Globally, land contamination with petroleum hydrocarbons (PHCs) represents a significant challenge as they pose serious risks to natural ecosystems and human health if left untreated. The first step in removing PHCs from the environment is performed by the indigenous microorganisms, a process termed bioremediation. The ability of soil's indigenous microbial population to degrade PHCs offers a natural bioremediation approach with many advantages over traditional techniques. Understanding the changes dynamics of the microbial communities occurring following contamination is a crucial process since microorganisms are the backbone of any microbial degradation process. Many factors affect the biodegradation rate of the contaminant, in particular, the presence of co-contaminants (e.g. heavy metals) which makes the natural bioremediation a very complex process as heavy metals may have toxic effects on the microbiota.
The aim of this study was to evaluate the ecotoxicity associated with co-contamination in bioremediated soil samples as the exposure to multiple contaminants might increase the toxicity. In addition, this study aimed to improve knowledge of the dynamics of bacterial communities in the co-contaminated soil to enable the more appropriate design of bioremediation strategies for co-contaminated soils.
Contaminated soils were collected from former diesel power stations located in Marble Bar – Western Australia. Toxicity, as measured using the Microtox test, showed that the presence of both PHCs and heavy metals significantly ((p ≤ 0.05) elevated the ecotoxicity. Toxicity was correlated with the presence of lead (Pb), zinc (Zn) and PHCs (89, 60, 49%), respectively. 16S amplicon sequencing revealed a lack of dominant genera; however, despite the variation in soil type, several genera were present in most soil samples such as Azospirillum spp. which has been previously found in uncontaminated soils from hot and extreme arid regions. Likewise, many genera of hydrocarbon-degrading bacteria were identified in all soil samples, such as Acinetobacter spp. which has been associated with co-contaminated sites. This study concluded that PHCs and heavy metal co-contamination significantly elevated the associated toxicity.