Life on Earth has existed for billions of years. For most of that time, all life was microbial.
Today, life abounds. And still, most living organisms on our planet are microbes. They size is tiny, but their impact is enormous - microbes play a vital role in the catalysis of global biogeochemical cycles.
The study of the geochemistry of modern environments, coupled with the examination of modern microbes can reveal how the complex evolution of life and Earth has unfolded. By working in the lab and in the field, we can study how microbes interact with their environment and leave behind distinctive chemical traces in the geologic record. This site details some approaches to better understand these biogeochemical signatures.
is the study of biologically-produced molecules that can carry information about living organisms into the sedimentary and geological records.
DNA is the information-carrying biomolecule. It is heritable, and encodes all the genes that can be expressed by an organism. Its information content is high, but its preservation potential is relatively low; under ideal conditions it may be preserved in environments for a few thousands or tens of thousands of years. Genomic DNA sequences are increasingly stored in databases such as GenBank. These databases may be queried for genes encoding synthesis of lipids - if the genes are known!
RNA is synthesized during gene expression. It is also used to build the ribosome. Like DNA, it contains sequence information, but it also carries information about the expression level of each gene. However, RNA is very unstable and is rarely preserved in the environment for more than a few minutes.
Protein is constructed of sequences of amino acids and is the major building block of enzymes. Amino acid sequences contain information, although less than DNA or RNA. It also contains expression information that can be preserved over moderate timescales: at least as long as DNA, and possibly much longer.
Lipids don't carry as much information as DNA, RNA, or protein. Unlike these other molecules lipids are not information-encoding polymers. However, they still contain information, since the structure of a lipid informs us about the genes that were required to make it. For instance, more than a dozen genes are involved in the synthesis of cholesterol. Thus, the detection of cholesterol indicates the presence of organisms with these genes. The carbon and hydrogen in lipids (as in DNA, RNA, and protein) also contains isotopic information that can tell us about the physiology of the organism from which it derives. But most importantly, lipids are stable: they can be preserved in rocks for billions of years.
The study of these molecules in modern environments, in the laboratory, and in ancient rocks can help to answer questions related to the coevolution of life and the Earth. What were the first ecosystems like? What were the first metabolic cycles? How can we better understand the evolutionary history of microbial life? What molecular and isotopic records are preserved in the rock record, and how can we interpret them? How do lipids reflect environmental parameters, such as temperature? How has the Earth's carbon cycle changed over geologic time, and what are the important contributions to it? Organic Biogeochemistry seeks to answer these questions.
Hopanoids are pentacyclic triterpenoids - polymers of isoprene with a backbone containing 30 carbon molecules and five rings. The figure at the right shows bacteriohopanetetrol (BHT) - an example of a hopanoid commonly found in the environment.
Hopanoids are derived from bacteria - but not all bacteria. By the most recent estimates, less than 10% of bacterial species, and perhaps less than 2% of the bacterial cells in the ocean contain hopanoids. Yet hopanoids are commonly detected in sedimentary rocks that are hundreds of millions of years old. In one case, hopanoids have been reported in rocks that are 2.7 billion years old.
Can these ancient hopanoids tell us something about the microbes that produced them? What kinds of environments did these organisms live in? What kinds of physiologies did they have? We know very little about the physiological function of hopanoids in bacteria living today - so examining these bacteria is a way to start.
In the Marx lab, we are investigating hopanoids in the model organism Methylobacterium extorquens. We are constructing mutant strains of this bacterium that are deficient in various aspects of hopanoid synthesis. Some of our mutants are unable to modify hopanoids with methyl groups or side chains, or can not make hopanoids at all.
We have found that Methylobacterium mutants deficient in hopanoid production exhibit a strong growth phenotype in liquid, i.e. they grow very poorly compared ot the wild type. This growth phenotype is particularly pronounced during growth on single-carbon compounds. One of the ideas we are exploring is whether the ability of Methylobacterium to grown on single-carbon compounds is related in part to its abundant production of hopanoids in its membrane.
The figure shows two flasks containing Methylobacterium grown in liquid media. The flask on the left contains wild-type (normal) Methylobacterium, while the flask on the right contains our mutant strain of Methylobacterium that produces no hopanoids. This figure demonstrates the severe flocculation and poor growth characteristics of hopanoid-free Methylobacterium. We are examining this trait, along with others, to try to determine what role hopanoids play in Methylobacterium, what this means for hopanoids in evolution, and what this can tell us about the hopanoids we detect in the environment and in the rock record
The Lost City Hydrothermal Field is an alkaline, low temperature hydrothermal field located near the Mid-Atlantic Ridge. Its fluids are rich in calcium, hydrogen, and methane and the microbial community in the vent chimneys is dominated by methane-cycling archaea from the family Methanosarcinales.
Are these archaea producing methane? Or consuming it? The abundant methane in the vent fluids might constitute a rich source for methane-consuming archaea, yet the abundant hydrogen in the vent fluids provide a thermodynamic drive for methane production. We analyzed biomarkers from the carbonate chimneys to determine the answer. The biomarkers are dominated by isoprenoidal and non-isoprenoidal diether lipids. Isoprenoidal diethers mainly derive from methanogenic archaea. Non-isoprenoidal dithers are commonly associated with sulfate-reducing bacteria, although their precise microbial source at Lost City is unknown.
It turns out that in environments where archaea are hypothesized to consume methane, lipids are always depleted in 13C relative to methane. In methane producers, lipids are enriched relative to methane. At Lost City, the latter pattern holds: archaea are producing methane.
The most unusual feature of the lipids at Lost City is that they lack the telltale depletion in 13C that is usually found in biologically-derived products. This is attributed to carbon-limitation in this ecosystem. Methane is also enriched in 13C, with values between -9‰ and -14‰ relative to VPDB.
The most likely explanation? Lost City Methanosarcinales are consuming H2 and CO2, and producing methane. If we examine the cell density in the towers, its obvious that they can't be producing all the methane that we observe in the vent fluids. Some of this methane must be coming from other sources, such as abiotic Fischer-Tropsch type (FTT) reactions, or Sabatier reactions. There is too much methane relative to ethane and propane for the reaction to be a pure FTT process, and the maximum temperatures are too low to catalyze this efficiently... yet the peridotite massif doesn't seem to be checmically reduced enough to effectively catalyze Sabatier reactions. So it's not yet completely clear what the precise source of methane at Lost City really is.
Yellowstone National Park is a fascinating ecosystem on many scales. It is probably the most famous concentration of hot springs and geysers in the world. My interest is in examining the microbial life inhabiting these hot springs, understanding their physiology and the potential for their preservation in the rock record, and trying to determine to what extent these ecosystems are good models for environments on early Earth.
We have examined bulk organic material and lipids from a number of hot springs in Yellowstone, with a particular focus on the alkaline hot springs in the Sentinel Meadows area of the Lower Geyser Basin. Interestingly, many of the biomarkers here are enriched in 13C as they are at Lost City - indicating that 13C enrichment may be a common feature in alkaline hydrothermal environments. The hydrothermal ecosystem here is much more complex however, with numerous inputs from photosynthetic organisms and also from the surrounding Sentinel Meadows ecosystem
The hydrothermal system is dominated by Crenarchaeota in the higher-temperature regions, and by photosynthetic organisms including cyanobacteria in the lower temperature regions. The Crenarchaeota produce abundant tetraether lipids, including some that appear to have central core moities of more than 40 carbons, which is highly unusual. Through examination of both the structures and the carbon isotope content of these lipids, we are starting to get a better picture of the carbon cycling in this system. By participation in an interdisciplinary team studying microbiology, community genomics, and geochemistry of this system, we hope to get a better understading of how this system functions.
Polar lipids are those lipids that are strongly amphiphilic, meaning that they contain both a non-polar part (such as fatty acids) that is fat-soluble, and also a strongly polar part that is water-soluble. Examples are phospholipids and glycolipids, both of which have very polar headgroups connected through gylcerol to a non-polar core moiety. Lipids such as sterols are not strongly amphiphilic and are termed "neutral lipids".
The polar headgroups of glycolipids and phospholipids are easily lost after cell death. Although the rate at which this happens is not precisely known, intact polar lipids (IPLs) are thought to be a good marker for living cells. The structure of the headgroup is also additional information that we can use to distinguish taxonomic groups and discern environmental processes.
Polar lipids are separated by High-Pressure Liquid Chromatography (HPLC) and analyzed by Mass Spectrometry (MS). By connecting these instruments together (HPLC-MS), we can rapidly assess the lipids in an environmental or laboratory sample.
For example, at Lost City we determined that the polar headgroups associated with non-isoprenoidal diethers derived from bacteria were glycolipids. This was highly unusual because bacterial glycolipid diethers have never been detected before - all known bacterial glycolipids had fatty acid cores, and all known bacterial diethers were phospholipids. The unusual combination in Lost City hydrothermal vents could be an evolutionary relic. Alternatively, it may reflect the unique chemistry of this environment. Lost City has an alkaline pH and vent chimneys contain abundant brucite. This combination of fluid and mineral is expected to render phosphate insoluble. Without any phosphate in vent waters, bacteria are likely to conserve this element (required for DNA & RNA, optional for lipids) as much as possible. One way to do that? Make your lipids with sugars instead of phosphate!
Future work will include examining the relative degradation rate of various polar lipids, testing the hypothesis that polar lipids constitute a biomarker for living cells, and examining the degree to which tetraether lipids with different polar headgroups contain differences in core moieties - with an eye towards testing proxies such as TEX86.
Methanogens have likely been an important part of Earth's biosphere for billions of years. In order to reconstruct their imprint on the Earth, I am interested in understanding how isotopes undergo fractionation as they pass through the biosynthetic networks of methanognes.
As elements are processed through biosynthetic networks, they undergo isotope fractionation. Carbon is the central element of biomolecules, and the distribution of carbon isotopes in natural products is critical to our interpretations of past environments.
In order to interpret the isotopic content of natural products in the environment or in the rock record, we must understand the source of isotope fractionation. In biosynthetic networks, carbon isotopes are particularly useful. The magnitude of carbon isotope fractionation depends on the 13C content of the substrates, along with the substrate concentrations, the kinetic or equilibrium isotope fractionations imposed by various enzymes involved in processing the substrate, and the amount of carbon that flows to different parts of the biosynthetic products. The ultimate destination of carbon is the pool of metabolites, which includes DNA, RNA, protein, and lipids, along with products such as carbohydrates, intracellular metabolites (e.g. NADPH), and biological byproducts like CO2 or CH4. Needless to say, it can quickly get complicated.
Luckily, we can learn a lot of useful information by looking simply at bulk end-products. For example, in methanogens we can examine the 13C content of bulk biomass, individiual lipids, and produced methane. If we know the starting isotope composition and concentration of the substrates, we can measure the difference in 13C content between substrate and products.
We have done work with the methanogen Methanosarcina barkeri, examining growth on four substrates: H2/CO2, acetate, methanol, and trimethylamine. By varying the concentrations of these substrates we have observed the how this variable affects the observed isotope fractionations. By examining a variety of biological products (biomass, methane, and lipids) we can look for patterns that let us work backwards in the environment, by examining products and inferring information about the availability of substrates.
Future plans include a more refined look at the isotopic composition of methanogen products and development of a new tracer method that will (perhaps) be able to distinguish the source of methane in geological environments.
Sulfate Reducing Bacteria are among the most important biogeochemical players in anaerobic environments. These bacteria respire by transferring electrons from a donor molecule to sulfate, thereby reducing the sulfate to sulfide. The pathway by which this happens, and how this bears on the fractionation of sulfur isotopes, has been a subject of debate for decades. This is important, because sulfur isotopes are an important tool for tracking the oxidation state of the crust through Earth's history.
Recently I have been involved in a project to try to work out the isotopic consequences of this pathway in greater detail. This work (with David Johnston and Wil Leavitt) draws upon recent crystal structures published in the biochemical literature. These suggest that the classic pathways for sulfur isotope fractionation, while generally correct, are not correct in detail. In fact, sulfate reduction is a highly branched pathway in which sulfite plays a critical intermediate role.
Well I don't want to give away all my secrets! However, there are some obvious directions to take my current work...
My interests are in applying organic geochemistry to understand modern and ancient environments, and in using geochemical tools to understand biogeochemistry, evolution, ecology, and climate.
I will continue to work on genes and biomarkers. Specifically, I am interested in hopanoids, and also very interested in tetraether lipids - lipids that span the bilayer in some Archaea as well as in some Bacteria. Identifying the genes involved in synthesizing these lipids, and the physiological controls on their regulation will be critical to understanding the environmental distribution of these lipids. These lipids are used as important paleo-indicators of temperature. This may be valid in many instances, but perhaps not in others. Genes should provide the insignt into understanding this.
Many classes of lipids used as biomarkers have little-known genetic bases. In this vein, I am collaborating with Gordon Love to identify the distribution of genes involved in the synthesis of certain sterols that may mark the first appearance of animals in the geochemical record.
I also plan to continue understanding how biosynthetic pathways control the isotopic content of biological products. I am particularly interested in major isotopes (C, H, N, O, S). In lipids, this is mainly limited to the distrbution of carbon and hydrogen isotopes. The D/H ratio of lipids is very interesting, since it is probably controlled both by the source water available to an organism, and by the energy metabolism of the organism (since lipid D/H derives from the NADPH pool). I have some ideas to test controls on this in living organims... check back in a few months
Lipids can be extracted from biological, geological, or environmental in a number of different ways, but all boil down to separating compounds soluble in organic solvents (like hexane, dichloromethane, or chloroform) from those soluble in water or not soluble at all. Lipids are found in the organic phase, while molecules like DNA and protein will be found in the aqueous phase. Insoluble lipids - those bound together in macromolecules like kerogen - can be further analyzed by techniques like pyrolysis.
The lipid extract contains many different compounds. To analyze these, the compounds must be separated from each other. The separation is achieved by chromatography. A crude separation can be done in the laboratory by passing the lipid extract over a column of silica gel, and separating it into fractions such as saturated hydrocarbons, aromatic hydrocarbons, alcohols, fatty acids, etc. After this crude separation, the resulting fractions can be further separated by a specialized instrument coupled to a mass-spectrometer used to detect them. There are different ways to do this, depending on the nature of the compounds being analyzed.
GCMS (gas chromatography-mass spectrometry) is a method by which compounds are separated in the gas phase on a capillary column. The capillary column is placed in a temperature-controlled oven and slowly heated, while a flow of helium passes through the column. As each compound elutes from the capillary column, it passes into the mass spectrometer, where it is ionized and fragmented into its constituent pieces. The mass of each of these pieces is detected, forming a mass spectrum that is used as a molecular fingerprint to determine the identity of the compound. GCMS is ideal for compounds that are volatile - i.e. that can be put into the gas phase at less than 300 degrees Celsius.
LCMS (liquid chromatography-mass spectrometry) separates compounds dissolved in a liquid solvent by passing them over a solid-phase column that binds to organic compounds. The organic compounds initially 'stick' to the column. As the chromatography proceeds, the polarity of the solvent is slowly changed, until compounds unstick from the column and are washed by the solvent into the mass-spectrometer. The mass spectrometer then ionizes and detects the compounds in a manner similar to GCMS. LCMS is ideal for nonvolatile compounds, including highly polar compounds such as hopanoids and Intact Polar Lipids (IPLs).
Most of my lipid analyses at Harvard are done in the Pearson Laboratory
Analyses of stable isotopes of carbon and nitrogen in bulk organic material is achieved by first combusting the biological material to CO2 and NOx compounds, followed by reduction of NOx to N2. The CO2 and N2 gases can then be introduced into an isotope-ratio-monitoring mass spectrometer (irMS), where the relative abundance of 15N/14N or 13C/12C can be measured. Other treatments can be used to prepare materials for analysis of their sulfur, oxygen, or hydrogen isotope ratios.
The irms functions by first ionizing the analyte gas (e.g. CO2) and accelarating the positively-charged ions along a flight tube. The flight tube is surrounded by a magnet, which deflects the ions from their trajectory. The degree to which they are deflected depends on their mass - lighter ions (e.g. those containg 12C) are deflected more than heavier (e.g. those containing 13C). Faraday cups then collect the ions at the end of their flight path, inducing a voltage that can be measured. The relative ratio of the voltages can be compared to a standard, yielding a measurement of the 13C/12C of the sample.
We can measure the 13C/12C ratio of individual lipids by connecting a gas-chromatograph to an in-line combustion interface coupled to an irMS. This means that we can simply inject our lipid extract onto the GC. The data we get out are a series of peaks with isotope ratio information. The irMS tells you only the carbon isotope ratio of each peak - not its identity. The identity can be associated with a particular structure by comparing the GC-irMS chromatogram to our GCMS data.
Genetics is a powerful tool that can be used to query the function of lipids. By disrupting individual genes that we think are associated with lipid biosynthesis, we can examine the suite of products that occur when that gene is absent. Subsequently, we can reintroduce the disrupted gene to the cell to be sure that the gene disruption is indeed associated with the phenotype we observed.
Most of the genetics I do in the Marx lab involves the targeted deletion of individual genes. We use a system that relies on single-crossover homologous recombination, selection on antibiotics (generally ampicillin, chloramphenicol, and tetracycline), and then counterselection against sucrose using a sacB locus.
Using these techniques, we have been able to disrupt a number of the genes involved in hopanoid biosynthesis in Methylobacterium. These genes are arranged in three clusters on the genome, in the structure shown below.
Experimental Evolution is a powerful tool that can be used in the laboratory to gain a better understanding of the function of genes or molecules. The idea is simple: by disrupting a gene, some cellular function no longer operates. However, during many generations of evolution in the laboratory it is possible that compensatory mutations may arise in some cells. If these mutations confer a selective advantage, those cells will come to dominate the population. Essentially we "break" a bacterium... and then ask it to fix itself!
Experimental evolution works by simply growing cells in batch cultures, and performing a serial passage of cells from one flask to the next. Once a flask is fully grown, a small proportion of it is removed and serves as the inoculum for the next flask. This is repeated... and repeated... and repeated.
The prediction is that compensatory mutations will reflect the function of the genes that have been disrupted. So, for example, if hopanoids are involved in membrane permeaility, than an evolved hopanoid-free mutant might be expected to acquire mutations in functions that deal with transporting things across the membrane. Mutations can be identified by using next-generation sequencing techniques to resequence the entire genome of individual evolved isolates. The resequenced genome is compared to the ancestor, and mutations are identified. These can then be reintroduced into the ancestral strain, one-by-one, to identify those mutations which confer the greatest selective advantage.
Bradley, A.S.*, Pearson, A., Sáenz, J.P., & Marx, C.J. 2010. Adenosylhopane: The first intermediate in hopanoid side chain biosynthesis Organic Geochemistry 41: 1075-1081
Bradley, A.S.* & Summons, R.E. 2010. Multiple origins of methane at the Lost City Hydrothermal Field Earth and Planetary Science Letters 297: 34-41
Bradley, A.S.*, Fredricks, H., Hinrichs, K-U., & Summons, R.E. 2009. Structural diversity of diether lipids in carbonate chimneys at the Lost City Hydrothermal Field Organic Geochemistry 40: 1169-1178
Love, G.D., Grosjean, E., Stalvies, C., Fike, D.A., Bradley, A.S., Kelley, A.E., Bhatia, M., Meredith, W., Snape, C.E., Bowring, S.A., Condon, D.J., Grotzinger, J.P., & Summons, R.E. 2009. Fossil steroids record the appearance of Demosponges during the Cryogenian Period Nature 457: 718-721
Bradley, A.S.*, Hayes, J.M., & Summons, R.E. 2009. Extraordinary 13C enrichment of diether lipids at the Lost City Hydrothermal Field indicates a carbon-limited ecosystem. Geochimica et Cosmochimica Acta 73: 102-118
Cohen, P.A., Bradley, A.S., Knoll, A.H., Grotzinger, J.P., Jensen, S., Abelson, J., Hand, K., Love, G., Metz, J., McLoughlin, N., Meister, P., Shepard, R., Tice, M., & Wilson, J.P. 2009. Tubular compression fossils from the Ediacaran Nama Group, Namibia Journal of Paleontology 83: 110-122
Londry, K.L., Dawson, K., Grover, H.D., Summons, R.E. & Bradley, A.S.* 2008. Stable carbon isotope fractionation between substrates and products of Methanosarcina barkeri. Organic Geochemistry 39: 608-621
Martinez, A., Bradley, A.S., Waldbauer, J.R., Summons, R.E., & Delong, E.F. 2007. Proteorhodopsin photosystem expression and photophosphorylation in a heterologous host. Proceedings of the National Academy of Sciences, USA 13: 5590-5595
Summons, R.E., Bradley, A.S., Jahnke, L.L., & Waldbauer, J.R. 2006. Steroids, triterpenoids. and molecular oxygen. Philosophical Transactions of the Royal Society B. 361: 951-968
Kelley, D. S., Karson, J. A., Fruh-Green, G. L., Yoerger, D. R., Shank, T. M., Butterfield, D. A., Hayes, J. M., Schrenk, M. O., Olson, E. J., Proskurowski, G., Jakuba, M., Bradley, A., Larson, B., Ludwig, K., Glickson, D., Buckman, K., Bradley, A. S., Brazelton, W. J., Roe, K., Elend, M. J., Delacour, A., Bernasconi, S. M., Lilley, M. D., Baross, J. A., Summons, R. E. & Sylva, S. P. 2005. A Serpentinite-Hosted Ecosystem: The Lost City Hydrothermal Field. Science 307: 1428-1434
A.S. Bradley Expanding the Limits of Life. Scientific American December 2009
Three in review, more on the way. Check back soon!
Links to the websites of several of my colleagues and mentors in geology, geochemistry, and microbiology