Successional processes have opposing effects on diversity and function of bacterial communities associated with the purple pitcher plant

Background/Question/Methods

Microbial organisms participate in the most important biochemical transformations driving nutrient cycles on Earth. The fields of environmental microbiology and ecosystem ecology have traditionally focused on large scale biochemical processing, we should also understand microbial function at the scale of community dynamics. Most microbial communities in nature are in a state of flux, recovering from disturbance, nutrient pulses, or recent invasions. Unfortunately, we currently lack a framework to generalize how different ecological processes can impact the biodiversity-function relationship. To address this gap, we studied the effect of successional processes on microbial diversity and function within pitcher plant leaves. Microbial communities within Sarracenia purpurea leaves undergo a successional shift from early top-down to late bottom-up ecological drivers, but it is unknown how these processes modify the relationship between microbial diversity and degradation. Importantly, this carnivorous plant relies on the microorganisms within its pitcher leaf fluid to degrade its prey and release their nutrients. We collected fluid from 300 leaves from a field south of Sumatra (FL) to conduct a microcosm degradation experiment. We assessed the effect of leaf age and bacterivore additions (top down effects) on bacterial diversity (16SrRNA gene amplicon sequencing) and degradation rates.

Results/Conclusions

Sarracenia leaf age had a positive effect on function and a negative effect on diversity. Microbial communities from older leaves were stronger degraders than those from younger leaves, while young leaves contained higher bacterial diversity. This response to leaf age suggests that successional processes decouple diversity from function in the pitcher plant bacterial community. Because bacterivores had no significant effects on our microcosms, a central driver of microbial function along succession must be nutrient availability, thus indicating a bottom-up effect. In the context of Sarracenianatural history, these findings are puzzling because older leaves have less contribution to the plant’s nitrogen budgets than intermediate leaves, and produce less nectar than younger leaves, thus capturing less prey. We propose that the plant plays a minor role in assembling its microbial community and that the provision of nitrogen to the plant is merely a byproduct of microbial metabolism in a suitable habitat within each leaf. In the broader context, we advocate for further identification of ecological mechanisms that decouple the biodiversity-ecosystem function relationship. These efforts should be central to advancing our understanding and predicting ability of a changing planet.