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¶ Introduction
Some soil laboratories are promoting analysis of soil PLFAs (phospholipid fatty acids) as a way to estimate “soil health”. Soil microbiologists have used PLFA analysis in research studies over the last ten to fifteen years, but like other methods, the technique has its limitations.
¶ PLFA Analysis
Phospholipid fatty acids are a main component of the membrane (or “skin”) of all microbes. These fatty acids break down quickly when a cell dies, so intact fatty acids can only be obtained from live cells. The chemical makeup of these fatty acids (called “biomarkers”) can differ between microorganisms, so the procedure can give researchers a type of biological “fingerprint”. The PLFA biomarkers extracted from a soil sample are concentrated in a solvent, then are identified by a technique called gas chromatography with flame-ionization detection (GC-FID).
The total amount and different types of these fatty acids in a sample can be used to provide information on three soil microbial characteristics: biomass (or total weight of microorganisms), makeup of the microbial population, and the metabolic activity of the soil microorganisms.
Certain bacterial groups produce fatty acids with a specific signature. The fatty acid profile in a soil sample can be used to characterize the community of microbes in that sample according to which fatty acids are found and in what concentrations. These profiles have been used to help identify the soil microorganism population, being able to separate bacteria from fungi or separate different families of bacteria from one another.
The biomarkers of some microbes change during times of slow growth or stress. Analysis of the fatty acid profile can help provide some indication of the microbial health or metabolic activity as it may be affected by environmental conditions.
PLFA analysis has been used in environmental cleanup projects to verify the progress of natural contaminant breakdown by soil microbes. The population of certain microbes may change if certain substances are added to soil. For example, PLFA analysis may be used to help identify the amount and comparative proportion of different microbial groups, like iron reducers, sulfate reducers, or fermenters. These microbes might increase after they start feeding on a substance that was added to the soil, then decrease as the substance is broken down and dissipates.
¶ PLFAs and Soil Health
The terms “soil health” or “soil quality” are often used interchangeably, but there is no universal definition for either term. Definitions of soil health or quality usually include some reference to biological productivity or activity, there is no specific measurement that separates a “healthy” soil from an “unhealthy” soil.
Microbiologists started using PLFA analysis as a common research technique about ten to fifteen years ago. Some researchers have proposed using the fatty acid profile from a soil sample to evaluate soil microbial activity as a routine soil test. Like any other type of testing, PLFA analysis has limitations.
Research studies suggest that good fatty acid signatures are found in very specific cases, but cannot be used across the board. Certain biomarkers may work well in forest soils, but are poor indicators in agricultural soils. A particular fatty acid profile may work well to identify fungi when soil bacterial populations are low, but then work poorly when bacterial populations are high.
One application of the method is to carry out a “census” of soil microbes to identify microbial diversity, but the PLFA results can lead to errors in some cases. For example, there may be no difference between the respective biomarkers for fungi, for soil bacteria, or for the roots of certain plant roots. The technique cannot separate one type from the other, affecting the diversity index.
Some studies show that PLFA patterns may change rapidly when environmental conditions change, which indicates the microbial community is under stress. A toxic substance or condition that kills microorganisms may also inhibit the enzymes necessary to break down fatty acids, thus delaying an obvious change in the fatty acid profile. This would mask the decline of a microbe population and also affect the diversity index.
Moisture, temperature, and other soil factors can affect the PLFA results. The U.S. Geological Survey conducted the Geochemical Landscapes pilot study in 2004 which included analysis of fatty acids, enzymes, and trace elements in soil samples collected across the continental U.S. (see Figure). Soil samples were collected along two transects: north-to-south (from Manitoba to Texas) and east-to-west (along the 38th parallel).
¶ Summary
Fatty acid profile analysis is a tool that can help identify the total amount, makeup, and activity of soil microbes. Researchers suggest that fatty acid analysis be combined with other methods, like RNA profiling, to properly evaluate microbial communities. The analysis results may be heavily influenced by the weather and climate conditions at the time of sampling. At the current time, there are no established standards for fatty acid levels in soils, but at some future date they may help contribute to our understanding of soil microbiology.
¶ Figure 1. Total Phospholipid Fatty Acid Map
¶ References
(accessed 19 Dec 2011) http://www.umb.no/statisk/nitrogengroup/publications/Use_of_PLFA_SBB.pdf
http://microbiology.usgs.gov/geographic_patterns.html#contin ental_patterns
Use and misue of PLFA measurements in soils. Frostegard, A., A. Tunlid, and E. Baath. Soil Biology and Biochemistry. In press, 18 November 2011. http://www.microbe.com/images/stories/plfa-overview.pdf