Diversity and funciton of microbial communities developing in the leaves of the Northern pitcher plant (Sarracenia purpurea)
The relationship between species diversity and ecosystem function is a fundamental ecological question that has been subject to much theoretical and empirical study, yet significant gaps in our knowledge remain. Microbial communities are especially understudied, despite the fact that they mediate many important ecosystem functions, especially those related to decomposition and nutrient cycling. We explored the diversity of microbial communities (using PCR-DGGE and sequencing of the 16S rRNA gene) in pitchers of the northern pitcher plant, Sarracenia purpurea. At the same time, we determined the decomposition potential of those communities, by measuring three enzyme activities in the pitcher fluid (phosphatase, beta-glucosidase, and chitinase). Fourteen newly produced pitchers were tagged and as soon as they opened, sterile artificial rainwater was added to them. Every day for the following 3 weeks, a water sample was collected from each pitcher. The sample was centrifuged (to collect all bacterial cells) and the remaining liquid was used to measure enzyme activities and pH. DNA was extracted from the bacteria containing pellets and used to generate a fingerprint of the microbial community through PCR-DGGE. A subset of samples from 3 pitches was used to perform high-throughput sequencing of the 16S rRNA gene.
Previous studies have shown that S. purpurea pitchers are sterile when they first open. No bacteria or enzyme activities were detected in the pitchers at the beginning of our experiment. Throughout the three weeks of the study, the species richness of bacterial communities in the pitchers steadily increased eventually reaching an average of 14 bands in DGGE fingerprints. Even though pitchers had a similar number of bacterial species at the end of the three weeks, very few of those were shared between different replicates (also seen in our sequencing data), i.e., each pitcher appeared to develop a unique bacterial community. Enzyme activities increased in pitchers during the first week after opening. Afterwards, they varied widely exhibiting very different trends in replicate pitchers. As a result, when we attempted to correlate enzyme activities to bacterial species richness, no significant relationship was found. We conclude that during the first week of pitcher development, when bacterial species richness is relatively low, enzyme activities increase as species richness increases. However, as pitchers mature, the identity of bacterial species, rather than the increasing number of species, becomes the most important driver of enzyme activity in the pitcher fluid.