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.