25th International Meeting on Organic Geochemistry IMOG 2011

O-70
Deciphering the physiological significance of hopanoids in the
marine geologic record
James Saenz 1,2 , Roger Summons 1 , Timothy Eglinton 2 , Stuart Wakeham 3,4 , John
Waterbury 2
1 Massachusetts Institute of Technology, Cambridge, MA, United States of America, 2 Woods Hole
Oceanographic Institution, Woods Hole, MA, United States of America, 3 Skidaway Institute of Oceanography,
Savannah, GA, United States of America, 4 University of Washington School of Oceanography, Seattle, WA,
United States of America (corresponding author:jsaenz@whoi.edu)
Lipids have a legacy in the geologic record
extending back to the Archaean. Since the
phylogenetic diversity of life is reflected in the
structural diversity of biomolecules, lipid biomarkers
that are shown to be diagnostic of certain organisms
that carry out specific biochemical processes or that
are demonstrated to have unique physiological roles
can be used to trace the biogeochemical influence of
bacteria in modern and ancient environments.
Hopanoids have been broadly applied in the marine
sedimentary record as taxonomic markers for certain
groups of bacteria and their associated
biogeochemical processes. However, our ability to
rigorously interpret the significance of hopanoids in
the geologic record has been greatly limited by a
dearth of knowledge surrounding the sources of
hopanoids in marine environments. Despite the
ubiquity of hopanoids in modern and ancient marine
sediments, their provenance in the modern oceans is
unknown. In this study, we present a survey of
bacteriohopanepolyols (BHPs) in a diverse selection
of marine and proximal marine environments, and in
marine cyanobacterial cultures and enrichment
cultures. Our work establishes the presence and
ubiquity of hopanoids in the oceans, and provides
fresh insight on the environmental sources and
biogeochemical significance of hopanoids in marine
sediments.
In the marine realm, we observe pronounced
heterogeneity in the spatial and temporal distribution
of BHPs, indicating the potential for the application of
hopanoids as biomarkers for biological processes in
the upper ocean and as tracers for organic matter
input to sediments. In particular, BHPs appear to be
relatively abundant and structurally diverse in low
oxygen and oligotrophic environments and in
particulate organic matter (OM) transported by rivers
from terrestrial environments. Given the rich structural
diversity of BHPs in terrigenous OM, interpretations of
the sedimentary record of hopanoids in coastal
marine settings must resolve inputs from marine
pelagic and terrestrial sources. Furthermore, BHPs
produced in suboxic and anoxic pelagic environments
likely represent an important input to the sedimentary
hopanoid inventory in upwelling environments and
anoxic marine basins. Our findings reveal that marine
suboxic and anoxic environments can be important
sources of hopanoids to the marine sedimentary
record.
There is evidence to suggest that hopanoids
may be diagnostic of cyanobacteria in the
environment, and therefore could serve as biomarkers
for cyanobacteria in past and present environments.
However, very little is known about the distribution or
physiological roles of hopanoids in marine
cyanobacteria. Our survey of a diverse range of
marine cyanobacterial cultures, and cyanobacterial
enrichment cultures indicates that hopanoid
production may be unique to, or at least widespread
among, the nitrogen-fixing cyanobacteria in marine
environments. We observe a very high structural
diversity of BHPs in the marine cyanobacterial
enrichment cultures, which stands in stark contrast to
the low structural diversity observed in pure cultures.
This demonstrates that there are diverse and
presently un-accounted for marine bacteria, possibly
cyanobacteria, that are capable of synthesizing a
diverse range of BHP structures in the marine
environment.
Interpreting the significance of hopanoids in
the sedimentary record relies on a presently
incomplete understanding of their phylogenetic
associations, biological functions, and spatial and
temporal disposition within the environment. From the
perspective of the marine geologic record, the
environmental distribution of BHPs is particularly
important since it provides constraints on the
provenance of hopanoids that are present in marine
sediments, which may in turn provide clues about the
biological source or function. We explore the current
state of knowledge of BHPs in modern environments,
and entertain the question: ―What can the vast
majority of hopanoids preserved in the sedimentary
record tell us about life in ancient environments?
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