Saturday, December 28, 2013

Chlamydomonas nivalis: Its Psychrophilic Physiology and “Red Snow”

By Ben Petrie

Since more than 2000 years ago explorers have marveled at “red snow,” a snow notable for its characteristic tinge that is typically encountered in the alpines or the polar regions of the world (Remias, Lütz-Meindl, & Lütz, 2005). With recent advances in molecular biology scientists have determined that of all the microorganisms that exist within these habitats, snow algae are responsible for this phenomenon, as well as similar green, yellow, orange, and grey tinges in the snow (Remias et al., 2005). These colors have been attributed to a variety of carotenoids within the snow algae, which are pigmented molecules that are commonly utilized for obtaining light energy at a range of wavelengths during photosynthesis (Kvíderová, 2012; Remias et al., 2005). Although, these carotenoids are believed to play an additional role in the snow algae, as their presence has been implicated in enabling snow algae to resist cold (Remias, Karsten, Lütz, & Leya, 2010).
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The snow algae are psychrophiles, capable of thriving at the freezing temperatures of the frigid tundras of the world. Of these algae, the genus Chlamydomonas makes up most of the pigmented microbes in “red snow” (Remias et al., 2005). Since the physiologies that snow algae maintain to tolerate this environment likely differ between different microorganisms, the studied eukaryote Chlamydomonas nivalis will be the focus of this blog post. C. nivalis often contains large amounts of carotenoid-associated vesicles in its cytoplasm. Of these pigments in C. nivalis, the most common carotenoid is astaxanthin, which has been attributed to the presentation of “red snow” (Hoham & Duval, 2001). The interesting connection between the caroteniod-associated vesicles and C. nivalis’ psychotropic physiology is that C. nivalis tends to dramatically increase production of these vesicles and carotenoids when it forms hypnoblasts, an immobile form that is resistant to cold temperatures (Hoham & Duval, 2001; Remias et al., 2005). These hypnoblasts contain large quantities of unsaturated lipids on both their intra and plasma membranes, which makes them better able to avoid solidifying at low temperatures (Remias et al., 2005). The function of the associated carotenoids is less clear, and this will be a major discussion point of this post.
            Researchers were curious about astaxanthin in C. nivalis, so they set out to determine how C. nivalis hypnoblast cells’ rate of photosynthesis is affected by temperature and hypothesized how astaxanthin fits in to the results (Remias et al., 2005). These researchers subjected C. nivalis in its hypnoblast form to a variety of temperatures and measured its rate of photosynthesis. As a result, they found that C. nivalis actually had a significantly higher photosynthetic rate at moderate temperatures than the low temperatures it would typically be found within in its hypnoblast form (Remias et al., 2005). I thought this was curious: why would an microbe that is essentially just trying to survive stressful conditions be so well suited to less stressful moderate temperatures? Clostridium, for example, forms highly resistant, inactive spores in harsh conditions that germinate under more permissive conditions (Wiegel, Tanner, & Rainey, 2006). Clearly this is not the case with C. nivalis, but why would such adaptability and activity be favorable for this stressed microbe? The answer to this question lies in the specific environment that C. nivalis exists in. In the arctic, the sun tends to melt the top layer of snow during the day. This top layer of melted snow refreezes during the night (Remias et al., 2005). This constant cycle of melting and freezing creates a significantly different microbial habitat than one that is constantly cold. While in a consistently cold environment a microorganism would likely be better off maintaining low activity (or even no activity as it waits for better conditions to arise) to survive, C. nivalis is able to take advantage of exposure to the sun (using the energy for photosynthesis) during the thaw cycles (Remias et al., 2005). The researchers connect the photosynthetic capabilities of hypnoblasts to carotenoids by stating that the carotenoids protect the photosynthetic apparatus by absorbing harmful light rays like UV rays (Remias et al., 2005). This made some sense to me, as carotenoids are known to fulfill this protective function as well as absorbing different wavelengths for photosynthesis. But, why ramp up production of these carotenoids after forming a hypnoblast and not before? Don’t C. nivalis vegetative cells need protection from harmful sunrays as well?
            Although I do not have definitive answers to these questions, information covered in this next paper provides some food for thought regarding this topic. 5 years after the previously discussed paper, researchers from that paper again looked at C. nivalis hypnoblast cells. However, this time they were concerned mainly with how the hypnoblasts as a whole present under stressful conditions (Remias et al., 2010). The researchers noted that upon forming the hypnoblasts the cells grew about 30 % in size, lost their flagella, and stopped dividing completely. The researchers also found that in addition to the increased number of carotenoid-associated vesicles in hypnoblast compared to vegetative cells, the chloroplasts also decreased in size significantly. Other cytoplasmic changes indicated a decreased metabolism, including alterations to the nucleus, ER, and Golgi (Remias et al., 2010). Generally speaking, these changes are indicative of reduced activity, which is consistent with how most microbes function in a stressful environment. Again, the researchers assert that the main function of secondary carotenoids like astaxanthin is protection from harmful radiation such as UV radiation (Remias et al., 2010). However, the question still remains: why the massive increase of secondary carotenoids in hypnoblasts from vegetative cells? As stated above, there are significant changes to the host cytoplasm following the hypnoblast transformation, one of which being the formation of numerous large vesicles. These vesicles take up most of the cytoplasm, and one seemingly significant benefit to using astaxanthin as a protective pigment is that it forms mono-esters with the unsaturated fatty acids that make up the vesicles, which causes astaxanthin to associate tightly with the vesicles (Remias et al., 2010). Therefore, it would make sense that astaxanthin (and potentially other similar carotenoids) is an effective protective pigment in hypnoblasts because it packs efficiently in an environment where space is limiting. Other researchers have hypothesized that since certain proteins that protect the cell from harmful radiation are produced at high concentrations only when a cell is photosynthetically active, secondary astaxanthin fills this void for the cell when it is not photsynthetically active in its hypnoblast form (Hoham & Duval, 2001).
            In situations like these where there are only a handful of primary literature papers on the topic of interest there are plenty of questions and few answers regarding the topic. There seems to be a lot different processes at work when C. nivalis forms its cold resistant hypnoblast form, and the literature has only scratched the surface. One of the fun things about these circumstances is that there is plenty of room for hypothesizing, and, with adequate resources, testing these hypotheses. There seems to be a tight link between the “red snow” first observed by explorers more than 2000 years ago and the ability of C. nivalis to persist as a psychrophile, but further research is required to truly uncover this mysterious link.

References:

Hoham, R., & Duval, B. (2001). Microbial Ecology of Snow and Freshwater Ice with Emphasis on Snow Algae. Snow ecology: an interdisciplinary examination of …. Retrieved from http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Microbial+Ecology+of+Snow+and+Freshwater+Ice+with+Emphasis+on+Snow+Algae#0
Kvíderová, J. (2012). Research on cryosestic communities in Svalbard: the snow algae of temporary snowfields in Petuniabukta, Central Svalbard. Czech Polar Reports, 2(1), 8–19. doi:10.5817/CPR2012-1-2
Remias, D., Karsten, U., Lütz, C., & Leya, T. (2010). Physiological and morphological processes in the Alpine snow alga Chloromonas nivalis (Chlorophyceae) during cyst formation. Protoplasma, 243(1-4), 73–86. doi:10.1007/s00709-010-0123-y
Remias, D., Lütz-Meindl, U., & Lütz, C. (2005). Photosynthesis, pigments and ultrastructure of the alpine snow alga Chlamydomonas nivalis. European Journal of Phycology, 40(3), 259–268. doi:10.1080/09670260500202148


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