Date of Award

2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Science

First Advisor

Dr. Syed Salman Ashraf

Second Advisor

Dr. Yusra Othman

Third Advisor

Dr. Ahmed M. Almehdi

Abstract

Soil is a natural resource that is utmost importance from an agricultural as well as environmental perspective. Good soil health not only helps improve the quality and yield of agriculture products, but is also important for recycling of significant amounts of organic biomass. However, pollution of soil by various pollutants, such as heavy metals, pesticides or petroleum hydrocarbons, can lead to serious environmental as well as biological effects. This pollution can directly harm animals and human beings, leach into drinking water source, damage plants and vegetation, as well as affect the endemic microorganisms and insects living in the soil.

Microorganisms that are normally found in the top soil play key roles in enzymatic recycling of nitrogen, phosphorous, and sulfur, as well as the decomposition of organic matter; thus they impact global recycling of nutrients, carbon and other elements. In addition to nutrient recycling, soil microbes can also affect physical properties of soil as they secrete extra-cellular polysaccharides which stabilize soil aggregates, which in tum affect water retention, infiltration rate, crusting, and erodibility of soil. Thus, microbial health is considered a good indicator of soil health. Heavy metal pollutants found in the soil con cause their deleterious effects by one of four ways: 1) These heavy metals can undergo redox cycling and in the process lead to oxyradical production which then causes oxidative stress in organisms, 2) they can bind directly to sulfhydryl groups of proteins and lead to protein inactivation and denaturation, 3) they can bind to intracellular glutathione (GSH) or antioxidant enzymes (e.g. SOD, Catalase, GSH reductase/ peroxidases) and reduce the antioxidant ability of cells; and 4) these heavy metals can also compete for metal cofactor binding of metallo-enzymes and lead to their inactivation. Recent work on the effect heavy metal pollution on soil has shown that like microorganisms, activity of the enzyme (produced by microorganisms), can also be used as indicators of soil health. The soil enzymes that have been used for these studies are dehydrogenase, beta-glucosidase, cellulose, phenol oxidase, urease, amidase, phosphatase and arylsulphatase. Most of these studies have been carried out by monitoring enzyme activities of pollutant treated soil. Though a valid approach, the pollutants in these studies could have a systematic effect on the microorganism and so that the decreased or increased soil enzyme activity could be due to other reasons, and not due to the direct effect of the pollutant on various enzymes. Additionally the tested pollutants could be metabolized by the microorganisms or broken down by the environmental factors, and so their effect on microbial enzymes could be secondary product. A few studies have been published in which the effect of heavy metal pollutants have been examined on purified enzymes in vitro, however they are very few and not very thorough. In this thesis, we have systematically examined the effect of the various of heavy enzymes such as (Co2+, Cd2+, Hg2+, Cu2+) on alkaline phosphatase (ALP) activity, and show that the heavy metals such as Hg2+, Cu2+ inhibited the enzyme more strongly than Cd2+, and Co2+. However, the positive bivalent alkaline-earth metals such as Ca2+, and Mg2+ activated the enzyme, with Mg2+ being a stronger activator of the enzyme than Ca2+. Also, the monovalent alkali metal ions such as Na+ had no effect on the ALP Activity. We also examined the effect of combining heavy metals on ALP activity; such as Ca2+ + Cd2+, Ca2+ + Hg2+, and Cu2+ and Hg2+.

In the second part of the study, we examined the effect of oxy-radicals such as those generated by UV/H2O2 on the ALP activity. We also looked at the effect of UV together with heavy metals on ALP activity. We observed that UV radiation by itself did not affect the enzyme, however the combination of UV and Cu2+, Hg2+ or Cd2+ inhibited the activity of the enzyme more than when the heavy metals were incubated by themselves alone. Moreover, H2O2 alone or together with heavy metals significantly inhibited the ALP activity as well.

The data presented here show that the important bacterial enzyme, alkaline phosphatase is very sensitive to heavy metal exposure and is readily inactivated by them. Furthermore, we show that conditions that lead to oxyradical production increase the extent of enzyme inactivation by various heavy metals. In summary, we show that heavy metal pollution could be of serious concern for good soil microbial health, which has the potential to affect agriculture and food production directly

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