The functional roles of mucus during aggregation

Soil organisms influence pedogenesis on a molecular level through the production of biopolymers which potentially interact with soil minerals depending on their molecular properties. Specifically, biopolymers can inhibit aggregation as separation agent or promote aggregation as bridging agent (Guhra et al. 2022). Mucus, a biopolymer excreted by earthworms consisting mainly of proteins, polysaccharides, and, to a lesser extent, lipids, has often been neglected so far, despite earthworm's fundamental contribution to soil quality and structuring via bioturbation. In our study, we investigate the role of cutaneous earthworm mucus (CEM) of L. terrestris during the formation of organo-mineral associations and aggregates. For this purpose, batch experiments were carried out with goethite and CEM at different pH values and increasing CEM concentrations resulting in the formation of mucus-goethite associations. Afterwards, the (homo/hetero) aggregation of these newly formed mucus-goethite associations with quartz particles was investigated in response to mucus-C loadings on mineral surfaces and CEM concentration in solution.

Our experiments showed a pH dependent CEM structure and an adsorption to goethite controlled by concentration and pH. Polysaccharides from CEM adsorb preferentially under acidic conditions (pH 3) and low CEM concentration (6 mg mucus-C/l). In contrast, a stronger adsorption of proteins was observed at higher CEM concentrations (30 mg mucus-C /l). In subsequent aggregation experiments, the hetero-aggregation rate of organo-mineral associations and quartz was decreased at low C-loadings and increased at high loadings in comparison to the CEM-free reference. Furthermore, the aggregation between goethite particles was inhibited by electrostatic/steric repulsion (separation agent) when high CEM concentrations were present in solution (mineral:mucus ratio of 17), while CEM functions as bridging agent at low relative CEM supply (mineral:mucus ratio of > 83).

The formation and the aggregation behavior of mucus-mineral associations contribute to nutrient/carbon storage as well as structure formation in soil. The composition, function, and (im-)mobilization of CEM and corresponding organo-mineral associations in earthworm-influenced soil structures is shaped by CEM availability and the structure/reactivity of CEM affected by environmental parameters.

 

 

References:

Guhra, T., Stolze, K. and Totsche, K.U. 2022. Pathways of biogenically excreted organic matter into soil aggregates. Soil Biology and Biochemistry, 164, 108483.