# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 3 Homeoviscous Adaption
### Temperature  
_Sulfolobus_ spp. are thermophilic organisms with an optimal cultivation temperature of about 80 \({}^{\circ}\)C. In order to ensure or control membrane fluidity and permeability at these harsh cultivation conditions, Archaea developed adaption strategies. The archaeal cell membrane can be distinguished from the bacterial due to characteristic ether bonds in the membrane lipids. Numerous thermophilic Archaea also combine the presence of tetraether lipids in their membrane. However, _Methanopyrus kundleri_, a hyperthermophilic species, growing at 90 \({}^{\circ}\)C, only contains diether lipids in its membrane, contradicting the proposal that GDGT is needed for survival at high temperatures. However, no further \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Cultivation Conditions**} & \multicolumn{3}{c}{**Modifications**} \\ \cline{2-4}  & **Ring Index (RI)** & **DEL:TEL Ratio** & **Headgroups** \\ \hline \multirow{2}{*}{**Temperature**} & \multirow{2}{*}{positive [42; 46]} & impacted, but no distinct & impacted, but no distinct \\  & & trend determined [42] & trend determined \\ \hline \multirow{2}{*}{**pH**} & \multirow{2}{*}{not investigated} & \multirow{2}{*}{not investigated} & calditol important for \\  & & & acid resistance [39] \\ \hline \multirow{2}{*}{**Growth Rate**} & \multirow{2}{*}{Negative [43; 78; 79]} & negative [43] & \\  & & impacted, but no distinct & impacted, but no distinct \\ \multirow{2}{*}{**Temporary**} & \multirow{2}{*}{positive [42; 46]} & trend determined [42] & trend determined [42; 43] \\ \hline \end{tabular}
\end{table} 
Table 2: Correlation between changes in temperature, pH and growth rate and the responses ring index (RI), diether to tetraether lipid ratio (DEL:TEL ratio) and headgroups in _Sulfolobus_ spp. exceptions like _M. kandleri_ have been discovered yet. Due to the low phase-transition temperature of archaeal membranes, they remain in liquid crystalline phase from 0 \({}^{\circ}\)C to 100 \({}^{\circ}\)C [80; 81; 82]. Already, in 1980, a connection between temperature and ether lipid composition was observed in _S. solfataricus_, then known as _Caldariella acidophila_. In this early research, a trend of increased incorporation of cyclopentane rings in TELs with rising cultivation temperatures was observed. The RI increased from around 2 to 2.5, in respect to a temperature change from 75 \({}^{\circ}\)C to 89 \({}^{\circ}\)C. Interestingly, GDNT was stronger affected by cyclization processes than GDGT. IP-GDGTs were minimally affected by temperature with regards to the average cyclization [46]. For two other species, _S. islandicus_ and _Sulfolobus tokodaii_, the temperature dependent lipid composition was also investigated. In both species the lipid species differed for the applied growth conditions. Albeit, no clear connection between temperature and the presence of specific lipid classes varying in the polar headgroups was observed. Generally, more cyclopentane rings embedded in the GDGT lipid occurred, with increasing temperature in both species. GDNT could not be detected due to a lipid standard issue. Hexose-GDGT-inositolphosphate's RI increased from 3.7 to 4.5 in _S. islandicus_ and from 4.7 to 5.9 in _S. tokodaii_, as a result of a 10 \({}^{\circ}\)C increase. In contrast to the above-mentioned study with _S. solfataricus_, the same trend was observed in IP-GDGT, with a similar RI shift. Sulfono-Hex3-GDGT-IP cyclization showed the least changes. The RI in _S. tokodaii_ is generally higher than in _S. islandicus_, which can also be a result of the lower cultivation pH of 3 rather than 4, respectively [42]. Nontheless, apparently there are significant difference in the average cyclization number in Sulfolobales (_S. solfataricus_: RI = 1.74 [46], _S. acidocalarius_: RI = 4.6 [43]). Different DEL:TEL ratios were observed in _S. tokodaii_ and _S. islandicus_, in respect to the applied variations of temperatures. However, no clear trend was detected [42], as it is found in the hyperthermophilic archaeon _Thermococcus_ spp. Both _T.