Tremendous progress has been made in the last decade in the genetic characterization of microorganisms, both in culture and in the environment. However, our knowledge of microbial membrane lipids, essential building blocks of the cell, has only marginally improved. This is remarkable since there exists a dichotomy in the distribution of lipids between the three Domains of Life (Bacteria, Archaea, and Eukaryotes). Diacyl glycerols based on straight-chain fatty acids are produced by bacteria and eukaryotes, whereas archaea synthesize isoprenoidal glycerol ether lipids. From a microbial evolutionary perspective, this ‘lipid divide’ is enigmatic since it has recently become clear that eukaryotes evolved from the archaea. The aim of the MICROLIPIDS project was to systematically characterize prokaryotic intact polar lipids (IPLs) with state-of-the-art analytical techniques to bring our knowledge of microbial lipids to the next level. This approach was complemented by the characterization of functional genes for lipid biosynthesis. This involved both mapping of known genes, based on the analysis of published whole (meta)genome data, as well as the identification of as yet unknown genes in selected groups of prokaryotes. The results of the project make a fundamental contribution to our understanding of the evolution of biosynthesis of membrane lipids, their application as microbial markers in the environment, and in the development and application of organic proxies in earth sciences.
A wide range of microbial cultures were analyzed as part of WP1. Through this work many novel lipids have been identified, deepening and broadening our knowledge of the wide diversity of bacterial and archaeal membrane lipids. A wide range of environmental samples were analyzed for the lipid composition as part of WP2. These data sets allowed us to develop a new methodology for environmental lipidomic data workflow using an information theory framework combined with molecular networking based on the similarity of the mass spectra of lipids, enabling the capture of lipidomic diversity and specificity in the environment. As part of WP 3, a range of studies focused on the biosynthetic genes involved in microbial lipid synthesis. These include the biosynthesis of iso-diabolic acid, a major membrane-spanning lipid, and hopanoids in Acidobacteria. We also demonstrated that bacteria of the Fibrobacteres–Chlorobi–Bacteroidetes superphylum encode a putative archaeal pathway for ether-bound isoprenoid membrane lipids in addition to the bacterial fatty acid membrane pathway.
