2009 Annual Science Report
Arizona State University Reporting | JUL 2008 – AUG 2009
Stoichiometry of Life, Task 4: Biogeochemical Impacts on Planetary Atmospheres
The abundance of molecular oxygen in planetary atmospheres may be a useful way to look for evidence of life. The amount of photosynthetically produced oxygen that accumulates in an atmosphere depends in part on the export of photosynthetically produced organic carbon from the ocean surface to the seafloor, which in turn may depend on the availability of bioessential elements. We are using a computer model to determine how this carbon export processes might operate in an ocean dominated by prokaryotes rather than eukaryotes, as may have been common in Earth’s past and as an analog for hypothetical extrasolar planets.
Oxygenation of Earth’s atmosphere must have involved an efficient mode of carbon burial in the early ocean. In the modern ocean, carbon export is dominated by fecal pellets and aggregates produced by the animal grazer community. However, during most of Earth history the oceans were dominated by prokaryotes causing a substantially altered biogeochemistry and mode of aggregate formation.
In a first step of modeling early ocean’s carbon cycle, collaborator Watson Gregg, coordinating with co-I Neuer and PI Anbar, applied the NASA Ocean Biogeochemical Model (NOBM) using cyanobacteria as the only photosynthetic group in the oceans (i.e., modern diatoms, chlorophytes, and coccolithophores were removed from the standard NOBM configuration). They were consumed by primitive heterotrophic bacteria.
First results show that total primary production decreased about 19% from the standard configuration because cyanobacteria are relatively slow growers compared to the more modern phytoplankton. The reduced growth produced increases in nitrate and dissolved iron in the surface ocean, as the reduced growth resulted in less efficient uptake.
A second experiment involved the removal of consumption of the cyanobacteria by excluding heterotrophic bacteria. This was intended to simulate a scenario in which primitive autotrophs are present but heterotrophs are not. The experiment resulted in model instability. This was due to lack of ammonia regeneration through the process of consumption. The source of the error causing the instability is not known at this time and is under investigation.
In future experiments we will continue optimizing this early ocean model of the prokaryotic carbon cycle using the NOBM, and include parameters on stickiness and aggregate formation of a model cyanobacterium which we are currently growing in the laboratory as a function of nutrient limitation.
PROJECT INVESTIGATORS:Susanne Neuer
PROJECT MEMBERS:Ariel Anbar
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Production of complex life.
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems
Biosignatures to be sought in nearby planetary systems