  Organic material in anoxic sediment represents a globally significant carbon
reservoir that acts to stabilize Earth's atmospheric composition. The dynamics
by which microbes organize to consume this material remain poorly understood.
Here we observe the collective dynamics of a microbial community, collected
from a salt marsh, as it comes to steady state in a two-dimensional ecosystem,
covered by flowing water and under constant illumination. Microbes form a very
thin front at the oxic-anoxic interface that moves towards the surface with
constant velocity and comes to rest at a fixed depth. Fronts are stable to all
perturbations while in the sediment, but develop bioconvective plumes in water.
We observe the transient formation of parallel fronts. We model these dynamics
to understand how they arise from the coupling between metabolism, aerotaxis,
and diffusion. These results identify the typical timescale for the oxygen flux
and penetration depth to reach steady state.
