ABSTRACT
Crude oil spill is encountered everyday in Nigeria and this has adverse effect on the environment and organisms. The effect crude oil has on Pleurotus tuber-regium is investigated and the degradative potential of the mushroom determined using different inocula (spawn and sclerotium) and substrate composition. The fungus was cultivated in crude oil contaminated soil and the substrate composition altered by adding supplements and agricultural wastes to enhance the growth of the mushroom in the contaminated soil. The supplements added were NPK 15:15:15 and poultry manure. The agricultural wastes were sawdust and shredded dried banana leaf blades. A combination of the agricultural wastes and the supplements formed a set of treatments.
Crude oil contamination affected the yield and morphometric characteristics of the mushroom significantly at P<0.05. There was significant difference between the two supplements used. The addition of poultry manure to contaminated soil enhanced the yield of the mushroom significantly, P<0.05. The highest yield of P. tuberregium among treatments was recorded in poultry manure + contaminated soil. In sawdust treatments average fresh weight values was significantly higher in spawn treatments than sclerotia inoculations. In poultry manure treatments average fresh weight was significantly higher in the sclerotia option at P<0.05. The addition of NPK fertilizer reduced the yield (least yield) and also caused total inhibition of fructification. Poultry manure encouraged proliferation of stipe and had the tallest fruit bodies. Sawdust significantly enhanced the size of the pileus at P<0.05, as the largest pilei were from treatments with sawdust in contaminated soil but not significantly different from the poultry manure treatment. Shredded banana leaf blades encouraged vegetative growth but reduced fructification in contaminated soil. The substrate composition and inoculum type also affected the nutrient status of the mushroom significantly at P<0.05. There was more carbohydrate, than protein, fat, ash and crude fibre in all treatments. Control soils showed fruit bodies that had the highest percentage of carbohydrate, which was significantly higher at P<0.05 than those harvested from any other substrate. The highest concentration of protein was recorded in fruit bodies harvested from poultry manure + contaminated soil. Fat was highest in fruit bodies harvested from contaminated soil only substrates, while moisture was highest in those from control soils. In poultry manure + contaminated soil and contaminated soil only treatment, the moisture content was significantly higher in spawn grown fruit bodies. The highest concentration of vitamins A, B and C was recorded in sawdust + contaminated soil. Sawdust + contaminated soil accumulated the highest concentration of sodium. Poultry manure + contaminated soil accumulated the highest concentration of Potassium and calcium in fruit bodies. Phosphorus was highest in sawdust + NPK + contaminated soil. Pleurotus tuber-regium was able to reduce the total petroleum content significantly at P<0.05. The fungus was also able to degrade the four fractions of petroleum crude oil (saturates, aromatics, resins and asphaltenes) to varying degrees. The type of inoculum used and the substrate composition affected the degradative potential of the mushroom. The best substrate composition for the degradation of saturates was banana leaf blades + contaminated soil. Sclerotium inoculum was better than the spawn inoculum in degrading saturates, as degradation was significantly higher with sclerotium treatment, P<0.05. The best substrate for the degradation of aromatics was sawdust + poultry manure + contaminated soil. Resins were also degraded but at a lower percentage than saturates and aromatics. Banana leaf blades + contaminated soil recorded the highest percentage degradation for resins. Spawn option was preferred for the degradation of the resins as this was significantly higher than the sclerotia inoculum at P<0.05. The asphaltenes were also degraded, highest degradation was 40% and this was recorded in poultry + contaminated soil substrate. There was increase in the asphaltic fraction in some of the substrate combinations. The highest level of metabolite formation was recorded in contaminated soil only substrate. There was bioaccumulation of heavy metals and subsequent significant reduction of metal concentration in soils at P<0.05. The substrate composition and inoculum type affected the bioaccumulation of these heavy metals - iron, manganese, cobalt, nickel, zinc, copper, lead and chromium in the tissues of the mushroom. Cadmium, mercury and arsenic were not detected in the soils and tissues of mushroom. The substrate sawdust + poultry manure + contaminated soil recorded the lowest residual heavy metals in remediated soils. The mushrooms harvested from this substrate also recorded the highest concentration of iron, cobalt, nickel, zinc and chromium in their tissues. The concentration of copper in tissues was highest in mushrooms harvested from sawdust + NPK + contaminated soil. Lead was highest in tissues from poultry manure + contaminated soil. Manganese was highest in mushrooms from banana leaf blades + contaminated soil. The transfer factor for copper was the highest compared to other metals. There was consistently higher concentration of elements in the pileus than in the stipe. The mushroom caused mineralization, as the concentration of metals in the exchangeable state was significantly higher in treatments with mushroom (i.e. with remediation) than the control without the mushroom. The bioavailability factor was highest in the substrate sawdust + poultry manure + contaminated soil for the metal manganese. The spawn option caused more mineralization than the sclerotia inoculum as the metals in exchangeable state in the spawn treatments was significantly higher at P<0.05. Phytoassessment of the remediated soil with Vigna unguiculata shows that some of the substrates formed harmful metabolites. The substrates sawdust + NPK + contaminated soil did not encourage the growth of the test plant. There was no growth in this substrate and also NPK + contaminated soil. The substrate sawdust + contaminated soil had the highest percentage germination and vine height of the test plant and thus was the best combination. Sawdust + poultry manure + contaminated soil had the highest leaf area and is also a good combination for the remediation of crude oil polluted soils. The addition of poultry manure enhanced the degradation of the four fractions of petroleum hydrocarbons.
Pleurotus tuber-regium is a good agent for the bioremediation of crude oil polluted soils. Agricultural wastes like sawdust boosted the degradative potential of the fungus and ensured that there was total clean up as even the very recalcitrant asphaltenes was degraded. Mycoremediation with P. tuber-regium is better with the addition of poultry manure or sawdust. The fungus as a bioremediating agent should not be made to grow directly in polluted soils only as this causes increase in the asphaltic fraction. Mushrooms from crude contaminated sites can accumulate harmful levels of heavy metals and are not safe for consumption. There was negative correlation between fresh weight, pileus diameter and total petroleum content in sawdust + contaminated soil substrate (P<0.05).
