Fungi
Aquatic fungi are the primary decomposers of refractory organic matter in the simulator. They fill an ecological niche that heterotrophic bacteria cannot: breaking down tough, complex substrates like humic polymers, cellulose, and lignin-like compounds via specialized extracellular enzyme systems. This is a one-way relationship -- fungi process what bacteria cannot, but in doing so they produce labile dissolved organic matter that bacteria then exploit. This handoff -- the fungal conditioning effect -- is the central ecological function of fungi in the model and the mechanism that drives the fungal-to-bacterial decomposition succession observed in Walstad-style aquaria.
The model represents fungi as a single functional group combining aquatic hyphomycetes (filamentous fungi that colonize solid substrates) and chytrids (small, often unicellular fungi that produce motile zoospores). Hyphomycetes dominate biomass; chytrids contribute the grazeable zoospore fraction.