Assessment of the Responsiveness to Different Stresses of the Microbial Community from Long-Term Hydrocarbon-Contaminated Soils

Soils exposed to long-term contamination with hydrocarbons may present extreme challenges to maintain the biological resilience to the stress. To elucidate the relationships between the initial event of contamination and the responsiveness to the stress, we investigated the extent of the microbial resilience of biological functions from two contaminated soils sampled from a petrochemical area (S1, underwent diffuse hydrocarbon contamination, and S2, from a land farming unit where an alkaline petrochemical sludge was treated) after the Cd, saline, and acid stresses. Both contaminated soils were characterized by low organic matter content compared with a pristine soil. Although similar Shannon diversity index and heterotrophic bacterial count were observed, different bacterial community structures (PCR-DGGE) and less enzymatic activities characterized the contaminated soils. Particularly, functional diversity determined by Biolog EcoPlates (TM) was not detected in S2 soil. Only the S1 soil showed resilience of the enzymatic activities and functional diversity, suggesting the presence of a well-adapted microbial community able to face with the stresses. The S2 was the most disturbed and less responsive soil. However, an increase in the functional diversity was evidenced after acidification, and it is possible to correlate this responsiveness with the sludge properties treated in the land farming unit. In addition, if the selected stress can reverse the soil condition provoked for the first disturbance, responsiveness could be expected.