Abstract
The branched glycerol monoalkyl glycerol tetraether lipids of bacteria (brGMGTs, sometimes referred to as H-brGDGTs) are found in marine, lacustrine, and terrestrial mesophilic environments. The abundance of brGMGTs relative to their dialkyl analogues (brGDGTs) has been proposed as a proxy for past continental air and lake water temperature. However, the source(s) of brGMGTs remain unknown. brGDGT production has been described in multiple cultivated Acidobacteria, but so far no brGMGT producer has been identified. We hypothesize that the stable carbon isotopic composition (δ13C) of brGMGTs, as a tracer of carbon source and metabolic pathway, could be used to elucidate the source(s) of brGMGTs relative to brGDGTs. However, the large monoalkyl moiety of these compounds impedes carbon isotopic analysis using conventional techniques based on gas chromatography. Here, we used Spooling-Wire Microcombustion to analyse the stable carbon isotopic composition of brGMGTs and brGDGTs found in peats and lignites. We show that the δ13C of brGMGTs is within ∼ 2 ‰ similar to that of brGDGTs from the same samples, as well as the δ13C of the total organic carbon. This suggests that the source organisms use heterotrophic metabolisms. However, offsets in the δ13C of brGDGTs and brGMGTs in some samples suggest that not all brGDGT producers also produce brGMGTs. Further, in modern peats we observe downcore increases in brGMGT relative abundance and decreases in the carbon isotopic offsets with brGDGTs, which indicates that brGMGT producers occur throughout the oxic-anoxic continuum and may metabolize different organic carbon pools. Based on this indirect evidence for diverse brGMGT sources, we suggest that in addition to homeostatic responses to temperature, brGMGT relative abundances may be influenced by changes in bacterial community composition in response to additional environmental and biogeochemical parameters.
| Original language | English |
|---|---|
| Article number | 104558 |
| Journal | Organic Geochemistry |
| Volume | 178 |
| Early online date | 18 Jan 2023 |
| DOIs | |
| Publication status | Published - 1 Apr 2023 |
Bibliographical note
Funding Information:We thank Dr. Angela Gallego-Sala and Dr. Vera Korasidis for providing samples used in this work. We thank associate editor Rienk Smittenberg and two anonymous reviewers for constructive comments that helped improve an earlier version of this manuscript. Funding for this work was provided by the US National Science Foundation ( 1702262 and 1843285 , to A.P.). L.K. was supported by Deutscher Akademischer Austauschdienst (DAAD RISE Worldwide Fellowship #US-ES-4055). F.J.E. was supported by the Deutsche Forschungsgemeinschaft (grant 441217575). B.D.A.N. acknowledges funding through a Royal Society Tata University Research Fellowship. V.L. was supported by funding from the Royal Society through the URF enhancement award of B.D.A.N.
Publisher Copyright:
© 2023 Elsevier Ltd