Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service. The name comes from Glomales, an order of fungi.
Discovery and controversy
Glomalin eluded detection until 1996 because, “It requires an unusual effort to dislodge glomalin for study: a bath in citrate combined with heating at 250 F (121 C) for at least an hour.... No other soil glue found to date required anything as drastic as this.” - Sara Wright. However, in 2010 using advanced analytical methods the citrate-heating extraction procedure was proven to co-extract humic substances, so it is still not clear if this "glue effect" comes from glomalin or the other substances that are co-extracted using that method .
The specific protein glomalin has not yet been isolated and described. However, glomalin-related soil proteins (GRSP) have been identified using a monoclonal antibody (Mab32B11) raised against crushed AM fungi spores. It is defined by its extraction conditions and reaction with the antibody Mab32B11.
The discoverer of glomalin, Sara Wright, thinks the “glomalin molecule is a clump of small glycoproteins with iron and other ions attached... glomalin contains from 1 to 9% tightly bound iron.... We’ve seen glomalin on the outside of hyphae, and we believe this is how the hyphae seal themselves so they can carry water and nutrients. It may also be what gives them the rigidity they need to span the air spaces between soil particles.” Glomalin takes 7–42 years to biodegrade and is thought to contribute up to 30 percent of the soil carbon where mycorrhizal fungi is present. The highest levels of glomalin were found in volcanic soils of Hawaii and Japan.
There is other circumstantial evidence to show that glomalin is of AM fungal origin. When AM fungi are eliminated from soil through incubation of soil without host plants, the concentration of GRSP declines. A similar decline in GRSP has also been observed in incubated soils from forested, afforested, and agricultural land and grasslands treated with fungicide. Concentrations of glomalin in soil are correlated with the primary productivity of an ecosystem.
The chemistry of glomalin-related soil protein (GRSP) is not yet fully understood, and the link between glomalin, GRSP, and arbuscular mycorrhizal fungi is not yet clear. The physiological function of glomalin in fungi is also a topic of current research.
Glomalin-related soil proteins (GRSPs), along with humic acid, are a significant component of soil organic matter and act to bind mineral particles together, improving soil quality. Glomalin has been investigated for its carbon and nitrogen storing properties, including as a potential method of carbon sequestration.
Glomalin is hypothesized to improve soil aggregate stability and decrease soil erosion. A strong correlation has been found between GRSP and soil aggregate water stability in a wide variety of soils where organic material is the main binding agent, although the mechanism is not known.
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