Under aerobic conditions, degradation of fatty acids may occur by beta-oxidation, with a consequent chain-length reduction by multiples of 2-carbon units. The normal operation of the beta-oxidation system is common to all microorganisms. Simple alkanoic and alkenoic acids undergo beta-oxidation readily. Ester formation does take place with aliphatic carboxilic acids in microorganisms. In soil, the only significant reaction of the ester group is hydrolysis reaction. The microbial hydrolysis of ester is catalysed by hydrolases. Many of these may be extracelullar as well as intracellular. Extracellular enzymes may not normally diffuse into soil environment. They can remain tightly bound to the microoganisms either within the cell wall or on the outside of cytoplasmatic membrane. However, when the microorganisms are grown in solution culture substantial diffusion does occur and the enzymes can be isolated from these cultures. If the esters, are lipids (glycerol ester s of fatty acids) or waxes (monoalchol ester of fatty acids), enzymic hydrolysis can occur very readily. These compounds are among the most highly reduced substrates available to microogansims and, as such, are potential sources of energy. Other degradation reactions mediated by soil microorganisms can occur in the hydrocarbon chain and are summarised as follows: 1.- long chain fatty alkanoic acids and related compound also undergo omega-hydroxylation, i.e hydroxylation at the opposite end of the chain. Fatty acids and related compounds also undergo omega-1 hydroxylation. Where both omega and omega-1 positions are available for hydroxylation, the relative portions of the hydroxylation products depend upon the substrate chain length and to some extent on the nature of the polar terminal group. The relative importance of beta-oxidation and omega or omega-1 hydroxylation depends upon the chain length of the acid. Implications are that acids longer that C16 –C18-alkanoic acids are near the optimum length for hydroxylation. Acids longer than this are largely (C19-C20) or completely (C22-C24) metabolised by beta-oxidation to a chain more favourable for hydroxylation. Acids shorter than C16 are too short for efficient hydroxylation and are largely metabolized by way of beta oxidation. 2.- Unsaturated aliphatic hydrocarbons serve as substrates for a wide variety of microorganisms. The molecule may be oxidised at a terminal methyl or methylen group, it may be oxidised at the double bond, multiple double bonds may be isomerised to conjugated systems, and double bonds may be reduced Oxidative attack at a terminal methyl group appears to be the major degradation pathway for unsaturated aliphatic hydrocarbons. The corresponding -unsaturated acids and primary alcohols are products of this reaction. Oxidation at the terminal methylene group and double bond of unsaturated aliphatic hydrocarbons can lead to a variety of products. Some of these reactions lead to cleavage of the molecule at the double bond., while others lead to the addition of an atom of oxygen or two hydroxyl groups to the ethylenic linkage.