Bacterial community succession in leaves of the Northern pitcher plant, Sarracenia purpurea

Background/Question/Methods

One of the oldest questions in community ecology is what determines the assembly of communities in nature? In the broadest of terms, two main types of theories have been proposed. Community assembly can be driven by niche-based mechanisms, commonly referred to as habitat filtering and competitive exclusion.  In contrast, neutral community assembly models assume that all species in a community are equivalent in terms of their dispersal and competitive abilities, and their requirements. The composition of the community is therefore a stochastic process – individuals that are dying are randomly replaced by immigrants or reproducing individuals from within the community.   We hypothesized that natural bacterial assemblages in small patchy aquatic habitats assemble following a neutral model.  An ideal system, in which we can study bacterial community assembly is presented within the pitchers of the northern pitcher plant, Sarracenia purpurea. Ten newly opened S. purpureapitchers were marked in June, 2012.  Changes in the bacterial community were then monitored for 2 months using two culture independent molecular techniques: PCR-DGGE (generates a presence-absence fingerprint of the dominant bacterial species in a sample) and sequencing of the 16S rRNA gene (provides phylogenetic information on the dominant bacterial species present in a sample).

Results/Conclusions

Immediately upon opening, the pitchers of S. purpurea were found to be sterile (no bacterial DNA could be detected using conventional techniques). Within 48 hours, the pitchers were colonized by diverse bacterial communities, dominated by members of the Proteobacteria. At this early stage, individual pitchers contained a mixture of common (found in all pitchers) and unique bacterial phylotypes (bands on DGGE fingerprints).  As time progressed, the bacterial assemblages of indiviual pitchers became increasingly different. At the final sampling time (2 months) there were very few bacterial phylotypes that were shared between all pitchers (<10%) and individual pitchers appeared to contain random bacterial assemblages.  We would expect significant similarities between pitcher bacterial communities, if their composition was largely determined by niche-related processes.  Our data is therefore consistent with a model of neutral assembly of bacterial species in the pitchers of S. purpurea.