Research in the Fischer Group.
Fundamental geobiological transitions in Earth’s history—like the evolution of oxygenic photosynthesis and rise of oxygen—present big and complex problems that demand a blend of insights from comparative biology, biochemistry, ecology, geology, geochemistry, and paleontology. Each of these perspectives has strengths and weaknesses. For example, comparative biology lends a high level of mechanistic detail and enables rigorous hypothesis testing but can only help ordinate evolutionary events in relative time, and because it relies on extant organisms, is blind to extinction. Geology and geochemistry on the other hand can provide an empirical historical record of geobiological processes but have challenges associated with accurately reading paleoenvironmental chemistry from sedimentary rocks and can only paint metabolic and ecological processes in broad and coarse brushstrokes. But because geological and biological records present different views of the same history, there is substantial opportunity in combining understanding gained from these different perspectives. We leverage these different ways of knowing to tackle a range of historical geobiological problems. Below are brief descriptions of several ongoing projects in the group.
The rise of atmospheric oxygen circa 2.35 billion years ago is one of the most marked environmental changes in Earth history, and this transition ultimately stems from a major biological innovation. The evolution of oxygenic photosynthesis conferred the ability to use water as a photosynthetic substrate (earlier photosynthesis was anoxygenic and required reduced iron, sulfur, carbon, or hydrogen). Primary productivity was no longer limited by a source of electrons. Molecular oxygen became widely available for use in anabolic and catabolic metabolisms, forming a rich cascade of evolutionary potential and consequence. This innovation profoundly altered biogeochemical cycles, led to the buildup of oxidants in the atmosphere and oceans, and ultimately paved the way for modern surface environments bathed in free oxygen. However, there are still many things that we do not understand about this transition, and perhaps chief among these are fundamental issues regarding its timing and proximal cause. Different paleoenvironmental and paleobiological proxies suggest conflicting time frames, ranging from 3.5 billion to 2.3 billion years ago—highlighting the possibility that oxygenic photosynthesis might predate the appearance of oxygen by over a billion years. We're studying several Archean and Paleoproterozoic-age sedimentary successions to inform and resolve of these contradictions.
One of the reasons that fundamental geobiological transitions in Earth history like the evolution of photosynthesis and rise of oxygen remain relatively poorly understood, at least in a mechanistic sense, is related to the way in which they are studied: commonly from the perspective of a single angle, approach, or geochemical proxy. This work has been valuable insofar as it revealed that data sets from different methods often yield conflicting results. These discordances result from the distinct redox and mass flux behavior of individual proxies, complicated by biases inherent to the sedimentary record—in particular, incomplete records of many paleoenvironments, changing lithologies and facies, and ever-present diagenesis and metamorphism affecting ancient rocks. In the Fischer Group we are working with a range of powerful and complementary techniques that allow us to better evaluate the quality of proxy data generated from ancient sedimentary rock samples collected from stratigraphic sections in South Africa and Western Australia. All rocks of early Precambrian age have complications introduced from post-depositional processes, but we do not have a strong understanding of how these processes might have impacted geochemical data. A common limitation of virtually all proxy measurements employed to date is that they operate on ‘bulk’ samples, typically gram-sized or larger pieces. As such, they lose the ability to relate geochemistry to petrography (which allows one to tell time from cross-cutting relationships) at the scale of mineral grains. A central theme of our work is bringing to bear analytical techniques that allow mapping of properties at micrometer scales, in essence “imaging” the proxy data. We use light and electron microscopy for petrography, electron microprobe and synchrotron XRF for elemental composition, synchrotron X-ray spectroscopy for redox state, scanning SQUID microscopy for remnant magnetism and the timing of iron mineralization, and secondary ion mass spectrometry (SIMS) to make isotope ratio measurements and inform bulk rock isotope data. The combination of these tools, and the ability to move back and forth between them working on the same samples is essential for disentangling the complex histories associated with ancient sedimentary rocks and recovering accurate paleoenvironmental signals.
Detrital pyrite in in Paleoproterozoic deltaic sandstones. These grains are common in Archean and Paleoproterozoic clastic sedimentary rocks and provide prima facie evidence for low oxygen concentrations in Earth surface environments prior to 2.35 Ga. Scale bars are 100 microns.
Evolution of photosynthesis. Oxygenic photosynthesis is the most important bioenergetic innovation in the history of life and it truly transformed our planet. Oxygenic photosynthesis is also an evolutionary singularity—it evolved once. The critical photochemical innovation here was the water-oxidizing complex (WOC) of photosystem II—a cubane cluster of four Mn centers and a Ca center, bound by oxo ligands—that acts as a capacitor to link the single electron photochemistry of the reaction center to the four electron chemistry needed to oxidize two waters to one molecule of oxygen. The processes that led to the innovation of the WOC have remained largely unknown. From an evolutionary perspective, it is notable that modern biological water-splitting in photosynthesis begins with Mn oxidation in the water-oxidizing complex of photosystem II, suggesting several evolutionary scenarios wherein Mn(II) once played a key role as an electron donor for photosynthesis prior to the evolution of oxygenic photosynthesis. To test this hypothesis, we're studying the behavior of the ancient Mn cycle using newly obtained scientific drill cores through an early Paleoproterozoic succession of sedimentary rocks preserved in South Africa. These strata contain substantial Mn enrichments (up to ∼17 wt %), well before those associated with the rise of oxygen. However, all early Precambrian successions have been impacted by metamorphic and metasomatic processes—a challenge that demands thoughtful approaches that connect chemistry and mineralogy to petrography and leverage the insights available cross-cutting relationships to evaluate the quality of redox proxy data. Using both bulk and novel microscale X-ray spectroscopic techniques coupled to optical and electron microscopy and stable carbon isotope ratios, we determined that the Mn is hosted exclusively in carbonate mineral phases derived from reduction of Mn oxides during diagenesis. Additional observations of independent proxies for molecular oxygen—multiple S isotopes (measured in bulk by isotope-ratio mass spectrometry and in situ by secondary ion mass spectrometry) and redox-sensitive detrital grains—reveal that the original Mn-oxide phases were not produced by reactions with oxygen, which points to a different high-potential oxidant. These results illustrate that the oxidative branch of the Mn cycle predates the rise of oxygen, and support the hypothesis that the WOC evolved from a former transitional photosystem capable of single-electron oxidation reactions of Mn.
Left: WOC of PSII. Right: Mn redox map of the Paleoproterozoic iron formation. Hot colors correspond to Mn-oxides and cool colors Mn-carbonates (field of view is ca. 2 mm).
We’ve also begun using genomics to study the acquisition of photosynthetic characters within the Cyanobacteria clade—the clade responsible for effectively all of the molecular oxygen on the modern fluid Earth. It is becoming clear from environmental sequencing efforts that Cyanobacteria are more diverse than previously realized. Molecular markers commonly used to asses environmental diversity demonstrate the existence of many uncharacterized clades of deeply-branching Cyanobacteria, many of which do not appear to be phototrophic on the basis of their environmental context (e.g. within the animal gut). Incomplete genomes assembled from metagenomic datasets suggest these organisms have a strict fermentative physiology. However, working with G. Dick and colleague J. Leadbetter, we’ve discovered several members of these deeply-branching Cyanobacteria that have high potential electron transport metabolisms. This particularly exciting because they have machinery shared with phototrophy (e.g. complex III/cytochrome b6f) and will offer valuable insight into the evolution of photosynthesis within the Cyanobacteria.
Cyanobacterial diversity from genomic and metagenomic datasets based on RpoB, a phylogenetic marker gene. Note the diversity of deeply branching groups.
Evolution of aerobic respiration. Tthe evolution of photosynthesis and aerobic respiration are inexorably intertwined, not simply because one provides the substrate for the other, but they rely on common molecular machinery and are wired in similar ways. We have begun examining how changes in the fluxes and concentrations of oxygen in Earth surface environments impacted the evolution of aerobic respiration (specially the heme-copper oxidoreductases). Again, genomic and metagenomic data reveal that this superfamily of proteins is extremely diverse and widespread with members playing key roles in both aerobic and anaerobic respiration (including novel microbial processes like NO dismutation for intra-aerobic metabolisms). New data, experiments, and analyses regarding the diversity, distribution, kinetics, and phylogenetic relationships of HCOs should allow us to ordinate oxygenic photosynthesis and aerobic respiration, place absolute time constraints on these transitions from fresh observations of the geological record, and also address longstanding hypotheses about eukaryotic evolution and oxygen concentrations in the atmosphere and oceans throughout Precambrian time.
Isotope mass balance approaches to understanding oxygen. Two key observations of the oxygen cycle present an apparent paradox: 1) atmospheric oxygen is remarkably dynamic with a residence time measured in hundreds of years, and 2) yet we observe atmospheric oxygen concentrations to be stable on timescales of hundreds of millions of years. The tension between these observations implies the operation of a set of feedbacks, albeit enigmatic, that maintain oxygen concentrations on Earth. In spite of incomplete understanding of the mechanics at work here, it is still possible to evaluate the history of oxygen in the atmosphere by constructing budgets for its carbon and sulfur cycle sources (organic carbon and pyrite burial) and sinks (rock weathering and volcanic outgassing) from stable isotope ratio data collected from sedimentary rocks. Isotopes are powerful tracers of mass flux, but require assumptions in interpretive frameworks that deserve thoughtful testing.
Carbon and sulfur cycle perturbations, as measured by stratigraphic trends in isotope ratio data, provide a set of historical experiments and the opportunity to test and explore the quality of these assumptions. We're studying a number of these events in sedimentary successions preserved across the world. The largest recorded carbon isotope excursion in Earth history is observed globally in middle Ediacaran carbonates—the Shuram excursion. The causes of this event have been the subject of much debate. Through a number of studies we’ve shown that this excursion appears to record the time-series history of marine DIC in carbonates but not in coeval organic phases. This behavior is not predicted, and consequently the Shuram excursion provides an important historical example that changes how we view the limits of carbon cycle function and redox change, during a key time in animal evolution.
Rank order of carbon cycle perturbations measured by the trends in the isotopic composition of marine carbonate rocks, colored by age. The largest perturbations in Earth history occur in Neoproterozoic sedimentary basins.
The sulfur cycle is commonly considered to share the same isotope mass balance topology as the carbon cycle, with subequal sinks of evaporites and pyrite, but assumptions of the isotopic composition of the inputs and biogeochemical fractionations are far less certain than for carbon. This framework can be tested by taking a different approach and leveraging independent constraints on these mass fluxes from the sedimentary record. We measured the sedimentary fluxes of evaporites over the past 500 million years and arrived at a different set of mass balance solutions that better fit our current knowledge of the Phanerozoic sulfur isotope record. This study has thus far revealed two key results: 1) Sulfate evaporite burial is unsteady and largely controls secular changes in the isotopic compositions of seawater sulfate and sedimentary rocks more than pyrite burial, and 2) we were able to untangle gross from net sulfur cycle fluxes on million year timescales and observe that the sulfur cycle is effectively a pyrite weathering and burial machine that holds a tighter grip on atmospheric oxygen than previously thought.
Early Mars and comparative planetology. Over the past decade, rover and orbiter data have provided insights into the Martian sedimentary record that contextualize Earth’s early environmental history. The challenges here are teasing out paleoenvironmental insight from observations of the structures, textures, stratal geometries, mineralogy, and geochemistry of Martian sedimentary rocks, and then evaluating this information in the context of mass balance frameworks that have been so useful for understanding interactions between the rock cycle and the atmosphere and oceans on early Earth. A range of observations suggest that Mars was once warmer and wetter than today. With M. Lamb, R. DiBiase, and reading group participants we discovered the erosional remnants of deltaic deposits within a thick clastic wedge of sedimentary rocks that onlaps the crustal dichotomy. The location of this ancient delta is notable because it does not sit in a confined basin, and supports hypotheses for a large ocean basin across the northern lowlands of Mars at one point in its history. With J. Eiler and I. Halevy we were able to constrain the temperature at which four billion-year-old regolith carbonates precipitated in the martian meteorite ALH84001 using clumped isotope paleothermometry, finding 18 ± 4 ºC. With Joel Hurowitz, we examined the fundamental acid-base problem presented by acidic mineralogy of the Burns Formation at Meridiani planum. The production of acid in these sedimentary environments was intimately related to redox processes—the oxidation of iron in emerging groundwaters. Estimates of the magnitude and timing of the process from orbital data provided a fascinating but counterintuitive result: though Mars surface environments were oxidizing, the atmosphere was more H-rich than today. These redox constraints offer an important reflection on the redox status of the early Earth. It has become common to infer the presence of molecular oxygen at very small concentrations in the paleoenvironment as a marker for Cyanobacteria. Mars provides us the opportunity to test the quality of this assumption.
HiRISE digital elevation model superimposed over HiRISE imagery of deltaic deposits at Aeolis Dorsa, Mars, now exposed in inverted topography. Inset shows a slope map of exposed delta foresets, dipping ca. 4 degrees to the southeast. These deposits are unconfined and sit north of the crustal dichotomy.
Additional ongoing research projects include: Origin of Archean and Proterozoic iron formation. Distribution and evolution of lipid biomarker synthesis. Limits of high potential phototrophy. Coupled behavior of redox and acid-base processes at critical transitions in Earth History, including Neoproterozoic Snowball Earth(s) and the P-T mass extinction. Late Ordovician mass extinction and climate change.
Research opportunities in the Fischer Group.
