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Rising Atmospheric CO2 Concentrations Help Sequester Potentially Toxic Elements
Reference
Gonzalez-Chavez, M.C., Carrillo-Gonzalez, R., Wright, S.F. and Nichols, K.A.  2004.  The role of glomalin, a protein produced by arbuscular mycorrhizal fungi, in sequestering potentially toxic elements.  Environmental Pollution 130: 317-323.

Background
Certain elements such as Cu, Cd, Fe, Mn, Pb and Zn are sometimes found in soils in large enough quantities to be toxic to different forms of life; and, hence, they are coming to be known as potentially toxic elements.  However, to quote the authors of this important study, "naturally occurring soil organic compounds stabilize potentially toxic elements (PTEs);" and in the work described in this report, they explore the hypothesis that "an insoluble glycoprotein, glomalin, produced in copious amounts on hyphae of arbuscular mycorrhizal fungi (AMF) sequesters PTEs."

What was done
In a set of three experiments, the authors tested various aspects of their hypothesis.  As they describe it, "experiment 1 examined PTE concentration in glomalin extracted from polluted soils ? experiment 2 tested the efficacy of glomalin produced by an isolate of Gigaspora rosea to sequester and bind Cu in vitro ? experiment 3 examined the efficacy of glomalin produced by two different isolates of Glomus mosseae to sequester or bind Cu in vivo."

What was learned
Gonzalez-Chavez et al. report that (1) "glomalin participates in the sequestration of different PTEs," that (2) "the glomalin pool in the soil may have a potential to sequestrate PTEs, not only by the colonized roots, but also by the hyphae and through deposition of glomalin in soil," and that (3) "this glycoprotein may be stabilizing PTEs, reducing PTE availability and decreasing the toxicity risk to other soil microorganisms and plants."

What it means
The enormity of these findings becomes evident when one realizes, to quote the authors, that (1) "glomalin is a glycoprotein copiously produced by all AMF tested to date (Wright et al., 1996, 1998; Nichols, 2003)," that (2) "AMF colonize 80% of vascular plant species (Trappe, 1987)," and that (3) AMF "are found worldwide in almost every soil."  What makes these observations even more significant, however, is the fact that the production of glomalin by AMF has been shown to be dramatically enhanced by atmospheric CO2 enrichment.  In the study of Rillig et al. (1999), for example, soil glomalin concentrations in the root zones of natural ecosystems rose linearly as atmospheric CO2 concentration rose in incremental fashion from 250 to 750 ppm; while in going from an atmospheric CO2 concentration of 370 to 670 ppm in the study of Rillig et al. (2000), total soil glomalin experienced a five-fold increase.  Hence, it can be appreciated that as the air's CO2 concentration continues to rise in the years and decades ahead, the entire terrestrial biosphere should experience the benefits described by Gonzalez-Chavez et al., i.e., reduced PTE availability and decreased toxicity risk to soil microorganisms and plants.

References
Nichols, K.  2003.  Characterization of Glomalin - A Glycoprotein Produced by Arbuscular Mycorrhizal Fungi.  PhD Dissertation, University of Maryland, College Park, Maryland, USA.

Rillig, M.C., Hernandez, G.Y. and Newton, P.C.D.  2000.  Arbuscular mycorrhizae respond to elevated atmospheric CO2 after long-term exposure: evidence from a CO2 spring in New Zealand supports the resource balance model.  Ecology Letters 3: 475-478.

Rillig, M.C., Wright, S.F., Allen, M.F. and Field, C.B.  1999.  Rise in carbon dioxide changes soil structure.  Nature 400: 628.

Trappe, J.M.  1987.  Phylogenetic and ecological aspects of mycotrophy in the angiosperms from an evolutionary standpoint.  In: Safir, G.R. (Ed.). Ecophysiology of VA Mycorrhizal Plants.  CRC Press, Boca Raton, Florida, USA, pp. 5-25.

Wright, S.F., Franke-Snyder, M., Morton, J.B. and Upadhyaya, A.  1996.  Time-course study and partial characterization of a protein on hyphae of arbuscular mycorrhizal fungi during active colonization of roots.  Plant and Soil 181: 193-203.

Wright, S.F., Upadhayaya, A. and Buyer, J.S.  1998.  Comparison of N-linked oligosaccharides of glomalin from arbuscular mycorrhizal fungi and soils by capillary electrophoresis.  Soil Biology and Biochemistry 30: 1853-1857.


Reviewed 3 November 2004