294
the DWH oil spill was on the same order of magnitude as natural/seasonal fluxes
elsewhere (Zarriess and MacKensen 2011; Theodor et al. 2016).
17.2.5 Microbial Lipid Biomarkers
Lipid biomarkers found in recent marine sediments are valuable biogeochemical
proxies which abundance and distributions can be related to their source microorganisms and various marine system variables which control the composition of
marine microbiome – nutrients, redox state, water temperature of the surface waters,
and nutrients to name a few.
In the past decade, a specific class of lipid biomarkers, known as glycerol dialkyl
glycerol tetraethers (GDGTs), has been extensively studied as (paleo)environmental
proxies (Tierney 2012; Schouten et al. 2013a). GDGTs are microbial membrane
lipids found ubiquitously in different environmental settings – marine, lacustrine,
and terrestrial (Schouten et al. 2013b). There are two main groups of GDGTs: isoprenoid GDGTs (iGDGTs), which are comprised of two C 40 isoprenoid units with a
varying number (0 to 8) of alicyclic moieties, bounded via glycerol groups in a
head-to-head configuration, and branched GDGTs (bGDGTs), which are comprised
of 4 to 6 methyl groups attached to the ether-bounded C 28 alkyl chains and with up
to two cyclopentyl moieties formed by internal cyclization. In general, the main
sources of iGDGTs are marine (planktonic archaea), while bGDGTs are predominantly from terrestrial bacteria, which is why they are used to calculate the BIT
index (branched and isoprenoid tetraether index), a proxy to assess marine vs. terrestrial inputs to a given marine sediment (Damsté et al. 2002; Hopmans et al.
2004). Typically, concentrations of GDGT species are determined by normal-phase
liquid chromatography (NP-HPLC) coupled to mass spectrometry (MS) detectors
using atmospheric pressure chemical ionization (APCI) (Schouten et al. 2013a).
Methods have also been developed for analysis of the intact ether lipids (i.e., still
bound to polar head groups), via LC separation using aqueous eluents, followed by
tandem MS with electrospray ionization (ESI) interface (Sturt et al. 2004), and
simultaneous detection of both intact and core ether lipids, by reversed-phase LC
(Zhu et al. 2013). A useful complementary tool to the abovementioned methods is
the ultrahigh-resolution Fourier transform mass spectrometry (FTICR-MS) because
of its broad range and high resolution of mass detection and diverse ionization
modes. FTICR-MS is a more comprehensive analytical method, targeting multiple
chemical species present in complex environmental matrices, after minimal sample
preparation, ideal for exploratory, non-targeted detection of various polar species,
including lipids.
More recently a simplified APPI-P (atmospheric pressure photoionization in
positive mode) FTICR-MS workflow was developed (RADAR, Rapid Analyte
Detection and Reconnaissance) and applied to analyze iGDGTs, bGDGTs, and
other lipid biomarkers, such as pigments, in northern GoM sediments collected in
2014 (Radović et al. 2016a, b). Interestingly, fractional abundances (f) of iGDGTs
I. C. Romero et al.
the DWH oil spill was on the same order of magnitude as natural/seasonal fluxes
elsewhere (Zarriess and MacKensen 2011; Theodor et al. 2016).
17.2.5 Microbial Lipid Biomarkers
Lipid biomarkers found in recent marine sediments are valuable biogeochemical
proxies which abundance and distributions can be related to their source microorganisms and various marine system variables which control the composition of
marine microbiome – nutrients, redox state, water temperature of the surface waters,
and nutrients to name a few.
In the past decade, a specific class of lipid biomarkers, known as glycerol dialkyl
glycerol tetraethers (GDGTs), has been extensively studied as (paleo)environmental
proxies (Tierney 2012; Schouten et al. 2013a). GDGTs are microbial membrane
lipids found ubiquitously in different environmental settings – marine, lacustrine,
and terrestrial (Schouten et al. 2013b). There are two main groups of GDGTs: isoprenoid GDGTs (iGDGTs), which are comprised of two C 40 isoprenoid units with a
varying number (0 to 8) of alicyclic moieties, bounded via glycerol groups in a
head-to-head configuration, and branched GDGTs (bGDGTs), which are comprised
of 4 to 6 methyl groups attached to the ether-bounded C 28 alkyl chains and with up
to two cyclopentyl moieties formed by internal cyclization. In general, the main
sources of iGDGTs are marine (planktonic archaea), while bGDGTs are predominantly from terrestrial bacteria, which is why they are used to calculate the BIT
index (branched and isoprenoid tetraether index), a proxy to assess marine vs. terrestrial inputs to a given marine sediment (Damsté et al. 2002; Hopmans et al.
2004). Typically, concentrations of GDGT species are determined by normal-phase
liquid chromatography (NP-HPLC) coupled to mass spectrometry (MS) detectors
using atmospheric pressure chemical ionization (APCI) (Schouten et al. 2013a).
Methods have also been developed for analysis of the intact ether lipids (i.e., still
bound to polar head groups), via LC separation using aqueous eluents, followed by
tandem MS with electrospray ionization (ESI) interface (Sturt et al. 2004), and
simultaneous detection of both intact and core ether lipids, by reversed-phase LC
(Zhu et al. 2013). A useful complementary tool to the abovementioned methods is
the ultrahigh-resolution Fourier transform mass spectrometry (FTICR-MS) because
of its broad range and high resolution of mass detection and diverse ionization
modes. FTICR-MS is a more comprehensive analytical method, targeting multiple
chemical species present in complex environmental matrices, after minimal sample
preparation, ideal for exploratory, non-targeted detection of various polar species,
including lipids.
More recently a simplified APPI-P (atmospheric pressure photoionization in
positive mode) FTICR-MS workflow was developed (RADAR, Rapid Analyte
Detection and Reconnaissance) and applied to analyze iGDGTs, bGDGTs, and
other lipid biomarkers, such as pigments, in northern GoM sediments collected in
2014 (Radović et al. 2016a, b). Interestingly, fractional abundances (f) of iGDGTs
I. C. Romero et al.