Dipteran larvae and microbes facilitate nutrient sequestration in theNepenthes gracilispitcher plant host

Abstract

The fluid-containing traps of Nepenthes carnivorous pitcher plants (Nepenthaceae) are often inhabited by organisms known as inquilines. Dipteran larvae are key components of such communities and are thought to facilitate pitcher nitrogen sequestration by converting prey protein into inorganic nitrogen, although this has never been demonstrated in Nepenthes. Pitcher fluids are also inhabited by microbes, although the relationship(s) between these and the plant is still unclear. In this study, we examined the hypothesis of digestive mutualism between N. gracilis pitchers and both dipteran larvae and fluid microbes. Using dipteran larvae, prey and fluid volumes mimicking in situ pitcher conditions, we conducted in vitro experiments and measured changes in available fluid nitrogen in response to dipteran larvae and microbe presence. We showed that the presence of dipteran larvae resulted in significantly higher and faster releases of ammonium and soluble protein into fluids in artificial pitchers, and that the presence of fluid microbes did likewise for ammonium. We showed also that niche segregation occurs between phorid and culicid larvae, with the former fragmenting prey carcasses and the latter suppressing fluid microbe levels. These results clarify the relationships between several key pitcher-dwelling organisms, and show that pitcher communities facilitate nutrient sequestration in their host.

Ecology and natural history of swimming pitcher mites (Creutzeria spp., Histiostomatidae) from the traps of Nepenthes pitcher plants

“…In vitro, model pitcher set-ups were established using 15-mL Cellstarâ centrifuge tubes with 3 mL of filtered pitcher fluids and 3.22 mg of D. thoracicus carcasses. In vitro experiments are a convenient yet realistic method for simulating conditions in Nepenthes pitchers and the methodology used in this study was the same as that used in other published Nepenthes inquiline community studies (Lam et al, 2017(Lam et al, , 2018aLeong et al, 2018). Ten randomly selected Creutzeria mite individuals were then introduced into each tube and allowed to feed and reproduce.…”

“…'phytotelma']) to specialized aquatic metazoan and microbial species known as inquilines (Clarke & Kitching, 1993;Chou, Clarke & Dykes, 2014;Chan et al, 2016;Adlassnig, Peroutka & Lendl, 2011;Gaume et al, 2019). Both aquatic metazoan and microbial inquilines have been shown to facilitate pitcher nutrient sequestration by assisting with the physical and biochemical breakdown of prey carcasses (Lam et al, 2017;Adlassnig et al, 2011), although the nature of this facilitation is still poorly understood and highly contextually variable (Leong, Lam & Tan, 2018).…”

“…Interspecific interactions clearly shape the inquiline communities within Nepenthes phytotelmata (Clarke & Kitching, 1993;Kitching, 2001). The diversity of such interactions, ranging from facilitation (Bradshaw & Creelman, 1984;Lam et al, 2017) to competition (Leong et al, 2018) and predation (Mogi & Chan, 1996;Chua & Lim, 2012;Lam et al, 2018a), make inquiline communities potentially valuable model systems for the study of community ecology and biodiversity phenomena (Srivastava et al, 2004). Nevertheless, knowledge gaps in the basic ecology and life history of some inquiline species greatly limit such a potential utility.…”

“…All pitcher fluids naturally contain microbes (Chou et al, 2014;Chan et al, 2016;Sickel et al, 2016;Adlassnig et al, 2011), and these proliferate to different degrees under different pitcher biotic and abiotic conditions. Pitcher fluid pH is an important determinant of microbe community (Bittleston, 2018), and filter-feeding mosquito larvae reduce their overall microbe densities (Lam et al, 2017), but it remains unclear if and how microbes may interact with other pitcher inquilines such as mites.…”

“…E. schuitemakeri was chosen over other metazoan inquiline taxa because E. schuitemakeri are the most similar in size and adult morphology to chloropid frit flies, which were dispersal agents to Madagascar Creutzeria sp. (Ratsirarson & Silander, 1996), and because the messy feeding habits of E. schuitemakeri make this species a potential processing chain commensal to other filter-feeding detritivorous inquilines (Lam et al, 2017), which we hypothesized Creutzeria mites to be.…”