Abstract
Bacterial lipids are well-preserved in ancient rocks and certain ones have been used as indicators of specific bacterial metabolisms or environmental conditions existing at the time of rock deposition. Here we show that an anaerobic bacterium produces 3-methylhopanoids, pentacyclic lipids previously detected only in aerobic bacteria and widely used as biomarkers for methane-oxidizing bacteria. Both Rhodopila globiformis, a phototrophic purple nonsulfur bacterium isolated from an acidic warm spring in Yellowstone, and a newly isolated Rhodopila species from a geochemically similar spring in Lassen Volcanic National Park (USA), synthesized 3-methylhopanoids and a suite of related hopanoids and contained the genes encoding the necessary biosynthetic enzymes. Our results show that 3-methylhopanoids can be produced under anoxic conditions and challenges the use of 3-methylhopanoids as biomarkers of oxic conditions in ancient rocks and as prima facie evidence that methanotrophic bacteria were active when the rocks were deposited.
Highlights
Hopanoids are triterpenoid lipids that support membrane integrity and permeability in certain bacteria (Ricci et al 2017)
Lipid analyses and genomic studies were performed on axenic cultures of four purple nonsulfur bacteria: Rhodopila strain LVNP, Rhodopila globiformis 7950 isolated from Yellowstone National Park (YNP), Rhodoblastus acidophilus 7050, and Rhodopseudomonas palustris DSM127 (Table 1)
Our results are the first to show the production of 3-methylhopanoids in bacteria grown anaerobically, refuting the contention that these lipids are only produced by obligately aerobic bacteria
Summary
Hopanoids are triterpenoid lipids that support membrane integrity and permeability in certain bacteria (Ricci et al 2017). While hopanoids are produced by metabolically diverse bacteria, hopanoids methylated in the A-ring are more restricted in Communicated by Erko Stackebrandt Their distribution and linked to particular bacterial taxa or aerobic metabolisms. PNS bacteria are a phylogenetically diverse group of anoxygenic phototrophs that preceded cyanobacteria on Earth by at least 500 million years and whose photosynthetic metabolism is strictly anaerobic (Hohmann-Marriott and Blankenship 2011). These Alpha- and Betaproteobacteria inhabit various aquatic environments, including lakes, wastewaters, hot springs, and marine and hypersaline waters and typically can conserve energy from both photosynthesis (anoxic/light) or respiration (oxic/dark) (Madigan and Jung 2009). Subsequent metagenomic analyses showed that hpnP, the gene encoding the enzyme that methylates hopanoids at
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