Bacterial cheaters, and investors: Non - polymer degraders, extracellular enzyme producers, and their roles in leaf decomposition

Background/Question/Methods

During the process of decomposition, extracellular enzymes catalyze breakdown of polymeric compounds into smaller molecules that can be taken up by cells. Little is known about the influence of “cheating” bacteria that do not secrete extracellular enzymes but use monomers resulting from the activity of extracellular enzymes secreted by other organisms (“investors”). The goal of this study was to isolate and identify cheating and investing bacteria from environmental samples. We hypothesized that higher levels of fungal biomass would result in greater extracellular enzyme activity and consequently, a greater population of cheating bacteria. We also hypothesized that increased moisture content would increase diffusion of extracellular enzymes and monomers from investors to cheaters, and therefore would favor cheating bacteria. Samples were collected from Jennings Woods, an experimental forest in Northeast Ohio, USA. We sampled decayed leaves from three habitats in and around vernal pools comprising a complete factorial design of two treatments: high or low fungal biomass based on the observation of fungal hyphae, and high or low moisture. Samples were used to inoculate bi-layer gradient agar plates where the lower layer contained monomers and the upper layer contained polymers as the carbon source.

Results/Conclusions

After inoculation on the bi-layer gradient agar plates, the numbers of bacterial colonies growing on each plate were counted. Results showed that colony counts were higher on the side of the plate containing the higher concentration of monomers (P<0.01). However, differences among treatments (high or low fungal biomass and high or low moisture) were not statistically significant (P>0.05). Screening was carried out for the growth of the isolates on the polymer cellulose and its monomer cellobiose and results showed that 40 percent of the total isolates did not grow on cellulose while all of the isolates grew on cellobiose. Fungal biomass and moisture treatments did not differ significantly in the proportion of isolates that could grow on cellulose (P>0.05). Non-polymer degrading bacteria were isolated and appear to be cheaters, however, there was no detectable difference among treatments.