Hydrocarbon contamination and plant species determine the phylogenetic and functional diversity of endophytic degrading bacteria

Salt marsh sediments are sinks for various anthropogenic contaminants, giving rise to significant environmental concern. The process of salt marsh plant survival in such environment is very intriguing and at the same time poorly understood. The plant–microbe interactions may play a key role in the process of environment and in planta detoxification. In this study, a combination of culture-dependent and culture-independent molecular approaches [enrichment cultures, polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), DNA sequencing] were used to investigate the effect of petroleum hydrocarbons (PH) contamination on the structure and function [polycyclic aromatic hydrocarbon (PAH) dioxygenase genes] of endophytic bacterial communities of salt marsh plant species (Halimione portulacoides and Sarcocornia perennis) in the estuarine system Ria de Aveiro (Portugal). Pseudomonads dominated the cultivable fraction of the endophytic communities in the enrichment cultures. In a set of fifty isolates tested, nine were positive for genes encoding for PAH dioxygenases (nahAc) and four were positive for plasmid carrying genes encoding PAH degradation enzymes (nahAc). Interestingly, these plasmids were only detected in isolates from most severely PH-polluted sites. The results revealed site-specific effects on endophytic communities, related to the level of PH contamination in the sediment, and plant-species-specific ‘imprints’ in community structure and in genes encoding for PAH dioxygenases. These results suggest a potential ecological role of bacterial plant symbiosis in the process of plant colonization in urban estuarine areas exposed to PH contamination