Abstracts |
We investigated extraction from soil of glomalin, a glycoprotein produced by
arbuscular mycorrhizal fungi, and we examined its measurement. The most commonly used protocols
for extracting glomalin require autoclaving of soil in citrate solution, followed by
centrifugation to separate the supernatant, and then measurement by either Bradford protein assay
or enzyme-linked immunosorbent assay (ELISA). We found that lengthening the time of autoclaving
increased easily extractable glomalin extraction. Delay of centrifugation after autoclaving,
however, diminished Bradford-reactive substances in the supernatant, suggesting that extracted
substances might be reversibly immobilized on soil particles. Surprisingly, increasing the volume
of extraction solution did not accelerate extraction of ''total glomalin'', but instead,
substantially increased the amount extracted. Multiple autoclave cycles nevertheless denature
glomalin, which may not be as heat-resistant as thought. Repeated 1-h autoclaving of supernatant
diminished both its Brad ford-reactive substances (7.3% h(-1)) and immunoreactive protein (22%
during the first hour and 9.5% h(-1) of the remainder thereafter), although a large initial
volume of extractant could reduce the loss of immunoreactive protein. Proteins and polyphenols
that survive the extraction process are measured non-specifically by the Bradford assay. When we
added other glycoproteins to dry soils, we recovered a maximum 34% bovine serum albumin and 22%
bovine mucin, primarily in the first two, 1-h extraction cycles. These added proteins may adhere
to soil organic matter and thereby be protected from denaturation. In addressing the endpoint of
glomalin extraction, we found that the Michaelis-Menten equation closely fits cumulative glomalin
per extraction cycle such that its asymptote provides an objective estimate of total extractable
glomalin for a given set of extraction conditions. Additionally, the equation provides a
curvature parameter that reflects the soil-specific efficiency of an extraction protocol.
Although the soils that we investigated with 7.6% or more soil organic matter had the most
asymptotic total glomalin, they were extracted the least efficiently. (c) 2007 Elsevier Ltd. All
rights reserved.
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