California State Polytechnic - Humboldt, United States
Background/Question/Methods
Microorganisms engage a variety of metabolic pathways to aid in degradation, driving global biogeochemical cycling. These microbial functional roles can be studied using a classical biodiversity ecosystem function (BEF) approach; yet the complexity of plant and animal tissue degradation may require us to expand on current BEF theory. We focus on the degradative function of the inquiline microbial communities found within pitcher plants. Darlingtonia californica is a carnivorous pitcher plant in Northern California that relies on the microorganisms living inside its pitcher to digest insect prey. We previously demonstrated negative BEF relationships in this system but did not determine specific metabolic pathways. We hypothesized that a single or few specific pathways are rate limiting for degradation and explain which species drive the broader degradation function. The purpose of this study was to identify whether chitin, protein, or ammonia metabolic pathways are negatively impacted by bacterial diversity. Bacteria was collected from D. californica pitchers and acclimated in the lab, and serial dilution was performed to produce a diversity gradient. Communities were given fruit flies as food to compare degradation over 11 days using the metrics fly mass loss, solubilized protein, ammonia, and nitrate, and enzymatic activity of chitinase and protease.
Results/Conclusions
Throughout our study, we found that bacterial diversity was negatively correlated with all pathways involved in nitrogen cycling. Bacterial communities with lower diversity contributed to higher fly degradation and higher solubilized protein than did bacterial communities of the highest diversity. This slower cycling of nitrogen resources at high bacterial diversity may increase stability in plants that rely on carnivory and delaying prey degradation may be an important function in these communities. Alternatively, plants may take advantage of increased nitrogen availability in leaves with lower microbial diversity, while leaves with higher diversity may represent a nitrogen sink. In contrast with classical BEF studies, where higher diversity is often correlated with higher ecosystem function, our results suggest a negative relationship between bacterial diversity and degradation function in inquiline pitcher plant communities. In fact, we found that underlying specific pathways reflect this negative relationship. We further identify the taxa that may be driving these transformations and propose potential coexistence mechanisms in the context of the pitcher plant model. We believe that an expanded view on BEF relationships will strengthen understanding of microbial roles in global biogeochemical cycles.
