Australians love eating mushrooms. Every year we consume nearly 70,000 tons of button mushrooms (Agaricus bisporus), which are a valuable source of dietary protein, anti-oxidants and vitamins C and D. Button mushrooms are farmed commercially on a specialized mushroom compost that is produced from recycled agricultural wastes (wheat straw and chicken manure). The controlled, industrial-scale microbial process of mushroom composting includes three stages: mesophilic wetting, thermophilic breakdown of cellulose and hemicellulose, and mesophilic conditioning. Each stage is characterized by a succession of typical microbial communities, and the combined process therefore constitutes a fascinating system for the study of microbial interactions under well-defined conditions on an industrial scale.
The later stages of composting see a rapid increase in the thermophilic fungus Mycothermus thermophilus, which makes up >90% of the compost fungal population by the end of conditioning. This ascomycete assimilates the cellulose supplied by the wheat straw, and the accumulated fungal biomass provides the initial food source for the Agaricus mycelium when it is added to the conditioned compost. This Mycothermus biomass is associated with several specific bacterial species, including heterotrophic nitrifiers (Pseudoxanthomonas), and a novel genus of chitinolytic bacteria (Mycovorax). The presence of Pseudoxanthomonas stimulates cellulase production by Mycothermus, though the bacteria cannot themselves degrade cellulose. Mycovorax, in contrast, is a predator of Mycothermus, producing enzymes that actively degrade its mycelium. The bacterial ecosystem is complemented by a broad range of bacteriophage that attack specific strains of Pseudoxanthomonas. Our work on the specific fungal/bacterial/phage interactions that mediate mushroom composting aims to understand these interactions in detail, while also maximizing output and quality of the mushroom crop and addressing questions of nutrient retention, greenhouse gas release and water conservation.