Microbes require carbon for respiration and growth. Microbes present in terrestrial ecosystems carry out decomposition of plant polymers using extracellular enzymes to fulfill their carbon requirement. During degradation of a specific plant polymer, microbes can be categorized as investors or cheaters based on their contribution to the extracellular enzyme pool. Investors are microbes that contribute exoenzymes to decompose that polymer. On the other hand, cheaters do not contribute exoenzymes, but still take up monomers produced through the action of exoenzymes.
Temperature may impact cheating by affecting microbial physiology and gene transcription as well as by altering the rate of aqueous diffusion of exoenzymes and degradation products. Temperature affects reactions rates, protein structure and function. Furthermore, at higher temperatures, exoenzymes and degradation products experience a greater rate of aqueous diffusion resulting in increased availability of degradation products to the microbe.
In this study we looked at the effect of temperature and competition on microbial physiology and gene transcription in relation to cheating in fungi involved in plant matter degradation. Three model fungi - Trichoderma reesei QM6a, Phanerochaete chrysosporium RP-78, and Rhodotorula mucilaginosa PTD3 were grown as monocultures and as co-cultures in sand microcosms containing either ground beech leaves or cellulose at either 20 °C or 30 °C. We analyzed gene expression (Illumina next generation sequencing) and many physiological parameters to find evidence of cheating. We determined whether a particular organism was cheating during growth in co-culture by measuring shifts in gene expression to reveal physiological responses and changes in gene copy number as a proxy for altered biomass of individual organisms.
Results from our study revealed evidence of cheating. During growth in co-culture with T. reesei, P. chrysosporium was cheating by significantly down-regulating its expression of glycoside hydrolase genes while significantly up-regulating its expression of transporter genes. Similarly, qPCR data revealed that R. mucilaginosa was cheating during growth in co-culture with P. chrysosporium on cellulose.
We also found evidence that temperature and substrate type have an effect on cheating. P. chrysosporium had a larger increase in transporter gene expression at 30 °C than 20 °C, suggesting that cheating by P. chrysosporium was greater at 30 °C than 20 °C. During growth in co-culture with P. chrysosporium, qPCR data revealed that R. mucilaginosa cheated when grown on cellulose but not on ground beech leaves, suggesting that substrate type affected cheating.