Microbial Membrane Components And Fatty Acids

Lipids occur in microbial membranes and as storage products. More than 1000 different individual lipids have been identified. Lipids can be extracted from the soil biota with a one-phase chloroform methane extraction. Separation of the extracted lipids on silicic acid columns yields neutral lipids, glycolipids, and polar lipids. The neutral lipids can be further separated by HPLC, derivatization, and gas chro-matography to yield quinones, sterols, and triglycerides. Glycolipids, on hydrolysis and derivatization followed by gas chromatography, yield poly-|3-hydroxybutyrate. The mixture of polymers can be analyzed to determine the nutritional status of bacteria such as those associated with plant roots. Polar lipids are separated by hydrolysis, derivatization, and gas chromatography to yield phospholipid phosphate, phospholipid glycerol, phospholipid fatty acids, and ether lipids (Table 3.4). Concentration of lipids in soil apart from those of microorganisms is low; the amount in organisms from different taxa is quite variable. It is thus difficult to relate the quantities extracted to biomass. Saponification and esterification after extraction followed by gas chromatography or HPLC produce a broad spectrum of the free plus bound cell fatty acids. This is known as fatty acid methyl esters and is useful in general community diversity analyses.

Phospholipids are found in the membranes of all living cells, but not in storage products, and are rapidly turned over upon cell death. They are therefore excellent signature molecules. The phospholipid fatty acid (PLFA) technique has been used to elucidate different strategies employed by microorganisms to adapt to changed environmental conditions under a range of soil types, management practices, climatic origins, and perturbations. PLFAs are extracted in single-phase solvent extractions

TABLE 3.4 Representative Fatty Acids Found in Microorganisms4'

Fatty acid

Example

Designation Name

Saturated

Monounsaturated

Polyunsaturated

CH3(CH2)14COOH CH3(CH2)5CH = CH(CH2)7COOH

CH3-C(CH2)12COOH

CH3(CH2)5CH = CHCH2CH = CH(CH2)6COOH 18:2w7

i16:0

16:0 16:1w7

Palmitate

Palmitoleate

Linoleate

Anteiso

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