# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 1 Introduction  
Membranes are an important barrier between the cell and extracellular space and thus act as a physical barrier as well as a gatekeeper. Their semi-permeable function is vital for the cell's survival. Hence, maintaining membrane permeability and fluidity throughout changes in environmental parameters like temperature, pH, pressure, mechanical stress, energy availability and others, is a key step for ensuring optimal membrane function and thus viability of the cell. Cells can achieve this by altering the lipid composition of their membranes [1]. This adaptive response, termed homeoviscous adaption, was first proposed by [PERSON] in 1974. He investigated the changes in membrane compositions of _Escherichia coli_, in response to increasing growth temperatures [2]. In prokaryotic organisms, it was shown that non-lamellar-forming membrane lipids are essential for specific membrane functions. This non-bilayer structure formation is influenced by growth conditions, such as the uptake of fatty acids and temperature. The phase transition temperature of the membrane lipids is modified by acyl chains and polar head groups, and therefore either a lamellar or non-lamellar phase constituted [3; 4]. A specific ratio between the lipids forming bilayer and non-bilayer was proposed to be maintained in _E. coli_[5; 6]. This phenomenon of organisms altering their membrane lipid compositions in response to changes in their environmental conditions has been observed in all three domains of life. Adjustments in unsaturation levels, hydrocarbon chain-length or polar head group compositions have been observed as an adaption mechanism to changes in temperature, pressure, pH and growth phase [7; 8]. This process conserves physiological homeostasis. In contrast to Bacteria and Eukarya, Archaea harbor fundamentally different membrane lipids comprised of hydrocarbon chains, linked by etherbonds rather than ester bonds [9]. In general, Archaea can thrive in extreme environments, such as in high/low temperature, high/low pH values, high pressure and high salinity, among others [10], abilities that are rooted in the unique archaeal membrane and lipid structure. Many studies demonstrated the modifications of these archaeal lipids following deviations of their typical growth conditions. As a result of these alterations, the membrane becomes either more rigid and tightly packed, or vice versa. The membrane barrier is especially important in the archaeal genus _Sulfolobus_, as these species maintain an almost neutral cytoplasmic pH of 6.5, albeit growing in an acidic environment with a pH of 2.0 to 4.0 [11]. Protons are continuously pumped out, thus creating a chemiosmotic gradient, which in return, rejects positively charged protons [12].  
Archaeosomes, liposomes made of archaeal membrane lipids, are a prosperous approach for oral drug delivery. Their stellar durability towards temperature and pH offers a superior alternative to liposomes made out of phospholipids. As growth conditions and environmental factors affect the membrane compositions of Archaea and thus the characteristics of their lipids, the very same approach can be used to steer the properties of the then assembled archaeosomes, in order to tailor their properties for specific applications [13,14].  
In this review, an overview of the membrane's homeoviscous adaption of Sulfolobales is given, as well as the current prosperous biotechnological application in the form of archaeosomes.