Rapid Screening of Glycerol Ether Lipid Biomarkers in Recent Marine Sediment Using Atmospheric Pressure Photoionization in Positive Mode Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Many of the molecular proxies commonly used for paleoenvironmental reconstruction are focused on a limited set of glycerol ether lipids, mainly due to the lack of more comprehensive analytical methods and instrumentation able to deal with a more diverse range of species. In this study, we describe an FTICR-MS-based method for rapid, nontargeted screening of ether lipid biomarkers in recent marine sediments. This method involves simplified sample preparation and enables rapid identification of known and novel ether lipid species. Using this method, we were able to identify complete series of core glycerol dialkyl glycerol tetraethers (GDGTs with 0 to 8 alicyclic rings), including the complete resolution of GDGT-4 and the unexpected detection of GDGTs with more than 5 rings, in sediments from mesophilic marine environments (sea surface temperature, SST, of 24-25 °C). Additionally, mono- and dihydroxy-GDGT analogs (including novel species with >2 rings), as well as glycerol dialkanol diethers, GDDs (including novel species with >5 rings) were detected. Finally, we putatively identified other, previously unreported groups of glycerol ether lipid species. Adequacy of the APPI-P FTICR-MS data for the determination of commonly used GDGT-based proxy indices was demonstrated. The results of this study show great potential for the use of FTICR-MS as both a rapid method for determining existing proxy indices and, perhaps more importantly, as a tool for the early detection of possible new biomarkers and proxies that may establish novel geochemical relationships between archaeal ether lipids and key environmental-, energy-, and climate-related system variables.

A rapid method to assess a broad inventory of organic species in marine sediments using ultra‐high resolution mass spectrometry

“…The performance of the extraction method adopted for GoM sediments has been discussed elsewhere . In this study, between 1 and 2 mg of sediment extracts were obtained from about 1 g of dry sediment.…”

“…The distributions of these lipid species have been typically used as proxies for sea surface temperature (SST), and a recently published FTICR‐MS method focusing on GDGT analysis in sediment extracts has proved its value as a rapid method for screening of GDGT analogs . Using the method proposed herein, an averaged mass resolution higher than 310,000 is found for peaks in the GDGT area, compared with 530,000 reported by Radović et al . Such performance still allows unequivocal molecular formula attribution for the most intense isoGDGT species commonly used in paleoceanography …”

“…Note that FTICR‐MS is not sufficiently precise to perform stable isotope ratio measurements, and that the global average carbon isotope distribution used to calculate the theoretical relative abundances of isotopologs has not been corrected for any local fractionations. The peak assignment algorithms implemented in the CaPA software package consider mass accuracy and isotopic patterns to attribute the most probable unique formulae corresponding to the m / z values of the peaks present in the mass spectra . The Ragnarök v.1.4 software package was employed for data visualization.…”

“…GDGT‐0 up to GDGT‐8. This is comparable with our previous FTICR‐MS study of ether lipids in GoM sediments …”

“…Recently, a method using laser desorption ionization (LDI) coupled to FTICR‐MS has shown high spatiotemporal organic species resolution along the vertical profile of a sediment core, and been used to identify and profile the main archaeal lipid biomarkers typically used for paleotemperature reconstructions . Similarly, a recent publication expanded the inventory of archaeal lipids detected in sediment extracts by atmospheric pressure photoionization (APPI) coupled to FTICR‐MS, suggesting the presence of many new compounds previously undetected by other techniques …”