Complex Lipid Biomarker Profiles of Microbial Mats and Stromatolites inHamelin Pool, Shark Bay, Australia

MassachusettsInstitute ofTechnology

EAPSEarth, Atmospheric and Planetary Sciences

Elise M. Myers1 *, Florence Schubotz2, Emily Matys3, Roger Summons3

1Department of Ecology and Evolutionary Biology, Princeton University, USA. 2Center for Marine Environmental Sciences, University of Bremen, Germany 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA *corresponding author: emmyers@princeton.edu

Biogenic stromatolites, indicated by the coexistence of microbial mats (active microbial com-munities) and stromatolites (lithified structures), comprise one of the best modern analogs to ancient microbial community assemblages. Membrane lipids serve as important biomarkers for microorganisms, particularly because of their varying degrees of recalcitrance (i.e. per-sistence in the rock record) and adaptability. While there have been numerous characteriza-tions of the fatty acid methyl esters (FAMEs) of microbial mats in Hamelin Pool (e.g. Allen et al., 2010) , there has been significantly less cross characterization with other lipid biomark-ers. By creating comprehensive lipid biomarker profiles and including FAMEs, it was pos-sible to develop a more thorough understanding of the lipids of these microbial mats and the corresponding lithified stromatolites, while also confirming our results with previous studies. In this study, microbial mat samples of three different morphologies (smooth, pustular, and colloform) were analyzed, with select samples being separated into upper and lower layers to test for microbial community stratification. In addition to the FAMEs, other lipids analyzed in this study are intact polar lipids (IPLs), glycerol dialkyl glycerol tetraethers (GDGTs), and bacteriohopanepolyols (BHPs).

Analyzing microbial mats that are known to form stromatolites allow for characterization of the earth’s earliest microbial communities, which formed despite the inhospitable environ-mental conditions of the early earth. Because of this, it is possible that early life forms on other planetary bodies could resemble these microbial accretions.

Introduction

Methodology

Research Question and GoalsQuestion:

• what do complex lipid biomarker profiles suggest about the microbial composition of ancient microbial communities?

Goals:

• create comprehensive lipid biomarker profiles of microbial mats to provide the basis for future analysis of ancient stromatolites • draw conclusions about the microbial communities in this area • explore morphologically induced differences in microbial community composition

Conclusions and Future Exploration

AcknowledgementsI would like to extend the deepest thanks to Roger Summons, Florence Schubotz, and Emily Matys for their mentorship throughout this project and to Vicki McKenna for finding the fund-ing for me to pursue a 4th year Master’s degree. This project was made possible through the generous support of the Summons Lab.

Results1. Intact Polar Lipids ~ IPLs

Rela

tive

Abu

ndan

ce

100%16

16:1

2Me-C18 stnd

1415 17

18:1

18:1

18:1

2022

24 26

23

60 62 64 70 726866

Retention Time (min)

20 30 40 50 60 70 80 85

Retention Time (min)

Colloform Mat 1 - Total Ion Chromatogram: FAMEs

26

27

2830 32

3. Glycerol Dialkyl Glycerols (GDGTs) and Glyco-GDGTs (G-GDGTs)

4. Bacteriohopanepolyols (BHPs)

ng

BH

P /

g T

LE

Normalized BHP Amounts (ng BHP/ g Total Lipid Extract)

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1

Logemann et al., 2011

A sample chromatogram of a colloform mat, which shows the length of the Carbon chain of each fatty acid

G Deichmann

Stromatolites in Hamelin Pool

Note the similar relative abundances of G-GDGTs and core GDGTs, which suggest that recalcitrant, core GDGTs are representative of the lipid signatures of living microbes.

i.e. GDGTs = reliable tracing lipids

Dry BiomassTotal Lipid Extract

Saturated Hydrocarbons Fatty Acid Methyl Esters Diols and BHP Aromatic Hydrocarbons Alcohols

Fractions:1 3 52 4

BHPsIPLs GDGTs

Total Lipid Extract(TLE)

Dry Biomass

2. Fatty Acid Methyl Esters ~ FAMEs

Polar Head Group

Why FAMEs? - widely analyzed, so useful for comparisonDominant FAMEs & Significance

Why IPLs? - useful for elucidating the composition and quantity of viable biomass(polar head group used in identification, but degrades quickly post mortem)Dominant IPLs & Significance • Trimethyl Ornithine Lipids (TM-OL) and OL signifying anaerobic autotrophic metabolism, with OL suggesting photosynthetic bacteria • Phosphatidylcholine (PC) minimizes membrane permeability (good for hypersalinity)

• C12 - C19 correlate to sulfate reducing bacteria (including dominant C16 and C18 peaks)

• C24-C30 is likely a vegetation signature (even > odd peaks)

• data closely resemble trends in ooids, (supporting ooid formation by microbial biofilms)

• lipid profile correlates to cyanobacteria• organic matter (cyano.) and alkalinity

(S-reducers) drive carbonate precipitation in organosedimentarybiofilms

Why GDGTs & G-GDGTs? - core GDGTs (sans polar head group) are highly recalcitrant; viewed together, they can show microbial/environmental transitionsDominant GDGTs/G-GDGTs & Significance • GDGT-0 signifies anaerobic metabolism (highest in smooth mats) • Crenarchaeol GDGT signifies aerobic metabolism (highest in colloform and pustular mats, suggests amm- onium oxidizing anaerobes) • general trend: higher Crenarchaeol GDGT in upper layers and higher GDGT-0 in lower layers • pustular and colloform mats had similar profiles • smooth mats < colloform mats for all G-GDGT amounts (i.e. less dominated by [recently] living microbes)

Why BHPs? - highly recalcitrant, likely structural, lipid degradation products that are resistent to abiotic, thermal, or pressure degradationDominant BHPs & Significance • bacteriohopanetetrol (BHT) most dominant, as in other studies • 2-Methyl BHTs in all mats are biomarkers for cyanobacteria (previous studies determined cyanobacteria from 2-Me BHTs in smooth and pustular mats) • aminotriol signifying purple, non-sulfur bacteria • potential presence of 3-Me BHT, which must be confirmed through further testing

Comprehensive Lipid Profile Suggests: • a microbial community including: cyanobacteria, sulfate-reducing bacteria, anaerobic autotrophs, and ammonium oxidixing bacteria • a potentially strong influence of morphology on microbial communities

Future Exploration • isotopic analysis to robustly characterize microbial metabolism • sampling and analyzing pustular mats that are separated in upper/lower layers • sampling and analysis at deeper layers within microbial mats • use degradation products in ancient stromatolites and this lipid profile to predict ancient microbial communities

Smooth Mats - flat, millimetric laminae of organic matter and fine-grained carbonateColloform Mats - coarse laminoid wavy fabric with curved voidsPostulate: lower porosity (smooth mats) limits oxygen diffusion, even at shallow depthsObserved: significant increase in GDGT-0 (anaerobically linked) in smooth mat lower layers, while no significant change in colloform mat layersPustular Mats - filamentous cyanobacteria bind and trap sand, with microorganisms boring into the surface; signifies greater porosityObserved: similar GDGT profiles for pustular and colloform mats (smooth mats distinct)

Smooth Mats Colloform MatsUpper Layers (0-5cm)

GDGT-0 ~ 49%Crenarchaeol GDGT~ 47%

GDGT-0 ~ 26%Crenarchaeol GDGT~ 61%

Lower Layers (5-10cm)GDGT-0 ~ 72%Crenarchaeol GDGT~ 15%

GDGT-0 ~ 23%Crenarchaeol GDGT~ 52%

This table shows weight percentages of each GDGT per total GDGT

David Daiku Trowbridge

colloform stromatoliteCantrell & Perez, 2013

smooth mat

BHT 2-Me BHT 3-Me BHT Aminotriol Adenosyl Hopane BHPentol

Morphologically Induced Differences

plots of relative abundances and normalized weight of GDGTs