| The South Pole is one of the coldest regions on the earth, and its basic characteristics are the extemely low temperatre. There is a large number of salt capsule and channel in the sea ice of South Ocean, so its dramatic changes of temperature and salinity will cause great damage to organisms. Antarctic sea ice algae is the typical microalgae living in extreme environments, such as the sea ice, sea ice edge, and seawater, which are the prominent primary producers in the Antarctic regions. Antarctic sea ice algae, have formed an unique physiological and biochemical mechanisms to adapt to the extreme environment. Under the extreme condition, it is a crucial challenge to maintaining the optimum membrane fluidity for microorganisms. Moreover, the compositions of plasmamembrane plays an important role in regulating membrane fluidity.Antarctic sea ice algae, Chlamydomonas sp. ICE-L, is a typical unicellular microalgae, which can directly respond to the surrounding environment at cellular levels. Because of the special physiological characteristics,it has been used as themodel organism to study the polar extreme environments. Therefore, we studied the adaptability of the plasma membralle in Antarctic sea ice microalgae Chlamydomonas sp. ICE-L to low temperature and high salinity.These results will help the reasonable exploitation of Antarctic ice microalgae,the increase of available microbial resources,discovery of valuable active substances and products, and the further development of fundamental researches.This paper stuied the effects of temperature and salinity on cell membrane fluidity by using Fluorescent Probe and the fatty acids contents and composition of the membrane by using GC-MS. Then, we analyzed the relationships between the membrane fatty acids and membrane fluidity. The results showed that the membrane fluidity of Chlamydomonas sp. ICE-L increased at both low temperature and high salinity conditions, which could maintain the normal function of the cell membrane and the stability of internal environment. With the increase of salinity and decrease of temperature, the amount of membrane unsaturated fatty acids(UFAs) increased. C20: 5, C20: 3 and C18: 3 were the main UFAs. There was a relationship between the cell membrane fluidity and the UFAs. Overall, fatty acids(FAs) play the major role to regulate the membrane fluidity by desaturation, isomerization and changes of acyl chain length.According to the transcriptome sequencing of Chlamydomonas sp ICE-L, 75,805 functional gene sequences were obtained. The received fatty acid desaturase genes(FADs) were cloned by conventional PCR. The transcriptional regulation mechanism of five key fatty acid desaturase were studied with real-time quantitative PCR technique.The results showed that the fatty acid desaturase gene regulation patterns varied at different temperatures and salinity conditions. Δ9CiFAD, as the rate-limiting enzyme in the conversion of saturated fatty acids(SFAs) to UFAs,inserted into SFA at the first double bond.The expression of Δ9CiFAD was increased rapidly at low temperature and high salt stress. The expression of Δ6CiFAD, ω3CiFAD1 and ω3CiFAD2 maintained at a high level at low temperature(-20°C and 0°C), which indicated that they could protect algal cells at low temperature. When coped with low temperature and high salt stress, the expression regulation of Δ12CiFAD and Δ6CiFAD delayed. Δ9CiFAD, Δ6CiFAD, and ω3CiFAD1 expressed more sensitively at high salt stress, whereas Δ12CiFAD and ω3CiFAD2 reacted more actively at low salinity. That was because that Chlamydomonas sp. ICE-L could produce more UFAs by increasing the fatty acid desaturase gene and fatty acid desaturase to maintain membrane fluidity under environmental stress.We explored the relevance between metabolic pathways of unsaturated fatty acids and membrane fluidity by cloning and analyzing the FADs and determined the FA desaturation metabolic pathway of Chlamydomonas sp. ICE-L, which would help to understand the functions of FADs and fatty acids compositions in response to temperature and salinity stress. |
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