Towards understanding the bacterial biosynthesis of branched GDGTs: Identification of iso-diabolic acid-based tetraester and mixed ether/ester, membrane-spanning lipid intermediates in members of the Bacillota

ABSTRACT Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane-spanning lipids (MSLs) resembling archaeal membrane lipids, as they form monolayers and are linked to glycerol backbones via ether bonds. Ubiquitous in soils, sediments, and aquatic environments, their distributions are widely applied as paleoclimate proxies for reconstructing past temperature and pH. Despite this, understanding of their biological origins and functional role in cells remain incomplete. While some Acidobacteria are known producers of brGDGTs, genomic evidence and environmental surveys indicate additional bacterial contributors. In this study, we report the first detection of potential brGDGT biosynthetic intermediates in Bacillota. Ultra-high pressure liquid chromatography high resolution multi-stage mass spectrometry (UHPLC-HRMS n ) revealed membrane-spanning diglycerol lipids which contained iso -diabolic acid (13,16-dimethyl octacosanedioic acid)-derived alkyl chains. These diglycerol lipids displayed diverse structures, including tetraesters, mixed ester/ether combinations, and vinyl ether bonds. Additionally, ‘open’ membrane spanning lipids analogous to brGTGTs were also identified. Notably, all brGDGT and brGTGT analogues were detected with a phosphatidylglycerol head group. Experiments showed that the two Bacillota strains, which produce these brGDGT biosynthetic intermediates, responded differently to changes in temperature and oxygen availability, suggesting that environmental regulation of brGDGT-related lipids is taxonomically dependent. Based on these findings, we propose a biosynthetic pathway for brGDGT formation and highlight the physiological implications for interpreting brGDGT-based paleoclimate proxies. This work expands the known diversity of bacterial sources of brGDGTs and provides new insights into the ecological and evolutionary significance of these lipids. IMPORTANCE Branched GDGTs (brGDGTs) are bacterial membrane spanning lipids which form a monolayer, linked through ether bonds to the glycerol backbone, characteristics more commonly found in archaeal membrane lipids. They are commonly used in paleoclimate proxies to assess past temperature and pH but their predictive power is hampered by the lack of information regarding their biological producers. Branched GDGTs have been detected in just a few species of the Acidobacteria but there are strong indications that other bacterial phyla also contribute to the pool of brGDGTs in the environment. Here, we report for the first time the production of potential brGDGT intermediates in Bacillota species. This study demonstrates that brGDGTs likely occur much more widespread in the bacterial domain than previously thought and opens a new chapter both in the understanding of the function of these membrane lipids and their use in paleoclimatology.