlamús, "cloak, mantle") and μονάς (monás, "solitary"),[12] while nivalis, from the Latin nivālis, translates to 'found growing in or near snow', as this species of algae are only found associated with snow or near snowy areas.[13][14]
Morphology
Red snow has a variety of forms with cell development stages being considered to correspond with cell form.[15] Cell wall structure is typically what differentiate cell types, with smooth, nippled, papillae, and wrinkled being among the most common.[15] Ribbed, rosette, ellipsoid, and star formations have also been observed.[15][16] Cell sizes vary with red spheres being 35.96 ± 4.9 μm in diameter, ellipsoid/oval shaped cells being 20–28 mm long and 11–15 mm wide, and star shaped cells being 40–50 μm.[17][18][15][19] Chlamydomonas nivalis have been observed to have primary and secondary cell walls, where the primary will be shed leaving a smooth cell wall behind.[20] Secondary cell walls tend to constitute to more mature cysts of Chlamydomonas nivalis and being ribbed where as younger cysts were associated with being rounder, lobed, and smooth-walled.[20][21] Furthermore, cell walls can be found to be 66 to 154 nm thick for smooth cell walled cysts.[22]
Dispersal
While snow algae are considered to be a cosmopolitan species and Chlamydomonas nivalis has been observed across bipolar regions, the mechanisms behind their dispersal and migrations are not clear. Additional research across lattitude-ranges of red snow patches on glacial and snow fields will be advantageous in understanding the colonizing habits of snow algae.[23]
Description
The seasonal life cycle of C. nivalis can be broken down to three stages based on the color of the cell as a result of carotenoid composition, which are green, orange, and red.[24] Orange cells and red cells are the most difficult to differentiate as they look similar while the red and green cells are easiest to differentiate as they have more significant differences in composition.[25] Cells at the red stage were previously described as a separate species than the green cells, but were later discovered to be different stages of the C. nivalis' complex life cycle.[2]
Small green coloured motile cells of the young C. nivalis at the green stage are produced in spring or early summer when temperatures are warmer and zygotes undergo meiosis in meltwater pools.[26] The biflagellated cells are slightly oval and about 5-15 μm in diameter.[25][27] In this asexually reproductive phase, the cells are sensitive to temperature and drought stress. They avoid unfavorable light and temperature by swimming in the snow until they reach more optimal conditions.[28][26][24] Chloroplasts of green cells are irregularly shaped.[24] The dominating pigment, chlorophyll, gives the cell its characteristic hue and facilitates maximum cell growth through light absorption. Secondary carotenoid concentrations are much lower at this stage as the cells need photosynthetically active radiation for energy and growth.[25] Cells in the green stage also have less organic and inorganic particles on their surface compared to mature cysts.[28]
Later in the season, when nitrogen and nutrients becomes limited and radiation stress increases, the green cells will develop into flagellated sexual gametes that mate and produce new zygotes that have lost their flagella and are capable of surviving the winter period.[29][27] Transformation into the zygote, or hypnoblast, is characterized by the production and accumulation of reserve materials that include sugars and lipids as well as the formation of esterified secondary carotenoids.[28] The secondary carotenoids will turn the green zygotes orange as they accumulate in the area around the plastids of the cell to protect the zygotes from UV radiation.[28][25] Orange and red spores can be seen throughout the summer. During this stage, the cell wall will also begin to thicken to help the cell tolerate freezing temperatures and UV light.[27][30] In addition, the color of these pigments reduces albedo such that individual cells may melt nearby ice and snow crystals to access limiting nutrients and water in an otherwise unavailable frozen state.[31]
History
The earliest documentation of red snow was made by Aristotle.[32] While he recognized that something must be contributing to the odd colouration, red snow was also commonly mistaken as mineral deposits or pollen up until the early 1900s.[33] In 1819, samples of 'red snow' were brought back for examination with a returning Arctic expedition under Sir John Ross. The samples were sent to Robert Brown and Francis Bauer for examination. Both men came to different conclusions on how to classify the specimens. Brown believed the specimen to be a unicellular alga while Bauer declared it a new species of fungus, Uredo nivalis.[33][1][34] Over the next century, many researchers disputed over whether these organisms were lichen, plants, alga, or animal. It was not until the early 20th century when researchers finally began to agree on the algal nature of the organism and gave its currently known name, Chlamydomonas nivalis.[1][34] In 1968 C. nivalis was officially recognized as a collective taxon.[35] Unfortunately, due to the lack of sequencing techniques, reliance on visually examining similarly looking snow alga, and complicated life cycle of this species, errors continued to be made in classifying this and other species of snow algae. Today, C. nivalis has become one of the most well-studied snow algae. Although its taxonomy is still being settled, the life cycle of this snow algae is now much better understood.[36][1][2][37] The historical disputes about the classification and misclassification of specimens have resulted in a number of names from older publications that all mean to refer to C. nivalis. These are: Uredo nivalis, Sphaerella nivalis, Protococcus nivalis, and Haematococcus nivalis.[1][2][3]
Furthermore, a new genus of snow algae has been proposed, Sanguina, with the predominant red snow algae found being classified as Sanguina nivaloides. Chlamydomonas nivalis is of close relation to Sanguina nivaloides. While using the name Sanguina nivaloides to be synonymous with Chlamydomonas nivalis is of interest, the two cannot be used interchangeably due to there not being a way to prove the historical species are conspecific.[38]