Effect of temperature on the composition and distribution of archaea and its membrane lipids

ABSTRACT

Temperature has a vital effect on the survival of microorganisms. Microorganisms synthesize diverse lipid structures with widely varying biophysical properties for adapting those changing conditions because the membrane lipids play a key role in energy conservation and the maintenance of homeostasis. Temperature not only affects the microbial diversity, but has a significant impact on the composition and degradation of lipids. The predominant core lipids (CLs) of archaea are isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) and bacteria can synthesize branched GDGTs (brGDGTs). H-shaped iGDGTs, also called glycerol monoalkyl glycerol tetraethers (iGMGTs), are a unique group found in several archaea. In living archaea, iGDGTs occurs with polar head groups, such as monohexose (MH) or dihexose (DH). Upon cell death, most of these intact polar lipids (IPLs) are transformed to CLs via hydrolysis of the polar head groups. Both pure culture and environmental surveys indicate that the number of cyclopentyl rings (Ring Index, RI) per iGDGT correlates with the temperature of their living environment. To study the effect of temperature on the microbial diversity and its membrane lipid composition, three sedimentary cores, named core1 (20cm, 64.2-90.8°C), core2 (36cm, 42.3-72.6°C) and core3 (28cm, 66.5-73.2℃) were sampled from Tengchong hot spring in Yunnan Province, China. We detected iGDGTs-0~8, crenarchaeal and its isomer (crenarchaeol’), iGMGTs-0~5 and brGDGT-Ia~IIIc of CLs and MH/DH-iGDGTs-0~4, MH/DH-crenarchaeal and MH/DH-crenarchaeal’ of IPLs in three cores. Then we calculated RI_i of iGDGTs, RI_H of iGMGTs and RI_IPL of MH-iGDGTs. For all three cores, the absolute abundances of archaeal and bacterial lipids decrease with increasing temperature. However, the relative abundances of archaeal lipids increase with temperature. It means microbial diversity decreases in high-temperature environment, while archaea are more adaptable than bacteria. For lipids, temperature had no significant effect on RI_i and RI_IPL in core1 and core2, but they had positive correlation in core3. RI_H had no correlation with temperature in core1 and core3. In core2, RI_H is almost zero above 14cm where temperature increases with depth, while the high value appears in the rest of deeper layers where temperature doesn’t change. The reason of those phenomena may be that: (1) the composition and distribution of microorganisms and their tetraether lipids are influenced by other environmental factors except temperature; (2) different layers have different microbial diversity so that they have different membrane lipids. Besides, we put forward a hypothesis that the influence of temperature on H-RI has a critical temperature value. When the environmental temperature is lower than this value, temperature has no effect on H-RI. Conversely, when the environmental temperature is above this critical value, the effect of temperature on the composition of iGMGTs is unusually significant. As there are few studies on H-RI currently, further studies are needed to confirm this hypothesis or we can find out this critical value in different environment.

Key words: Temperature, Microbial diversity, membrane lipids, Ring index