Abstract:
Soil pollution is a major anthropogenic problem which affects global food security and human health. Many bioremediation mechanisms have been used to ameliorate the problem, but there is dearth of knowledge on the use of combined plant and fungal actions. This study was therefore designed to investigate the synergistic potentials of Rhizosphere Fungi (RF), Spent Mushroom Compost (SMC) and Megathyrsus maximus (Mm) in remediation of hydrocarbon and pesticide polluted soils.
Four sites [Hydrocarbon Polluted Sites, HPS1 (Ugboroko), HPS2 (Ibadan) and Pesticides Polluted Sites, PPS1 and PPS2 (Akure)], were investigated. On each site, 5 kg of soil were collected from 20 points and composited while rhizosphere soils were also obtained from tussocks of 10 grasses (100 g each). The RF were isolated from rhizosphere soils and identified using morphological and molecular techniques. Frequently occurring RF were selected for 90-day synergistic remediation of the composited soil with SMC and Mm; pure RF cultures were mixed with SMC (1:10 w/w) and applied to sterilised composited soil (5 kg) at synergistic concentrations of 10%, 20%, 30% and 40% with two controls (Mm only and SMC+RF). Polycyclic Aromatic Hydrocarbons (PAHs) and pesticides [2, 2-dichlorovinyldimethylphosphate (dichlorvos) and γ-hexachlorocyclohexane (lindane)] in soil samples were analysed using GC/MS and values obtained were used to calculate Degradation Efficiencies (DE), degradation rates (K1) and half-life (t1/2). Responses of root and leaf structures were studied using standard anatomical methods. Genes encoding the production degrading enzymes in RF were determined by PCR while their expressions were assessed by RT-PCR amplifications. Enzyme activities were monitored using standard procedures and data obtained were subjected to ANOVA at α0.05.
Out of the 200 RF identified and characterised, 16 strains were most-frequently occurring (≥50%). In HPS1 and HPS2, synergistic treatment at 40% best reduced total PAHs of 851.61 and 805.00 mg/kg by 95.28 DE and 93.58 DE, respectively while Mm and SMC+RF gave 45.28 and 50.23 DE, respectively. In PPS1 and PPS2, 40% synergistic treatment reduced dichlorvos (30.00 mg/kg) and lindane (45.00 mg/kg) by 82.70 and 88.67 DE, respectively as compared to Mm only (62.20 DE) and SMC+RF (72.58 DE). The same treatment also gave the best K1 and t1/2 of 3.05 and 0.23 day-1 respectively in HPS1, 2.73 and 0.252 day-1 in HPS2. The 40% synergistic treatment gave the best K1 and t1/2 for dichlorvos (1.75 and 0.40 day-1) and lindane (2.18 and 0.32 day-1) in PPS1 and PPS2, respectively. The root and shoot structures showed increased cell size and reduced intercellular air-spaces as the synergistic treatment concentration increased. Degrading genes lig2-lig6, mnp, lcc, opd-A, mpd, afk2-afk4 and caM were over-expressed in all RF while lig1, cbh, trpC and cam were moderately-expressed in some RF. The pollutants significantly increased the enzyme activities in most RF. Activities (U/mL) of Laccase (168.00±7.49), Manganese peroxidase (111.00±13.01), Lignin peroxidase (105.00±1.00) and Catalase (87.00±3.00) were obtained in most RF.
Rhizosphere fungal strains acting in synergy with spent mushroom compost and Megathyrsus maximus had better degradation effect on Polycyclic Aromatic Hydrocarbons, dichlorvos and lindane in polluted soils than when applied alone.
