The primary goal within the group is to better understand the relationship between microorganisms and Earth surface evolution. Our work ranges from geologically rooted questions, where we aim to track the onset or environmental expression of different metabolic processes and follow atmospheric/oceanic oxidation, through to modern processes and environments (such as experimental work with extant organisms, purified protein, work in the modern ocean water column, and early diagenesis in marine sediments). A flavor for some of this work is demonstrated below.
(courtesy of Dr. Vivian Cumming)
The microbial reduction of seawater sulfate (dissimilatory sulfate reduction) is critically important to both the short- and long-term budgets of carbon, oxygen and sulfur on the Earth’s surface. Fortunately, these microorganisms leave a dramatic isotopic fingerprint of their activities in the sediments (and eventually rocks) they inhabit. We work to understand this metabolism and its isotopic consequences through microbial experiments and modeling exercises. Most recently, we performed a suite of chemostat (continuous flow and steady state bioreactor) experiments to calibrate the response of isotopic fractionation to growth and reduction rate. This relationship can then be parlayed through a simple model to explain the majority of the isotopic variance measured in sedimentary sulfides throughout the Meso- and Cenozoic (as presented in the Figure below from Leavitt et al., 2013 in PNAS). Interestingly, it is well understood that shallow marine sediments are the hotbed for biogeochemical activity. As such it is perhaps unsurprising that our predicted rates scale with estimates of size of the footprint for shallow marine settings.
The backbone of reconstructing Earth’s ancient oxidant budgets rests with the carbon cycle, and it has been long appreciated that Neoproterozoic (which is postulated to contain Earth’s second great oxidation event) carbonate records are complex. Through detailed analyses the carbon isotopic composition of carbonates and organic matter, work from within the group if forwarding a revised view of the chemical evolution of the ocean through this time interval. Depicted below, much of this work stems from field-work (see the Macdonald group page) in Mongolia, and by extension, northwest Canada and Alaska. In complement to work on the carbon cycle, studies focusing on the Fe cycle and coincident changes in paleontological records relates these storylines in a mechanistic fashion to the evolution of animals.
Underpinning all this work is a wide suite of specialized tools. In addition to analytical capacities supported within other research groups in EPS, in the Johnston Lab we regularly measure many different isotope systems, the most central of which are the multiple sulfur isotopes [32S, 33S, 34S, 36S] and oxygen isotopes in sulfates. Also coming soon is the capacity to measure 17O in oxides. Further, we supplement isotopic studies with a variety of elemental analyses (namely Fe and P).
Though these represent ongoing work and reflect the general lab philosophy, suggestions and new thinking is always welcome.