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Abstract

Microbial life plays a significant role not only in the biological surface but also in the geological subsurface carbon cycle as indicated by the widespread findings of microbial communities (deep biosphere) in the deep underground. Thereby, microorganisms occupy a wide range of different habitats determined by moderate to extreme environmental conditions. Suitable analytical tools are required to assess the presence, spatial distribution, abundance, and composition of microbial life in the many different natural environments on Earth, to understand the response and survival strategies of microorganisms to various environmental living conditions, and to unravel the role of microbial communities on the global biogeochemical cycles in natural habitats. From a biogeochemical perspective, such a tool is provided by microbial biomolecules such as phospholipids (PL) representing a significant part of microbial cell membranes. With their polar head groups and long hydrophobic side chains, they form the basic module of the membrane structure. PLs and especially phospholipid esters not only indicate the occurrence of microbial biomass but also the presence of living microorganisms, since they are only stable in viable microorganisms over longer periods of time. Therefore, PLs are also named microbial life markers. PLs can be used to quantify microbial life, to illustrate its spatial distribution, to provide taxonomic information at least on a broad level, and to assess microbial adaptation and carbon transformation processes. In this chapter we will present basic information on the utilization of phospholipids as life markers, will report on analytical methods to measure these biomolecules and elucidate their structures, and will provide examples for the application of these biomarkers in a geoscientific context.