Oil biodegradation by native bacteria is one of the most important natural processes that can attenuate the environmental impacts of marine oil spills. then quickly slows down, ultimately reaching a smaller percentage of degraded oil in longer time. The availability of the water-oil interface plays a key role in determining the rates and extent of degradation. We find that several parameters control biodegradation rates, including size distribution of oil droplets, initial microbial concentrations, initial oil concentration and composition. Under conditions relevant to the Deepwater Horizon spill, we find that this size distribution of oil droplets (mean and coefficient of variance) is the most important parameter because it determines the availability of the oil-water interface. Smaller oil droplets with larger variance leads to faster and larger extent of degradation. The developed model will be useful for evaluating transport and fate of spilled oil, different remediation strategies, and risk assessment. were calculated based on the theory of energetics and substrate partition into energy production and cell synthesis. With the general elemental composition of bacteria represented by the formula C5H7O2N, the half-reaction of cell synthesis can be written as follows [22]: is the mass of oxygen consumed per mass of hydrocarbon converted to carbon dioxide when microbial biomass is not synthesized (Table? 3). Table? 4 summarizes values of for individual hydrocarbons. These values are consistent with the thermodynamics of bacteria energetics and carbon balance which says that with the same electron acceptor, the yield coefficient should be similar for each unit carbon [23-25]. Table 3 Oxidation reactions of hydrocarbon compounds without including microbial growth is the microbial biomass produced per mass of oxygen consumed from the biodegradation of hydrocarbon type is the mass fraction of the hydrocarbon type represent the corresponding parameters for each individual hydrocarbon (the ratio of the total surface area over the total volume of all droplets) is usually given by: is the number of oil droplets of the same diameter (Dj). The total interfacial area (S) in the control volume is usually obtained by integrating equation (21) over the control volume (V). Assuming that oil droplets are uniform in the whole control volume: S?=?6Vo/ds, where the Sauter mean droplet diameter (ds) for the shrinking droplets is given by: Sirolimus inhibitor database math xmlns:mml=”http://www.w3.org/1998/Math/MathML” display=”block” id=”M23″ name=”1467-4866-14-4-i23″ overflow=”scroll” mrow mtext ds /mtext mo = /mo mfrac mrow mo mathsize=”big” /mo mrow msub mrow mi mathvariant=”normal” n /mi /mrow mrow mi j /mi /mrow /msub msubsup mrow mi mathvariant=”normal” D /mi /mrow mrow mi j /mi /mrow mrow mn 3 /mn /mrow /msubsup /mrow /mrow mrow mo mathsize=”big” /mo mrow msub mrow mi mathvariant=”normal” n /mi /mrow mrow mi j /mi /mrow /msub msubsup mrow mi mathvariant=”normal” D /mi /mrow mrow mi j /mi /mrow mrow mn 2 /mn /mrow /msubsup /mrow /mrow /mfrac mo Sirolimus inhibitor database = /mo mfrac mrow munderover mrow mi mathsize=”big” /mi /mrow mrow msub mrow mi mathvariant=”normal” D /mi /mrow mrow mn 0 /mn /mrow /msub /mrow mrow msub mrow mi mathvariant=”normal” D /mi /mrow mrow mtext max /mtext /mrow /msub /mrow /munderover mrow msup mrow mi mathvariant=”normal” P /mi mfenced close=”)” open=”(” mi mathvariant=”normal” D /mi /mfenced mi mathvariant=”normal” D /mi /mrow mrow mn 3 /mn /mrow /msup mtext dD /mtext /mrow /mrow mrow munderover mrow mi mathsize=”big” /mi /mrow mrow msub mrow mi mathvariant=”normal” D /mi /mrow mrow mn 0 /mn /mrow /msub /mrow mrow msub mrow mi mathvariant=”normal” D /mi /mrow mrow mtext max /mtext /mrow /msub /mrow /munderover mrow msup mrow mi mathvariant=”normal” P /mi mfenced close=”)” open=”(” mi mathvariant=”normal” D /mi /mfenced mi mathvariant=”normal” D /mi /mrow mrow mn 2 /mn /mrow /msup mtext dD /mtext /mrow /mrow /mfrac mfenced close=”)” open=”(” mrow mn 1 /mn mo – /mo mi mathvariant=”normal” X /mi /mrow /mfenced /mrow /math (22) Then, the number of cells per unit surface area of droplet is usually given by: math xmlns:mml=”http://www.w3.org/1998/Math/MathML” display=”block” id=”M24″ name=”1467-4866-14-4-i24″ overflow=”scroll” mrow msub mrow mi mathvariant=”normal” B /mi /mrow mrow mi s /mi /mrow /msub mo = /mo mfrac mrow mtext BV /mtext /mrow mrow msub mrow mn 6 /mn mi mathvariant=”normal” V /mi /mrow mrow mi mathvariant=”normal” o /mi /mrow /msub /mrow /mfrac mtext ds /mtext /mrow /math (23) where V is the control volume and Vo the volume of oil in the control volume. Because the biodegradation of oil droplets is usually assumed to take place at the water-oil interface, the accumulation rate of oil degrading microbes in the control volume is usually given as a function of the microbe concentration at the oil surface: math xmlns:mml=”http://www.w3.org/1998/Math/MathML” display=”block” id=”M25″ name=”1467-4866-14-4-i25″ overflow=”scroll” mrow mfrac mrow mi mathvariant=”normal” V EPLG1 /mi /mrow mrow mi mathvariant=”normal” S /mi /mrow /mfrac mfrac mrow mtext dB /mtext /mrow mrow mtext dt /mtext /mrow /mfrac mo = /mo msub mrow mi mathvariant=”normal” /mi /mrow mrow mtext max /mtext /mrow /msub mfrac mrow msub mrow mi mathvariant=”normal” C /mi /mrow mrow mtext oil /mtext /mrow /msub /mrow mrow msub mrow mi mathvariant=”normal” K /mi /mrow mrow mi mathvariant=”normal” s /mi /mrow /msub mo + /mo msub mrow mi mathvariant=”normal” C /mi /mrow mrow mtext oil /mtext /mrow /msub /mrow /mfrac msub mrow mi mathvariant=”normal” B /mi /mrow mrow mi s /mi /mrow /msub /mrow /math (24) Results Controlling parameters on biodegradation kinetics In microcosm experiments, a typical oil concentration is usually in the order of tens of mg/L. In marine oil spills like in the Sirolimus inhibitor database Gulf of Mexico, reported oil concentrations are between 0.1 to 1 1.0?mg/L [2,7]. Oil droplet size and its distribution have been reported to vary significantly at different sampling points [4]. Furthermore, concentrations of biodegrading microbes can differ extensively depending on the location of the spill. The goal of this section is to use the formulated model to assess the biodegradation time scale and its sensitivity to various factors, including initial oil and microbe concentration, maximum microbial density on oil droplets, oil droplet size distribution, and oil composition. For comparison, we also show the biodegradation kinetics of dissolved oil. The simultaneous biodegradation of dissolved and dispersed oil droplets are not included in this work. Except for the evaluation of the effect of chemical composition, the composition of dissolved oil and dispersed oil droplets in mole fraction used for calculations in all cases is usually 0.2 for Naphthalene, 0.2 for 1-Methylnaphthalene, 0.1 for 2-Methylnaphthalene, 0.1 for 2-Ethylnaphthalene, 0.1 for Phenanthrene, 0.1 for Anthracene, 0.05 for Pyrene, 0.05.