Abstract
Petroleum hydrocarbon-polluted environments harbour diverse microbial communities but their response to oil pollution is poorly constrained. Here, the effects of petroleum hydrocarbons (from seepages and spillages) and the associated heavy metals on oil-contaminated soils, including agricultural soils (grazeland and farmland soils), have been explored. Furthermore, the response and the distribution of the soil microbial community due to oil degradation have been assessed. Focus was made not only on the commonly studied phospholipid fatty acids (PLFAs) but also widened to include archaeal intact polar lipid (IPL), isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), and branched (bacterial) glycerol dialkyl glycerol tetraethers (brGDGTs) in oil-contaminated environments.Soil microbial communities represent an indispensable component responsible for the majority of soil ecosystem services. The production and decomposition of plant materials and the associated formation of humus (C-sequestration) would be hindered without soil microorganisms. Estimating microbial mass fractions is often used as a general indicator of soil biological activity to determine soil health and stress. In particular, how these microorganisms respond to petroleum hydrocarbon contaminants in an oil-contaminated environment remains an open question.
To begin with, the well-head crudes (onshore and offshore) and seepages were characterised by petroleum biomarkers to investigate their source(s). The PCA of the hopanes show that Ugwueme and Ehandiagu seepage, together with all the studied oil samples, have a common source (originating from a higher plants/terrestrial and open marine source rock). In contrast, Edda 1 seepage originates from plankton/algae and has a minimal contribution from terrestrial plant material.
The microbial population in seepage-contaminated soils was characterised. The distributions of bacterial and fungal microorganisms inferred by PLFA and GDGT soil biomarkers show that bacterial biomass dominated. The PLFA results revealed that gram-positive bacteria thrive more in an oil-contaminated environment. Ugwueme seepage soil has the highest relative biomass of bacteria compared to other seepage samples. The GDGT results revealed the dominance of isoGDGT-0 and crenarchaeol in the contaminated soil samples. This implies that isoGDGT-0 and crenarchaeol adapt more in oil-contaminated soils, and It is known that some archaeal species, such as those in the phylum Crenarchaeota, have metabolic pathways that can degrade petroleum hydrocarbons. Archaea feed on petroleum hydrocarbon as their source of carbon and energy and, as such, proliferate in oil-contaminated environments. Some isoGDGT-producing archaea might be involved in hydrocarbon degradation, contributing to their dominance in oil-contaminated environments.
A time course study of the impacts of oil degradation on the microbial community in agricultural soil in low and high oil concentrations revealed no significant increase in the microbial biomass prior to incubation, but as incubation proceeded, an increase in microbial biomass was observed, showing the presence of oil-degrading microorganisms. In control and oil-contaminated soil samples of grazeland and farmland, the fungi mass fractions remained relatively constant as incubation time increased. The PLFA and GDGT analysis results in both agricultural samples showed a shift within the soil's bacterial community due to low and high oil concentrations. Bacterial PLFAs and GDGTs dominated the farmland more than the grazeland soils (low and high oil concentrations); this correlates with the results of the pristane to phytane, acyclic isoprenoids to n-alkanes which showed an increased degradation rate in farmland soils, due to higher microbial activity. However, bacterial PLFAs and GDGTs are more known for their oil-degrading capabilities than fungi and archaea. The PLFA and GDGT results showed that increased oil concentration decreases the degradation rate in the agricultural samples.
Finally, the consequences of oil spillage and associated heavy metals on microbial biomass along a soil depth profile (vertical/north-south) and (horizontal/east-west) distribution in the Niger Delta were assessed. The oil-derived heavy metals were compared in different soil sections and control samples using the extent of oil contamination proxy (Cf). The results show that heavy metals affected the microbial biomass differently along north-south
( vertical) and east-west (horizontal) transects in spillage soil with depths. The point of spillage contamination (0-10 m depth) has the highest concentration of heavy metal and 20 m away from the point of contamination (V4 25-35 m depth), the least.
Also, there was a potential relationship between total carbon content and microbial biomass, indicating that higher carbon content corresponds to higher microbial biomass. A predominance of gram-negative over gram-positive bacteria was observed in metal-contaminated soils. The total mass fraction of PLFAs and totals for bacteria, fungi, and actinobacteria biomass are significantly higher in the north-south soil sections than in the control samples because these microorganisms utilize hydrocarbons as carbon and energy sources and proliferate in oil-contaminated environments.
| Date of Award | 18 Jun 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Heather L Buss (Supervisor) & Ian D Bull (Supervisor) |