Understanding the biodiversity and metabolic activities of culturable bacterial communities in the soil of a wooded riparian strip

Background: This research is part of a project aimed at verifying the potential of a specifically assessed wooded riparian zone in removing excess of combined nitrogen from a river flow for the reduction of nutrient input into Venice Lagoon. Objectives: 1. To compare microbial metabolic activities in the wooded riparian strip (internal) to that of external soil. 2. To characterize bacterial populations colonizing that soil at different depths. Methods: Cultivation on complex solid media, LIVE/DEAD BacLight stains and CTC assay were used to determine culturable, viable and actively metabolizing bacterial cells, respectively. Microbial diversity of colturable populations was analyzed by Amplified Ribosomal DNA Restriction Analysis (ARDRA). Results: As expected, the number of culturable, total living and metabolically active cells followed seasonal fluctuations and significantly higher values were always found for surface soils as compared to medium and deep samples. The main difference observed between the wooded riparian soil and the external samples was the number of metabolically working cells, although the total number of living bacteria was almost the same. Several hundreds colonies were isolated from the wooded riparian strip, 16S rDNA was amplified and HinfI- and HpaII-restricted. A variety of different DNA fragment patterns was revealed. Conclusions: The results obtained indicated that the plant play an important role in supporting microbial growth in soil by overcoming the main limiting factor, almost always represented by the availability of organic carbon. Microbial biodiversity was found directly related to the different soil layers, so providing important indications for the appropriate management of a phytoremediation site.