Identification of a Rhodopseudomonas isolate from Sarracenia purpurea pitchers and evaluation of its role in the nitrogen economy of the pitcher inquiline community

Background/Question/Methods

The fluid-filled leaves of the carnivorous pitcher plant Sarracenia purpurea are home to a diverse group of obligate symbionts that includes larvae of three dipterans, a mite and a bdelloid rotifer. Among numerous bacteria is a recurring photosynthetic facultative anaerobe, Rhodopseudomonas sp. We hypothesized that the species for this isolate would be either R. palustris or R. acidophilus, both of which share nutrient poor wetland environments. We used the Biolog (Hayward, California) Gen III system to produce a physiological profile for the Rhodopseudomonas isolate. Since this genus is not in the Biolog database, we are using the literature to match the physiological profile to a species. We looked at this microbe’s ability to fix nitrogen to determine its significance to the survival of the plant and the inquiline community. We tested for nitrogen fixation by growing the organism on media lacking a nitrogen source, using E. coli as a negative control.

Results/Conclusions

Rhodopseudomonas was found in over 80% of pitchers sampled during one growing season at Cedarburg Bog in southeastern Wisconsin. We isolated a pure culture of this bacterium and are in the process of determining the species identification based on its physiological profile. In addition to utilizing multiple carbon sources, the isolate also grew on nitrogen-free medium indicating that it is capable of nitrogen fixation. Since the leaves actively attract prey for only 28 days, a reliable source of nitrogen for both the plant and its inquilines after this period is not assured. Field sampling data show that Rhodopseuodomonas numbers peak at approximately the same time that prey capture declines. This suggests that nitrogen fixation may be an important reserve system for ensuring sufficient nitrogen for the inquilines and the plant.