Fig. 5: Adjustments of the molecular mechanisms used by the strain Collimonas pratensis PMB3(1) to weather mineral according to the solution chemistry, the source of carbon and the type of mineral.

The strain PMB3(1) is known to weather minerals using acidification and chelation related mechanisms, but their activation and effectiveness strongly depend on the solution chemistry (i.e., buffering capacity of the medium, the source of carbon and iron availability). The hypothetical model presented here illustrates the different adjustments done by the strain PMB3(1) based on the different experimentations done in the study and from the literature. A presents the mechanisms preferentially used by the strain PMB3(1) to weather minerals containing iron in their structure. B presents the mechanisms preferentially used by the strain PMB3(1) to weather minerals that do not contain iron in their structure. In each condition, the mechanisms involved are presented as follows: (i) GMC : GMC oxidoreductase allowing to the conversion of glucose to gluconic acid and protons, (ii) Malleobactin: siderophore involved in the mobilization of iron and iii) ? : unknown molecular mechanism(s). For all the minerals: i) In low buffered condition supplemented with glucose (BHm + glucose), the model strain mainly weather minerals by acidolysis through its GMC oxidoreductase activity. In this condition, the dissolution is the higher (+++) compared to the other conditions. (ii) In the same medium supplemented with mannitol (BHm + mannitol), the dissolution is quantifiable, but low (+). The molecular mechanisms used by the strain PMB3(1) to allow this low dissolution remain unknown (?) in this specific case. The strain PMB3(1) can also use complexolysis mechanisms, but the production of siderophore depends on the absence or low concentration of iron available in the solution. In low buffered condition and whatever the source of carbon, malleobactin is not produced due to the release of iron in solution by the GMC activity. In contrast, in highly buffered condition and whatever the source of carbon, the acidity produced by the GMC activity is neutralized, the malleobactin is consequently produced. The dissolution observed in such condition is mostly attributed to the action of the malleobactin. This mechanism is however less effective at weathering than acidolysis (+). For minerals devoid of iron, no active dissolution occurs in highly buffered medium (0), while malleobactin is produced.