Tropical pitcher plants (Nepenthes) act as ecological filters by altering properties of their fluid microenvironments

Abstract

Characteristics of host species can alter how other, interacting species assemble into communities by acting as ecological filters. Pitchers of tropical pitcher plants (Nepenthes) host diverse communities of aquatic arthropods and microbes in nature. This plant genus exhibits considerable interspecific diversity in morphology and physiology; for example, different species can actively control the pH of their pitcher fluids and some species produce viscoelastic fluids. Our study investigated the extent to which Nepenthes species differentially regulate pitcher fluid traits under common garden conditions, and the effects that these trait differences had on their associated communities. Sixteen species of Nepenthes were reared together in the controlled environment of a glasshouse using commonly-sourced pH 6.5 water. We analyzed their bacterial and eukaryotic communities using metabarcoding techniques, and found that different plant species differentially altered fluid pH, viscosity, and color, and these had strong effects on the community structure of their microbiota. Nepenthes species can therefore act as ecological filters, cultivating distinctive microbial communities despite similar external conditions, and blurring the conceptual line between biotic and abiotic filters.

Characterization and Comparison of Convergence Among Cephalotus follicularis Pitcher Plant-Associated Communities With Those of Nepenthes and Sarracenia Found Worldwide

“…Fluid pH and DNA concentration were not correlated with volume (linear models, p > 0.05); however, fluid pH and DNA concentration were correlated with each other (linear model, t value = 2.88, p = 0.0065), with more acidic pitchers having lower DNA concentrations ( Supplementary Figure 1B ). A similar effect of pH on DNA extraction success has been noted in a previous study in Nepenthes pitcher plant microbial communities ( Gilbert et al, 2020b ). A PCA biplot of pitcher pH, volume, and DNA concentration showed separation of the different factors among our samples ( Supplementary Figure 1C ).…”

“…Pitcher plants provide an opportunity to examine how hosts with similar morphology and life-history traits that have evolved independently in dramatically different geographic locations, can nevertheless attract and cultivate similar communities within their structures. Pitcher plant-associated communities from the genera Sarracenia (family Sarraceniaceae) and Nepenthes (family Nepenthaceae) have now been characterized across numerous studies ( Peterson et al, 2008 ; Koopman and Carstens, 2011 ; Gray, 2012 ; Gray et al, 2012 ; Kanokratana et al, 2016 ; Bittleston et al, 2018 ; Grothjan and Young, 2019 ; Gilbert et al, 2020a , b ). Many of the same bacterial and eukaryotic taxonomic groups are common in both Sarracenia and Nepenthes communities and their metagenomes show enrichment in specific degradation genes ( Bittleston et al, 2018 ), suggesting that convergent interactions ( Bittleston et al, 2016b ) may exist among these convergently evolved plants and the organisms living in their pitchers.…”

“…Pitcher microbiomes likely assist in degradation of prey and support plant nutrient access ( Butler et al, 2008 ; Luciano and Newell, 2017 ; Bittleston et al, 2018 ). In Nepenthes and Sarracenia species, pitcher-associated communities have been found to vary by host species and pitcher fluid pH and volume, among other characteristics ( Kanokratana et al, 2016 ; Bittleston et al, 2018 ; Gilbert et al, 2020b ). Pitcher plants, despite being unusual within the plant world, are an emerging model system for community ecology because their pitchers contain small, naturally replicated microcosms that assemble anew each time a pitcher opens ( Srivastava et al, 2004 ; Peterson et al, 2008 ; Miller et al, 2018 ; Ellison and Gotelli, 2021 ).…”

Transcriptomic and Proteomic Analyses of Nepenthes ampullaria and Nepenthes rafflesiana Reveal Parental Molecular Expression in the Pitchers of Their Hybrid, Nepenthes × hookeriana

“…The pitchers with acidic fluids and secreted enzymes are important for trapping and digesting invertebrate prey ( Ravee et al, 2018 ; Gilbert et al, 2020 ). Several digestive enzymes are commonly reported to be secreted into the pitcher fluids, which include aspartic proteases, nucleases, and pathogenesis-related (PR) proteins ( Athauda et al, 2004 ; Hatano and Hamada, 2012 ; Buch et al, 2014 ; Rottloff et al, 2016 ; Fukushima et al, 2017 ).…”

On the regulation of dissolved oxygen by Nepenthes pitchers

“…Oxygen regulation possibly varies by species, but the difference between pitchers of different ages seems more pronounced than interspecific differences. Understanding how fluid properties like DO are regulated by the plants has implications for understanding how the plant interacts with its symbionts (Adlassnig et al 2011;Bittleston 2018;Gaume et al 2019;Gilbert et al 2020). Interspecific differences in DO regulation could very well contribute to the ability of pitchers to act as filters to shape what insects and microbes live within them.…”