Oxygen Dynamics in a Vertically Stratified Microbial Community

Microbes in sediment naturally self-organize into distinct layers, each of which is composed of microbes that exhibit a common metabolism. This ubiquitous vertically stratified structure arises as microbes compete for oxidants and carbon sources, which diffuse from the surface. We study the dynamics by which the topmost stratum, corresponding to aerobic heterotrophy, emerges in homogenized salt marsh sediment. Using a fluorescent oxygen detector, we measure the 2D oxygen distribution with sub-millimeter spatial resolution at 1 min intervals over the course of several days. We vary the concentration of carbon sources in the sample. Assuming Michaelis-Menten kinetics for the metabolic rate of the constituent microbes, we invert the measured oxygen concentration for the rate of oxygen consumption. This analysis reveals how the oxygen penetration depth relaxes to steady state as microbes reproduce, die, and migrate through the sediment. These measurements give insight into the stability of microbial ecosystems in sediment.