Bacterial Diversity and Community Structure in Two Bornean Nepenthes Species with Differences in Nitrogen Acquisition Strategies

Abstract

Carnivorous plants of the genus Nepenthes have been studied for over a century, but surprisingly little is known about associations with microorganisms. The two species Nepenthes rafflesiana and Nepenthes hemsleyana differ in their pitcher-mediated nutrient sources, sequestering nitrogen from arthropod prey and arthropods as well as bat faeces, respectively. We expected bacterial communities living in the pitchers to resemble this diet difference. Samples were taken from different parts of the pitchers (leaf, peristome, inside, outside, digestive fluid) of both species. Bacterial communities were determined using culture-independent high-throughput amplicon sequencing. Bacterial richness and community structure were similar in leaves, peristomes, inside and outside walls of both plant species. Regarding digestive fluids, bacterial richness was higher in N. hemsleyana than in N. rafflesiana. Additionally, digestive fluid communities were highly variable in structure, with strain-specific differences in community composition between replicates. Acidophilic taxa were mostly of low abundance, except the genus Acidocella, which strikingly reached extremely high levels in two N. rafflesiana fluids. In N. hemsleyana fluid, some taxa classified as vertebrate gut symbionts as well as saprophytes were enriched compared to N. rafflesiana, with saprophytes constituting potential competitors for nutrients. The high variation in community structure might be caused by a number of biotic and abiotic factors. Nitrogen-fixing bacteria were present in both study species, which might provide essential nutrients to the plant at times of low prey capture and/or rare encounters with bats.

Ecological outsourcing: a pitcher plant benefits from transferring pre‐digestion of prey to a bat mutualist

“…The trapping organs of other carnivorous plants, for example Sarracenia purpurea, are inhabited by bacterial dinitrogen fixators (Prankevicius & Cameron 1991). Schulze et al (1997) also assumed nitrogen fixation processes in old Nepenthes pitchers due to microbial activities, which have already been confirmed for N. hemsleyana (Sickel et al 2016). Our results suggest that faecal enrichment in the digestive fluid could provide an optimal habitat for nitrogen-fixing bacteria.…”

“…() also assumed nitrogen fixation processes in old Nepenthes pitchers due to microbial activities, which have already been confirmed for N. hemsleyana (Sickel et al . ). Our results suggest that faecal enrichment in the digestive fluid could provide an optimal habitat for nitrogen‐fixing bacteria.…”

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

“…CPA models (Table 1; Fig. 3), as well as the ubiquitous nature of microbes (Takeuchi et al, 2011;Chou et al, 2014;Chan et al, 2016;Sickel et al, 2016), suggest that the contributions of microbes to pitcher nutrient sequestration may be the most reliable pathway of nutritional mutualism, while the diversity present in microbial communities (Chou et al, 2014;Chan et al, 2016;Sickel et al, 2016) may allow them to process a wide range of prey resource types.…”

“…The findings of study are likely to apply to the dominant microbial taxa present in pitcher fluids under our experimental scenarios, but weak interactions or interactions between microbe taxa that were present only in small quantities were likely to have been obscured. The microbial communities of Nepenthes pitchers are highly diverse (Chou et al, 2014;Chan et al, 2016;Sickel et al, 2016), and individual microbe species are likely to interact in species-specific ways with other members of the inquiline community (Cochran-Stafira & Von Ende, 1998). Future studies should thus aim to quantify the abundances of individual microbe species and elucidate species-specific trophic links within Nepenthes pitcher communities.…”

“…Another poorly understood inquiline group are the microbes. 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.…”