Aims: The objective of this study was to investigate the changes in the indigenous bacterial community structure for assessing the impact of biostimulation on spilled oil.

Methods and Results: Changes in the bacterial community structure were monitored by denaturing gradient gel electrophoresis (DGGE) and clone library methods based on 16S rRNA gene (rDNA) sequences. The results of DGGE, coupled with the use of the Shannon index and principal component analysis (PCA) and clone library analyses, were consistent. In the treated (fertilized) area, one operational taxonomic unit (OTU) became dominant during the fertilization period, and it was most closely related to Pseudomonas putida.

Conclusions: The bacterial community structure in the treated area was markedly different from that in the control (non-fertilized) area during the fertilization period, but in the two areas it became similar at 14 weeks after the end of fertilization.

Significance and Impact of the Study: The results suggest that the bacterial community structure was disrupted by the biostimulation treatment, but that it recovered immediately after the end of fertilization.