We present a three species model describing the degradation of substrate by
two competing populations of microorganisms in a marine sediment. Considering diffusion
to be the main transport process, we obtain a reaction diffusion system (RDS) which we
study in terms of spontaneous pattern formation. We find that the conditions for patterns
to evolve are likely to be fulfilled in the sediment. Additionally, we present simulations
that are consistent with experimental data from the literature. We focus on the interaction
between pattern formation and non-uniform forcing, which naturally occurs in our model,
since layers closer to the sea water are richer in particular chemicals than deeper layers.
We find that heterogeneous forcing has a strong impact on the structures that evolve, leading to
the formation of banded structures or even pulses instead of the well-known Turing patterns.