Abstract
Most components of petroleum oily sludge (POS) are toxic, mutagenic and cancer-causing. Often bioremediation using microorganisms is hindered by the toxicity of POS. Under this circumstance, phytoremediation is the main option as it can overcome the toxicity of POS. Cajanus cajan a legume plant, was evaluated as a phyto-remediating agent for petroleum oily sludge-spiked soil. Culture dependent and independent methods were used to determine the rhizosphere microorganisms’ composition. Degradation rates were estimated gravimetrically. The population of total heterotrophic bacteria (THRB) was significantly higher in the uncontaminated soil compared to the contaminated rhizosphere soil with C. cajan, but the population of hydrocarbon-utilizing bacteria (HUB) was higher in the contaminated rhizosphere soil. The results show that for 1 to 3% oily sludge concentrations, an increase in microbial counts for all treatments from day 0 to 90 d was observed with the contaminated rhizosphere CR showing the highest significant increase (p < 0.05) in microbial counts compared to other treatments. The metagenomic study focused on the POS of 3% (w/w) and based on the calculated bacterial community abundance indices showed an increase in the values for Ace, Cho, Shannon (Shannon-Weaver) and the Simpson’s (measured as InvSimpson) indices in CR3 compared to CN3. Both the Simpson’s and the Shannon values for CR3 were higher than CN3 indicating an increase in diversity upon the introduction of C. cajan into the contaminated soil. The PCoA plot revealed community-level differences between the contaminated non-rhizosphere control and contaminated rhizosphere microbiota. The PCoA differentiated the two treatments based on the presence or absence of plant. The composition and taxonomic analysis of microbiota-amplified sequences were categorized into eight phyla for the contaminated non-rhizosphere and ten phyla for the contaminated rhizosphere. The overall bacterial composition of the two treatments varied, as the distribution shows a similar variation between the two treatments in the phylum distribution. The percentage removal of total petroleum hydrocarbon (TPH) after 90 days of treatments with 1, 2, 3, 4, and 5% (w/w) of POS were 92, 90, 89, 68.3 and 47.3%, respectively, indicating removal inhibition at higher POS concentrations. As the search for more eco-friendly and sustainable remediating green plant continues, C. cajan shows great potential in reclaiming POS contaminated soil. Our findings will provide solutions to POS polluted soils and subsequent re-vegetation.
Highlights
The global over-dependent on fossil fuels as sources of energy increases the rate and extent of exploration, transportation, refining and storage of crude oil by the petroleum industries[1,2]
The total heterotrophic bacteria (THRB) counts for the 2% petroleum oily sludge (POS) concentration from 0 to 90 days show a significant (p < 0.05) increase in rhizospheric contaminated rhizosphere (CR) counts from 12.7 × 107 to 144.8 × 107 CFU/g, no significant change (p > 0.05) to the contaminated non-rhizospheric contaminated non-rhizosphere (CN) count, no significant change (p > 0.05) to the uncontaminated rhizospheric uncontaminated rhizosphere (UR) count (243 × 107 to 232 × 107) CFU/g and no significant change (p > 0.05) to the uncontaminated non-rhizospheric uncontaminated non-rhizosphere (UN) count CFU/g (Fig. 2(b))
The THRB counts for the 3% POS concentration from 0 to 90 days show that the contaminated rhizospheric counts CR were increased but not significant (p > 0.05), the contaminated non-rhizospheric CN counts were not increased significantly (p > 0.05) CFU/g, the uncontaminated rhizospheric UR counts were increased significantly (p < 0.05) from 152 × 107 to 232 × 107 CFU/g, and the THRB counts in the uncontaminated non-rhizosphere UN were not increased significantly (p > 0.05) CFU/g (Fig. 2(c))
Summary
The global over-dependent on fossil fuels as sources of energy increases the rate and extent of exploration, transportation, refining and storage of crude oil by the petroleum industries[1,2]. Culture-independent bacterial community in C. cajan rhizosphere after 90 days was determined using metagenomics analysis of the rhizospheric soil collected from the contaminated rhizosphere CR of 3% oily sludge and contaminated non-rhizosphere CN of 3% oily sludge as control.
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