LAUSR

Membrane lipids are thought to be a crucial part of the homeoviscous adaptation of archaea to extreme conditions. This article reviews the recent lipidomic studies of physiological membrane adaptations of archaea, assesses the biomolecular basis of an organic paleothermometer, TEX 86 , and contemplates the future directions of archaeal lipidomics. The studies of extremophilic archaea have revealed that at least three different molecular mechanisms are involved in membrane adaptation of archaea: (1) regulation of the number of cyclopentane rings of caldarchaeol, (2) alteration of the diether-to-tetraether lipid ratio, and (3) variation of the proportion of saturated and unsaturated lipids. However, most of the studies have focused on a limited number of archaeal ether-linked lipids, such as glycerol dialkyl glycerol tetraethers (GDGTs), which only represent a fraction of the entire lipidome. Environmental factors such as growth temperature and pH have been most frequently reported, but biotic factors, including growth phases, nutrition, and enzymatic activities affecting the membrane lipid composition are often overlooked. Membrane lipids of mesophilic ammonia-oxidizing marine Thaumarchaeota have been applied in the reconstruction of past sea surface temperatures. However, recent culture-based physiological studies have demonstrated that non-thermal biotic factors, including dissolved oxygen, ammonia oxidation rate and the growth rate, are the main drivers of GDGT cyclization in Nitrosopumilus maritimus . Moreover, other related strains or ecotypes exhibit a markedly different set of stress adaptations. A trend is now developing to examine the whole lipid profile (lipidome) for studies of archaeal physiology and biochemistry related to lipid biosynthesis (lipidomics) to gain a better understanding of the biological mechanisms underpinning the applications of membrane lipid-based proxies in biogeochemical or ecological research.