Fatty acids are generally classified into six categories: straight chain, straight chain cis monoene (cis2), straight chain trans monoene (tran2), branched chain saturated, cyclic and polyene fatty acids (these fatty acids are preferentially bonded to the intermediate C atom of phospholipid glycerol skeleton).
PLFAs are the main components of almost all living cell membranes, with extremely fast turnover rate and rapid degradation with cell death[1]The structure and types of fatty acids are diverse, sensitive to environmental factors, and the analysis results have good repeatability[2]。Both simple reagents and equipment can be used to measure the phosphate converted by PLFAs to determine the total amount of microorganisms, and special functional flora can be studied according to the differences in the length, saturation, hydroxyl and other substituent positions of specific fatty acid C-chain of different flora[1]。In terms of microbial quantification and activity determination, the results of PLFAs are very consistent with those of microbial biomass C, substrate induced respiration (SIR) and ATP[3]。Therefore, PLFAsSoil microbiologyIt has great potential in research.
The identification of PLFAs usually uses MIDI system (Microbial Identification System) and Gas Chromatography Mass Spectroscopy (GC-MS)[4]High Performance LiquidChromatography2Electrospray Ionization2Mass Spectrum, HPLC2ESI2MS[5]Identify PLFAs.
Generally, there are two ways and purposes to analyze PLFAs[6]: ① Analyze the composition of a single strain in order to compare or evaluate the characteristic phospholipids of different strains, and improve the multi classification ability of PLFAs database; ②To analyze phospholipid composition of individual microbial groups cultivated in natural environment or laboratory, and study structural changes and metabolic pathways of specific functional microbial groups.At present, PLFAs technology has been used in fields such as the response of pure bacterial culture to external stress, the effect of root excreta on rhizosphere microorganisms, the effect of nutrients on arbuscular vesicles, the characteristics of farmland soil microbial flora, and the induction of plant disease resistance[7]。
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More than 1000 phospholipid substances have been found[8]It can be divided into phospholipid fatty acids (PLFAs), glycolipid fatty acids (GLFAs) and neutral fatty acids (NLFAs) according to their polarity.Phospholipid fatty acid is the main component of microbial cell membrane, which is formed by esterification of the third hydroxyl group of glyceride with phosphoric acid and other hydroxyl groups with fatty acid. Its phosphate group part is called polar head, and two hydrocarbon chains become non-polar tail[9]。
Fatty acid is formed after methyl esterificationFatty acid methyl ester(Fattyacid methyl esters, FAMEs), which can be divided into: hydroxyl substituted FAMEs (OHFAMEs);Ester linked hydroxy substituted FAMEs (EL2OHFAMEs);Non ester linked hydroxy substituted AMEs (NEL2NYFAMEs);Saturated FAMEs (SAFAMEs);Monounsaturated FAMEs (MUFAMEs);Ester linked polyunsaturated FAMEs (PUFAMEs);Non ester linked unsaturated FAMEs (UNSFAMES)[6]。
Phospholipid fatty acid
Different kinds of PLFAs are often represented by a series of C atomic numbers and Greek letters.For example, 16 ∶ 1 ω 7t refers to a fatty acid containing "16 C atoms P, one double bond P, double bond distance methyl (ω) end 7 C atoms P trans configuration";The degree of unsaturation can also be placed after Δ x, which represents the double bond position nearest to the carboxyl end (Δ).The configuration of fatty acids can be represented by simple symbols, such as cis (cis) 2 (2H on both sides of the double bond is on the same side) and trans (trans) 2 (2H on both sides of the double bond is on the opposite side), which are represented by c2 and t2 respectively;A2 and i2 represent allotype (antiiso) 2 (methyl is on the third C atom at the end of C) and homotype 2 (iso2) (methyl is on the second carbon atom at the end of C);Br2 indicates the position of the unknown methyl branch chain;The number before ME2 represents the number of C atoms between the methyl substituent group and the carboxyl end of the molecule;Cy2 represents cyclopropane fatty acid;α 2 and β 2 represent the 2OH of hydroxyl fatty acid on the second and third C atoms respectively[6]。
