Exobiology and Evolutionary Biology

DESCOPE: Biosignatures in Evolving Planetary Atmospheres (Tasks 1 & 2 only)

PI: Paul Falkowski

We propose to investigate how the atmospheric composition of a planet, in particular the concentrations of two potential biosignature gases, O ₂ and N ₂ O, evolve during the transition from an anaerobic to aerobic environment. Although O ₂ is considered a biosignature gas in its own right, and can be sensed remotely, the presence of O ₂ and N ₂ O would be compelling evidence of non-equilibrium metabolic reactions that are related to life as we presently know it. Our proposed research seeks to develop a data driven model that represents key microbial processes (photoautotrophy, chemoautotrophy, and heterotrophy) which link the carbon, oxygen, and nitrogen cycles, and explicitly predicts the atmospheric concentrations of O ₂ and N ₂ O during the transition of a planet from mildly reducing to oxidizing conditions. The model will be parameterized from experimental data derived from microbial physiological investigations in the laboratory which focus specifically on the effect of O ₂ on the flux of N ₂ O during the oxidation of ammonium and the reduction of nitrate. To further understand the couplings and feedbacks between the C, N, and O cycles on Earth, we will measure ¹³ C and ¹⁵ N of organic matter in Archean and early Proterozoic rock samples, focusing on the transition from reducing to oxidizing condition approximately 2.3 Ga. On Earth, the coupling between these three cycles has likely played an important role in delaying the rise of atmospheric oxygen and the emergence of higher life forms, and may play a similar role on other Earth-like planets. The study of the aerobic-anaerobic transition process is highly relevant to a variety of NASA planetary characterization missions, such as TPF, where visible and IR planetary spectra may detect small amounts of O ₂ , or O ₃ and N ₂ O.