Enzyme activities of mesofaunal endosymbionts across host species and ecosystem

Background/Question/Methods

Organic matter decomposition plays an important role in soil carbon (C) and nitrogen (N) cycling. Of particular importance is the breakdown of large molecular-weight carbon compounds into smaller polymers and monomers, a process that is carried out primarily by microbially-produced extracellular enzymes. Detritivorous soil mesofauna also contribute to decomposition through feeding activities that fragment and redistribute litter, and by offering a unique, substrate-rich gut environment that may host a highly-active microbial community. The digestive capabilities of these endosymbiotic microorganisms could carry out a disproportionally large amount of the organic matter breakdown that occurs in the soil ecosystem and may constitute an as-of-yet uncharacterized mechanism explaining the well-known concept that mesofauna greatly affect soil C and N cycling. Thus, we tested the hypothesis that enzyme activities in oribatid mite guts were greater than, and different from, the enzyme activities in bulk soil or litter. Further, we contrasted the enzyme activities of endosymbionts in three oribatid mite species within and across three replicated forest and row-crop agricultural systems. Enzyme activities were tested using modified florescence and absorbance-based microplate methods with soil, litter, and mite gut material.

Results/Conclusions

Preliminary data show that while cellulase activities contributed similarly to overall enzyme activities (40% in litter vs. 36% in guts) the contributions of other enzymes varied considerably between corn litter and mite guts (chitinase: 22 vs. 50%; phosphatase: 26 vs. 14%). Additionally, some enzymes were detected within decomposing litter but were absent or only detected at low concentrations in mite guts (phenol oxidase and aminopeptidase). On a per-mass basis, mites had greater chitinase and phosphatase activities than litter (~90 and ~5 times, respectively). These data demonstrate the likelihood that we will find differences between enzyme activity of mite guts and soil. Furthermore, due to the predicted importance of microbes in mite gut enzyme production, as well as the observation that soil microbial community structures differ across ecosystems, it is plausible that gut enzyme activities may also vary by environment.  It is unknown whether mite species differ in gut enzyme activities, but such a phenomenon could have important consequences for how the composition of detritivorous soil mesofaunal communities affect C and N cycling. Our results will elucidate how mite endosymbionts affect decomposition dynamics, and how these effects may differ between species and across environments.