Large D/H variations in bacterial lipids reflect central metabolic pathways

Hydrogen isotopic fractionations between lipids and water are widely studied in biological systems. These fractionations are typically attributed to biosynthetic isotope effects. However, the assumption that these values remain constant across different conditions has not been thoroughly tested. Prior research has shown that D/H ratios can reflect environmental and metabolic conditions. Yet, the extent to which these ratios vary within a single organism remains unclear. This gap motivated an investigation into whether lipid D/H fractionations are influenced by metabolic state. No prior work had resolved the connection between lipid isotopic composition and central metabolism. Understanding this link could refine the use of lipid isotopes as environmental proxies. This uncertainty drove the current study to explore lipid D/H variations in Proteobacteria. The findings could reshape how lipid isotopes are interpreted in biogeochemical contexts.

This study aimed to determine whether lipid D/H fractionations are influenced by the metabolic state of the organism. The researchers focused on four cultured Proteobacteria species to assess the variability of D/H ratios in their lipids. They sought to test whether these variations could be attributed to biosynthetic pathways or growth substrates. The study also aimed to identify if different metabolic modes produce distinct isotopic signatures. The researchers hypothesized that lipid D/H ratios might reflect the isotopic composition of NADPH. This would suggest a link between lipid isotopes and central metabolism rather than biosynthetic isotope effects. The goal was to establish whether lipid D/H values could serve as a proxy for metabolic pathways. The study aimed to provide a framework for interpreting lipid isotopes in environmental and geological archives.

The researchers analyzed D/H ratios in lipids from four Proteobacteria species grown under different metabolic conditions. They measured the isotopic composition of lipids relative to growth water. The study compared chemoautotrophic, photoautotrophic, and heterotrophic growth modes. They used stable isotope analysis to determine the extent of D/H fractionation. The researchers controlled for substrate D/H ratios to isolate metabolic effects. They also examined lipid biosynthetic pathways to assess their role in fractionation. The study tracked changes in lipid D/H across different carbon sources. The data were compared to theoretical predictions of isotope effects in biosynthesis.

The study found that lipid/water D/H fractionations varied by up to 500 per thousand in a single organism. These variations were not linked to changes in lipid biosynthetic pathways. The researchers observed distinct D/H ranges for different metabolic modes. Chemoautotrophic growth yielded fractionations of approximately -200 to -400 per thousand. Photoautotrophic growth produced values between -150 and -250 per thousand. Heterotrophic growth on sugars resulted in fractionations from 0 to -150 per thousand. Growth on TCA-cycle intermediates produced values from -50 to +200 per thousand. The data suggest that lipid D/H ratios are influenced by the isotopic composition of NADPH.

The authors propose that lipid D/H ratios are primarily controlled by the isotopic composition of NADPH. This suggests that lipid isotopes reflect central metabolic pathways rather than biosynthetic isotope effects. The study shows that different metabolic modes yield lipids with distinct D/H signatures. These findings indicate that lipid D/H values could serve as a proxy for energy metabolism. The researchers suggest that lipid deltaD values may provide information complementary to 13C data. The results imply that lipid isotopes can be used to trace metabolic processes in natural systems. The study supports the idea that lipid isotopes are influenced by NADPH isotopic composition. These conclusions align with the observed systematic variation in D/H fractionations across metabolic states.