Li, Y.; Yu, T.; Feng, X.; Zhao, B.; Chen, H.; Connock, G. T.; Liu, X.-L.; Yang, H.; Kaiser, J.; Wittenborn, A. K.; Dong, L.; Wang, F.; Anderson, H. R.; Burns, N. Z.; Zeng, F.; Tao, L.; Zeng, Z.

Archaeal membrane lipids GDGTs (glycerol dialkyl glycerol tetraethers) are biomarkers used for tracking Earth\'s historical environmental changes. Among these GDGTs, the H-shaped GDGTs (H-GDGTs, or GMGTs) represent a less-explored and often overlooked subset, with its biosynthetic pathway and geological significance remaining elusive. Here, we identified the gene responsible for biosynthesizing H-GDGTs, which encodes to a radical S-adenosyl-L-methionine (SAM) enzyme, named as H-GDGTs bridge synthase (Hbs). Heterologous expression of the gene hbs in a methanogen, as well as in vitro activity assay using the purified Hbs enzyme were performed. Additionally, we found that the genes encoding Hbs are exclusively present in obligate anaerobic archaea genomes and the metagenomes obtained from oxygen-deficient environments, but not in oxic settings. The H-GDGTs lipids were also consistently enriched in the modern oxygen-deficient environments, and remarkably accumulated in ancient sediments during oceanic anoxic event-2 (OAE-2, ~94 million years ago) period. Our findings indicate H-GDGTs holds significant promise as a novel biomarker for studying historical ocean oxygen deficiency supported by a well-established biological basis.