# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 2 Cell Membrane and Lipids of _Sulfolobus_ Spp.
### Effect of Cyclopentane Rings, DEL:TEL Ratio and Length of Carbon Chain on the Archaeal Membrane  
The effects of cyclopentane rings on the properties of archaeal membranes is usually studied in vitro, by investigating liposomes formed of the respective diether lipids (DEL) and tetraether lipids (TEL).  
TELs, the main constituents of creenarchaeal membranes, can exhibit either GDGT or GDNT as hydrophobic core. Up to eight cyclopentane rings can be incorporated within this core [60; 61]. Differential scanning calorimetry and perturbation calorimetry were used to determine the effect of the incorporation of cyclopentane rings. Findings in liposomes made with extracted GDGT and GDNT lipids of _S. acidocalarius_ suggest that a higher number of cyclopentane rings in the hydrophobic core affects the membrane packing and increases the transition temperature. Albeit, the volume of the membrane is relatively constant between liposomes containing low and high amounts of cyclopentane rings [62]. In general, the compressibility, the relative change of volume in response to a change in environment conditions [63], of tetraether liposomes seems to be lower when compared to diester liposomes, meaning that TEL liposomes are less influenced by stressors such as temperature [64]. These experimental findings are backed by _in silico_ results. Molecular modeling proposed that the incorporation of cyclopentane rings results in a tighter and more rigid membrane [65]. However, it was shown that the isothermal compressibility values increase with the number of cyclopentane rings in a nonlinear manner. The most tightly packed membrane was not observed at the highest number of rings, but at the optimal growth temperature of the organism [62]. These results indicate that cyclopentane rings might not be the only aspect affecting the rigidity and packing of archaeal membranes [62; 65; 66].  
Diether lipids, like diester lipids, form bilayers, while tetraether lipids can span the lamellar structure, and thus, form monolayers [1]. Archaeosomes constructed from total polar lipid extracts of methanogens and halophiles, mainly consisting of diether lipids, are less stable against extreme pH treatment, measured by entrapped radioactive compounds, when compared to archaeosomes created from lipids extract of _T. acidophilium_. The incorporation of TEL increases the stability towards high temperature, pH treatment and serum proteins [67]. Liposomes with a high TEL content could even endure sterilization (121 \({}^{\circ}\)C, 1.034 bar, 15 min). There is a clear relationship between the leakage rate of a fluorescent dye and TEL presence in the membrane. Albeit, archaeosomes containing TEL or DEL can endure enzymatic treatment with pancreatic lipase [68; 69]. In general, the increase of TEL content in the membrane aids the flux regulation of solutes and protons [70].  
The chain-length variation of the hydrophobic lipid core is typical for Bacteria and Eukaryotes as a response to environmental stress [71]. Hereby, the difference between altering the length of hydrocarbon chains and changing from DEL to TEL has to be distinguished. While switching from diether to tetraether is a process in which two diethers are conjoined together, Bacteria alter their length of fatty acid chains; e.g., switching from C\({}_{16}\) to C\({}_{18}\)[72]. Membranes of alkaliophilic Archaea are composed of a mixture of C\({}_{20\text{-}20}\) and C\({}_{20\text{-}25}\) diether lipids, resulting in higher permeability and fluidity at low temperature. However, establishing a connection between this ratio and the framework homeoviscous adaption has not been possible yet [70; 73].  
In addition to high temperature adaption, methanogens, like _Methanococcoides burtonii_, can adapt to low temperature by unsaturation, forming double bonds in the hydrocarbon chain [74], a feature that appears to be absent entirely in Sulfolobales.