The Northern pitcher plant (Sarracenia purpurea L.) is a carnivorous plant (traps insects in pitchers) found in bogs, fens and other habitats with wet and nutrient poor soils throughout North America. Each growing season, pitcher plants produce several new pitchers. Most of the new pitchers cannot survive the harsh winter conditions, however, a portion of leaves can overwinter and function as an insect trap during a second growing season. At the base of the pitcher food web are drowned insects and detritivores (bacteria and several invertebrates) that mineralize them. Despite their importance to the pitcher plant, bacterial communities have received relatively little attention and in particular their composition and diversity have been rarely addressed. Our objective was to determine the composition of bacterial communities in second year pitchers and how their diversity might change during the growing season. We tagged twelve second-year pitcher plants in three different bogs (four plants per bog) located on Beaver Island (lake Michigan). At three times during the summer (May, June, July) we determined the bacterial diversity in the pitchers using a culture-independent molecular technique (PCR-DGGE of the 16S rRNA gene, variable V3 region) coupled with random sequencing.
Results/Conclusions
Both DGGE fingerprinting and random sequencing indicated that second year pitchers contain a very diverse bacterial community. The communities of individual pitchers appeared to be unique even within the same site; the average similarity of DGGE fingerprints between pitchers at a given site ranged from 25 to 65%. We determined spatial (based on location) and temporal (based on time of sampling) patterns in the diversity of the bacterial community using Multiple Response Permutation Techniques (MRPP). MRPP indicated significant grouping of pitchers by time of sampling but not by site. Therefore, we conclude that pitcher plant bacterial communities exhibit a significant seasonal shift. This could possibly be due to changes in the macroinvertebrate community and/or changes in the physical characteristics of the fluid in the pitchers (temperature, volume, pH, etc.). Sequencing results indicated that many different bacterial divisions are represented in the communities of pitchers. The predominant sequences belonged to Proteobacteria. We detected members of Verrucomicrobia, Acidobacteria, Bacteroides, Planctomycetes and a few unclassified groups, as well. Most of the sequences we obtained were only distantly related to known bacterial species, thus, they seem to represent novel, uncultured bacteria. Previous studies of bacteria in pitcher plants that use culturing techniques may have seriously underestimated their diversity.
