Chemical characteristics of glomalin-related soil protein (GRSP) extracted from soils of varying organic matter content

Abstract

Glomalin is described in the literature as a N-linked glycoprotein and the putative gene product of arbuscular mycorrhizal fungi (AMF). Since the link between glomalin and various protein fractions in soil is not yet clearly defined, glomalin-related soil protein (GRSP) more appropriately describes glomalin's existence in natural organic matter (NOM). The objective of this study was to examine the chemical characteristics of GRSP present in several mineral and organic soils of varying organic carbon content. GRSP was isolated using high temperature sodium citrate extraction followed by either trichloroacetic acid (TCA) or hydrochloric acid (HCl) precipitation. GRSP was characterized by quantitative solid-state 13C DPMAS NMR, infrared (IR) spectroscopy, elemental analysis, and the Bradford assay for protein content. GRSP accounted for 25% and 52% of total C in the mineral soils and organic soil, respectively. Molar C/N and H/C ratios reveal that GRSP has less nitrogen than bovine serum albumin (BSA), and that GRSP extracted from the Pahokee peat soil possessed a more unsaturated, and thus aromatic character relative to the mineral soil GRSP, respectively. GRSP's high aromatic (42–49%) and carboxyl (24–30%) carbon contents and low aliphatic (4–11%) and carbohydrate-type carbon contents (4–16%) suggests that GRSP does not resemble a typical glycoprotein. In fact, the NMR spectra of GRSP closely resemble that of humic acid. GRSP extracted from mineral and organic soils possessed the same NMR fingerprint regardless of the precipitation method used (i.e., either TCA or HCl). It is likely that the current GRSP extraction methods, because of their similarity to the method used to extract humic acid, are coextracting both materials.