UV treatment of the digestive fluid of Nepenthes hemsleyana pitcher plants affects their digestive process, possibly via reducing microbial inquilines.

Interactions with microbes are ubiquitous, and many of them are essential for the survival and success of plants. In Nepenthes pitcher plants, they occur as part of a diverse community of organisms, so-called inquilines, that live inside the digestive fluid of the pitcher traps. However, evidence is ambiguous regarding the role of microbial inquilines: they may complement the plants' prey digestion, fix atmospheric N, act as competitors that reduce plant-available nutrients or affect the plants in other ways unrelated to the breakdown of prey. In a field experiment on Borneo, we investigated the effect of UV disinfection of the digestive fluid on prey digestion of N. hemsleyana that captures and digests insects as well as bat faeces in its pitchers. We show that in the short term, the photosynthetic performance of plants with UV-treated digestive fluids decreases compared to untreated plants, likely due to lower abundances of microbial inquilines. However, at the end of 2 months, responses of pitcher plants with UV-treated and untreated digestive fluids tend to equalise. Nutrient source, whether from insects or bat faeces, does not influence prey digestion. We expect our findings to be a starting point for unveiling the ecological role of microbial inquilines in pitcher plants and how they interact with other inquiline groups of higher trophic levels. Ultimately, this will also help to improve understanding of the functioning and evolution of convergent interactions in other carnivorous plants.