2012 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2011 – AUG 2012
Geochemical Signals for Low Oxygen Worlds
We are studying the physiology of sulfate reducing bacteria, organisms that perform a key microbial metabolism in anoxic worlds. By calibrating microbial sulfur isotope effects, we can infer the redox level of paleoenvironments in the geologic past by studying sedimentary records. The sulfur cycle is intimately linked to the redox budget of the Earth’s surface, such that this study will help inform us about the evolution of aerobic environments, a key process that set the stage for animal evolution. Similarly, we also are studying the role of oxygen in controlling the budget and transformations of nitrogen in the ocean. Nitrogen is a critical nutrient limiting marine production, and the balance of its redox cycling controls how much nitrogen is added or removed from the ocean by redox-sensitive processes.
Microbial sulfate reduction (MSR) produces large and variable sulfur isotope effects, and the isotopic signatures can be preserved in rock records. Sulfur isotope records, thus, may inform us about the redox level of depositional environments as well as about the evolution of oxygen in the deep time since biogeochemical cycles of sulfur and oxygen are intimately related. The key to understanding the isotope record is the magnitude of isotope fractionation produced by microbial processes.
Shuhei Ono, Tanja Bosak and Min Sub Sim (graduate student) carried out a series of laboratory culture experiments for microbial sulfate reduction and demonstrated that 1) carbon source is the primary controlling factor for the magnitude of sulfur isotope effect (Sim et al., 2011a), and 2) the maximum isotope effect can reach the thermodynamic limit (Sim et al., 2011b). The result is applied to constrain the extent of the deep biosphere in the oceanic crust (Ono et al., 2012). A new publication (Sim et al., 2012) reports the effect of Fe and N limitation on MSR growth and isotope fractionation. We suggested that Fe limitation would be important in sulfidic environment that might contribute large sulfur isotope effect seen in euxinic environments.
The major pool of fixed nitrogen in the oxic, modern ocean primarily is the oxidized form, nitrate. This contrasts with anoxic periods of Earth’s history in which the dominant form would have been ammonium, a reduced species. One major question is whether such major shifts in nitrogen redox state influence the balance of primary productivity in the oceans between eukaryotic and prokaryotic plankton. To answer this, Ann Pearson, graduate student Meytal Higgins, and undergraduate student Leah Tsao conducted a study to examine the nitrogen isotopic composition of chlorophyll degradation products during major anoxic events of the Cretaceous. The conclusions are that even during this time, most productivity was eukaryotic, and that differences in the nitrogen isotopic composition of chlorophyll degradation products are diagnostic for the type of ecology (Higgins et al., 2012; Tsao et al., 2012). In related work, we also surveyed the microbial diversity of the extremely euxinic (sulfide- and ammonium-rich) Mahoney Lake, BC, Canada (Klepac-Ceraj et al., 2012), concluding that microbial diversity remains very high in sulfidic bodies of water.
PROJECT INVESTIGATORS:Shuhei Ono
Project InvestigatorAnn Pearson
PROJECT MEMBERS:Tanja Bosak
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Earth's early biosphere.
Production of complex life.
Environment-dependent, molecular evolution in microorganisms
Biosignatures to be sought in Solar System materials