Functioning of phosphatases in tropical soils: effect of the organo-mineral composition on the expression of enzymatic activity
Thesis defended on 20 December 2011 - Montpellier SupAgro and Cocody University, Abidjan, Ivory Coast
The role of catalytic activity of fungal phosphatases in the improvement of the phosphorus nutrition cannot be reliably predicted without taking into account the factors which influence their behaviour in the soil.
The objective of this thesis was to study various factors which could influence the effectiveness of ectomycorrhizal fungal phosphatases in soils. Adsorption and the activity in the adsorbed state of phosphatases produced by Suillus collinitus and Hebeloma cylindrosporum were studied in contact with several fractions of various tropical soils. The persistence of the activity of these enzymes immobilized on the soils was also studied.
These phosphatases showed a diversity of affinity for soil colloids, due to their origin and their characteristics. Moreover, no relation was found between adsorption and the resulting catalytic activity; there was generally no loss of activity in an adsorbed state. One of the enzymes which underwent rapid degradation in solution was protected by the presence of ferrallitic soils but not by the vertisols. These fungic phosphatase samples were purified and partially characterized. The fractions of S. collinitus phosphatases retained on hydrophobic chromatography column showed enhanced activity in contact with mineral clays with respect to solution. The hypothesis of dimerisation on the clay surfaces was advanced to explain the unexpected enhancement of catalytic activity in an adsorbed state of the purified fractions.