بررسي مقايسه اي اثر آلودگي نفت خام بر روي جمعيت ميكروبي خاك جنگل و خاك صنعتي

Comparative Study of Crude Oil Contamination Effect on Industrial and Forest Soil Microbial Community

هیدروكربن¬های نفتی آلاینده¬های گسترده¬ای هستند كه از طریق انتقال نفت خام، نگهداری، حوادث نشت نفتی و فرآیندهای تصفیه در پالایشگاه¬ها وارد خاك می¬گردند. آلودگی نفتی دارای اثرات اكولوژیكی بر روی خاك می¬باشد بطوری¬كه تركیب و تنوع جامعه میكروبی را بر هم زده و اثراتی نیز بر روی فعالیت ریزجانداران و آنزیم¬های خاك دارد. در این تحقیق جهت مطالعه اثر نفت خام بر روی جمعیت میكروبی خاك، دو نوع خاك متفاوت شامل خاك¬های صنعتی (مجاور تأسیسات نفتی و پتروشیمی شیراز) و جنگل تهیه و نمونه¬برداری شد و شش نوع میكروكازم طراحی گردید. هر خاك دارای سه میكروكازم با شرایط متفاوت شامل بدون آلودگی، آلوده به نفت و آلوده به نفت همراه با مواد غذایی نیتروژن و فسفر بود. شاخص¬هایی همچون جمعیت باكتری¬های هتروتروف، جمعیت باكتری¬های تجزیه كننده، آنزیم دهیدروژناز و میزان تجزیه نفت در مورد هر میكروكازم در یك دوره زمانی 120 روزه بطور جداگانه سنجش گردید. نتایج این تحقیق نشان داد كه بالاترین میزان باكتری¬های هتروتروف مربوط به خاك جنگل با ارزش 108 × 8 می¬باشد. بطور كلی تعداد باكتری¬های تجزیه كننده در خاك¬ها بطور قابل توجهی كمتر از تعداد كل باكتری¬های هتروتروف در خاك¬ها بود. كمیت باكتری¬های تجزیه كننده تا روز 60 آزمایش بصورت كاهشی و پس از آن تا انتهای آزمایش افزایش داشت. در بین سه نوع مختلف میكروكازم، میكروكازم آلوده به نفت همراه با افزودن منابع نیتروژن و فسفر بالاترین فعالیت آنزیمی دهیدروژناز را دارد. از لحاظ تجزیه زیستی نفت خام در خاك، بیشترین میزان تجزیه مربوط به خاك در میكروكازم صنعتی (95 %) بود. تحلیل آماری داده¬ها نشان داد كه یك ارتباط معنی¬دار بین تعداد كل باكتری¬های هتروتروف كه با روش MPN سنجیده شده با سایر شاخص¬های مورد بررسی وجود دارد. با بكارگیری نتایج حاصله از این تحقیق می¬توان بر حسب نوع خاك راهكارهای مناسبی جهت احیای زیستی آن¬ها پیشنهاد نمود.

Introduction: Petroleum hydrocarbons are widespread pollutant that enters to soil by some pathwayssuch as: Transportation of crude oil, conservation of oil compounds, crude oil spill and treatment process on refineries. Oil pollution has some ecological effect on soil that disturbed composition and diversity of microbial community. Also this pollution has some effects on microbial activity and enzymes of soil. Forests ecosystems may be polluted with petroleum hydrocarbons via different ways such as transportation and spill of crude oil from resource of petroleum storage. Industrial soil defined as the soils that located in industrial area such as petrochemical plant, mine, chemical factories and etc. These soils always contaminated to many pollutant such as: oil, diesel and heavy metals. These pollutants have some effects on the texture of the soil and microbial community. The aim of this research is to understand the effect of oil pollution on two different soils. Material and Methods: In order to evaluate the effect of crude oil on soil microbial community, two different soil samples were collected from industrial and forest soils. Six microcosms were designed in this experiment. Indeed each soil sample examined inthree microcosms asunpolluted microcosm, polluted microcosm, and polluted microcosm with nutrient supply of Nitrogen and PhosphorusSome factors were assayed in each microcosm during 120 days of experiment. The included study factors were: total heterotrophic bacteria, total crude oil degrading bacteria, dehydrogenase enzyme and crude oil biodegradation. For enumeration of heterotrophic bacteria nutrient agar medium was used. In this method serial dilutions were done from each soil and spread on nutrient agar medium then different colonies were counted. For enumeration of degrading bacteria Bushnel-Hass (BH) medium were used. The composition of this medium was (g/lit): 1 gr KH2PO4, 1gr K2HPO4, 0.2 gr MgSO4.7H2O, 0.02 gr CaCl2, 1 gr NH4NO3, and two drops of FeCl3 60% , the pH was 7. The carbon source of this medium was crude oil (1%). In MPN method microplates (24 well) were utilized and turbidity was calculated as positive index. Results and Discussion: The results of this study showed that the highest quantity of heterotrophic bacteria was related to forest soil (8 × 108). The quantities of degradative bacteria significantly were lower than heterotrophic bacteria in all soil microcosms. This result may be expected because heterotrophic bacteria can use other carbon sources instead of crude oil such as organic carbon, suger and some nutrients that exist in the soil, but degrading bacteria have some limit in the use of organic carbons and only capable to use crude oil hydrocarbons. Sothe quantity of these bacteria is lower than heterotrophic bacteria. The quantity of degradative bacteria have decrement pattern until 60th day of experiment but after this day these bacteria have increment pattern. This result can be interpreted as from beginning of experiment until 60th day of experiment the bacteria adapted to toxic effect of crude oil and after this time the quantity of bacteria increased and have ability to use pollutant in the soil. The best deydrogenase activity between different microcosms related to polluted microcosm with nutrient. This result confirms that nitrogen and phosphorus can decrease the damage effect of crude oil on soil microbial community. The mechanism of this attenuation of toxicity effect of crude oil on microbial community can be related to enhance bioavailability of essential elements for bacteria in the soil. So after oil pollution of an area, soil supply upto nitrogen and phosphorus demand must be mentioned as a necessary practice to decrease the toxicity effect of pollutants. The highest biodegradation of crude oil in all studied soils belonged to industrial microcosm (95 %). It can be explained by adaptation theory because the bacteria in the industrial soil were better adapted to different pollutants and these bacteria have more capability for biodegradation of crude oil. By this reasonthe rate of degradation of crude oil in the industrial soil were higher than forest soil. Statistical analysis of the results showed that there was a significant correlation between MPN quantity of heterotrophic bacteria and other assayed factors. Also, forest soil seemed to have significant difference with other soils. Conclusion: according to the obtained results by this study, it can be possibly proposed appropriate strategies for bioremediation of different studied soil types. The selection of best bioremediation strategies belong to specific types of soil. Just as this research confirmed that the type of soil plays significant role in the percentage of degradation.