25th International Meeting on Organic Geochemistry IMOG 2011

P-184
Have they lost their heads? Differential detection of archaeal
IPLs and GDGTs
Sara A. Lincoln, Edward F. DeLong, Roger E. Summons
Massachusetts Institute of Technology, Cambridge, MA, United States of America (corresponding
author:slincoln@mit.edu)
The use of intact polar lipids (IPLs) as tools for
obtaining ecological and taxonomic information has
burgeoned in the past decade. Notably, they have
been used as markers for living biomass in the
environment. This application is based on the
assumption that hydrolysis of labile polar head
groups, such as the cleavage of hexose from an
archaeal tetraether IPL yielding a glycerol dialkyl
glycerol tetraether (GDGT) core lipid, occurs shortly
after cell death. Consequently, the presence of intact
archaeal IPLs in environmental samples can be
viewed as evidence of an active archaeal community.
As part of an effort to understand the ecology of
modern marine archaea and the distribution of their
fossil lipids in the geologic record we have analyzed
archaeal IPLs and GDGTs in suspended marine
particulate matter from five sites. We report the
striking, apparent absence of archaeal IPLs in our
sample set, even in those samples in which abundant
GDGTs have been detected. This absence is
surprising for several reasons: living archaea are
nearly ubiquitous in the world‘s oceans; large volumes
(600-1500L) of seawater were filtered; bacterial IPLs
were detected in the same samples, indicating proper
sample handling and extraction; and synoptic
microbiological studies have shown high archaeal cell
counts and activity at a number of depths/sites.
Following established protocols, IPLs were analyzed
by high performance liquid chromatography mass
spectrometry (HPLC-MS) using electrospray
ionization (ESI), while GDGTs were analyzed using
atmospheric pressure chemical ionization (APCI). We
conclude that the limits of detection (LOD) for IPLs
and GDGTs analyzed using these two analytical
techniques differ significantly.
To determine the extent to which established IPL
analysis may underestimate active archaeal
communities several HPLC-MS experiments were
conducted. A laboratory stock seawater extract, a
commercially available intact GDGT mixture and acid
hydrolyzed aliquots of both were analyzed at a range
of concentrations in order to determine limits of
detection and to compare response factors. We also
compare LODs obtained using quadrupole-time-offlight,
single quadrupole and ion trap instruments.
Our results suggest that caution is warranted when
drawing conclusions about active archaeal
communities from IPL analysis, particularly in lowbiomass
environments such as the oligotrophic open
ocean.
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