# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 3 Homeoviscous Adaption
### Growth Rate  
Recent studies by [PERSON] et al. [42], [PERSON] et al. [79], [PERSON] et al. [78] and [PERSON] et al. [43] confirm that growth phase/rate also have an effect on the lipid composition of Sulfolobales. [PERSON] et al. observed that the lipid compositions of _S. islandicus_ and _S. tokodai_ not only changed with the cultivation temperature, but also with the growth phase. A distinction was drawn between lag, exponential and stationary phase. Although the presence of the 3 investigated lipid classes changed with the growth phases, it was not possible to identify a clear trend between the occurrence of each lipid class and the different growth phase and no apparent connection was found between the changes in growth phase and the increasing and decreasing DEL:TEL ratios. The average cyclization of the total lipid extracts underwent a dip from lag phase to exponential phase, followed by an increase in the stationary phase. A possible explanation is the overwhelmed machinery of cyclization incorporation or synthesis at such high growth rates [42]. A different experimental approach was taken by [PERSON] et al. _S. acidocalarius_ was deprived of its nutrients and thus the growth rate was reduced. The survival rate and the cytoplasmic pH, as well as the membrane composition, were monitored. A longer depletion led to a lower viability and a decrease in the cytoplasmic pH. Upon nutrient depletion, the average cyclization number stayed the same with an RI of 3.4, whereas when cultivated with nutrient rich medium the ring index slightly decreases. The amount of GDGT with 3 cyclopentane rings clearly increased with time when the cells were not nutrient deprived, while the same lipid class decreased in starved cells. This probably is part of the stress response, making the membrane less permeable by tighter packing. The following decrease in RI in the cells supplied with nutrient rich medium could be due to the fact that the cells were still in the exponential phase at the time point that the different conditions were applied. Harvesting and subsequent transfer into nutrient rich and depleted medium was performed at an OD\({}_{600}\) of 0.4 to 0.5, which might suggest that, at this time point, the cells were still in the exponential phase and the previously described drop in average cyclization still took place. A slight decrease in the DEL amount upon nutrient depletion was observed, as shown in their Supplementary Material [79]. [PERSON] et al. conducted continuous cultivations and regulated the growth rate via the feed addition while a constant temperature, pH and aeration rate was given [78]. Slower growth rates increased the cyclization, which is in conformity with the experiments conducted with the nutrient depleted medium by [PERSON] et al. [79] The RI changed from 2.0 in the highest applied growth rate of \(\mu=0.069\) h\({}^{-1}\) to 3.2 when a slow growth of \(\mu=0.01\) h\({}^{-1}\) was set. The amount of GDGT-4 to 6 increased with the decreasing growth rate. This confirms the implication that the availability of nutrients decreases the membrane packing and thus increases the membrane permeability towards ions, and vice versa. The tighter membrane packing and thus decreased membrane permeability at lower growth rates is important for reducing proton leakage. Hence, the pH homeostasis is maintained, which is vital for this thermoacidophilic archaeon. The amount of DEL and thus the DEL:TEL ratio was not discussed in this publication [78]. Similar results were obtained by [PERSON] et al., in a continuous stirred tank reactor with 2.0 L culture volume. Two different growth rates, \(\mu\) of 0.011 and 0.035 h\({}^{-1}\), were employed by controlling the feed rate. At the lower growth rate, the RI increased to 5.1 when compared to the RI of 4.6 in the higher growth rate [43]. This negative correlation is in accordance with the findings of the previously mentioned research by [PERSON] et al. [78] and [PERSON] et al.