25th International Meeting on Organic Geochemistry IMOG 2011

P-425
Insights into the microbial communities of modern stromatolites
(Highborne Cay, Bahamas) from bacterial and eukaryotic lipids
Sabine Mehay 1 , Joan Bernhard 2 , Anna McIntyre-Wressnig 2 , Virginia Edgcomb 2 , Roger
Summons 1
1 Earth, Atmospheric and Planetary Department, Massachusetts Institute of Technology, Cambridge, United
States of America, 2 Geology and Geophysics Department, Woods Hole Oceanographic Institution, Woods
Hole, United States of America (corresponding author:mehay_s@hotmail.com)
Stromatolites are the oldest visible form of
life on Earth. Widespread throughout the Proterozoic
Eon, these distinctively laminated microbial mats are
now relatively rare in contemporary environments. We
are investigating the biochemical processes involved
in stromatolite formation, and why their abundance
and variety drastically decreased at the end of the
Proterozoic, based on the study of modern analogs.
The role of cyanobacteria has been extensively
investigated since the first characterizations of
stromatolitic mats for evidence of particles
agglutination and carbonate precipitation and
cementation. However the microbial communities
involved are complex and this complexity may play a
significant role in various processes taking place
within the mat. The role of eukaryotic microorganisms
has been poorly appreciated in this respect. Algae,
but also non-photosynthetic protists could be
implicated in mat evolution and taphonomic
processes including the fabric modification of
laminated stromatolites into clotted thrombolites.
Here, we show lipid biomarker data from
various forms of microbialites collected in Highborne
Cay (Bahamas) in April 2009. This set of samples
includes intertidal and subtidal stromatolites,
thrombolites and ooids. Ooids, roughly spherical
sedimentary grains largely composed of aragonite,
are the major carbonate constituent of the Bahamian
stromatolites and thrombolites.
Steroids isolated from stromatolites show
that eukaryotes were present in every sample. We
also identified a diverse assemblage of
bacteriohopanepolyols (BHP) including BHT,
aminotriol, lactone, cyclitol ether and guanidine
substituted cyclitol ether. Methylated analogs for BHT,
aminotriol and lactone were also tentatively identified.
Thrombolite samples showed the same variety of
BHP as stromatolite samples. Ooid samples display
most of the same BHP found in stromatolites and
thrombolites although at lower abundance. We also
observed significant variations in the relative amounts
of hopanoids independently of the nature of the
samples.
The observation of similarly complex BHP
distributions in stromatolites, thrombolites and ooids
implies that similar bacterial taxa are present.
However differences in the relative abundance of
these BHPs indicate that the corresponding
physiological processes are variably active. We did
not find any hopanoid fingerprints that were specific to
stromatolites or thrombolites. Production of these
lipids, therefore, may be more related to biochemical
processes occurring in specific zones within the
microbial mat rather than to the laminated or clotted
nature of the mat.
Further work on the characterization of the
steroid distributions is in progress. Concurrently, we
are analyzing the lipids of cultured Allogromia
laticollaris, a foraminiferan closely related to some
previously identified in Bahamian stromatolites (JMB,
unpubl.), with the aim of identifying lipid biomarkers
specific to protists that are active with stromatolitic
and thrombolitic communities.
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