Linking microbial succession and elemental cycling in a natural micro-ecosystem

Background/Question/Methods

It is widely theorized that trophic interactions can influence rates of elemental flux in ecosystems. How such predictions fare in natural systems is a critical missing link in the unification of community and ecosystem-level processes. Using laboratory competition assays and field isotopic enrichments, I examined how primary succession within aquatic microbial food webs drives rates of nitrogen and carbon cycles in developing leaves of the pitcher plant Darlingtonia californica. I hypothesized that phytotelm communities undergoing primary succession should become more complex and homogeneous. Furthermore, I speculated that an increase in food web complexity with succession should facilitate a tighter N-cycle, leading to a decrease in plant-available N in the oldest, or “climax,” leaves. Laboratory competition assays among pitcher plant bacteria were conducted as part of an ongoing evolution experiment and were designed to test for both positive and negative interspecific interactions as well as diversity-productivity relationships. In the field, I traced the fate of prey-derived nitrogen by adding ¹⁵N-enriched fruit flies to pitchers of known ages (0 – 365 days). After 11 days, pitchers were harvested and their phytotelm communities thoroughly enumerated and assayed for rates of carbon respiration. Foliar tissue was analyzed for 15N uptake using isotope ratio mass-spectrometry. Microbial diversity was estimated via SSU-rDNA amplicon sequencing. Viral, bacterial, protist, and invertebrate diversity and biomass was also measured.

Results/Conclusions

Competition assays revealed a strong diversity-productivity relationship, though species identity explained approximately twice the variance as richness per se. Strong interactions, both facilitative and competitive were also observed in pairwise cultures, particularly among the bacterial taxa most ubiquitous among phytotelm communities. In the field, food web complexity increased with time, and peaked in the oldest pitchers. Bacterial diversity, however, peaked mid-season, likely responding to increased detrital biomass. Nitrogen uptake efficiency and carbon respiration both showed significant unimodal and linear relationships with time and detrital biomass, respectively, suggesting a role for microbial community succession in affecting these process rates. However, nitrogen uptake also correlated with leaf size and N-content, suggesting a role of plant traits in controlling N uptake kinetics independent of the actions of the microbial consortium. Although further investigation is required, these results suggest that host traits, as well as the composition of the pitchers' commensal microbiota are responsible for affecting ecosystem process rates. Future work will focus on disentangling the effects of these two drivers, as well as searching for metagenomic signals of microbial adaptation and succession within the phytotelm ecosystem.