The effect of irrigation with oil-polluted water on microbial communities in estuarine reed rhizosphere soils

The effect of long-term drip irrigation with oil-polluted water on microorganisms in reed rhizosphere soils was assessed. During five months of irrigation with oil-polluted water, n-alkanes and polycyclic aromatic hydrocarbons (PAHs) occurred primarily in topsoils. A petroleum origin was indicated by the predominance of n-alkanes with 13-26 carbons. Naphthalene, fluorene, phenanthrene and pyrene were the predominant components of the detected PAHs. After five months of irrigation, the oil contaminants decreased the abundances of bacteria, fungi and actinomycetes in the reed rhizosphere soils but promoted the growth and reproduction of hydrocarbon degraders. The counts of bacteria, actinomycetes and fungi at a depth of 20 cm in soils irrigated with oil-polluted water were only 1/4, 1/10 and 1/2, respectively, of those without oil-polluted water. A Pearson correlation analysis confirmed that ORP, PAHs and n-alkanes had substantial effects on the microbial populations. PCR-denaturing gradient gel electrophoresis profiles revealed that the structure of the bacterial community was generally less affected, but the dominant flora was changed. Several members of the phyla Proteobacteria and Bacteroidetes formed a stable, dominant community structure after five months of irrigation with oil-polluted water. The growth and reproduction of certain members of the phyla Firmicutes, Actinobacteria and Cyanobacteria were clearly decreased by oil pollution. Oil pollution tended to simplify the functions present in the microorganism communities.