Abstract:
The widespread use of the toxic heavy metal chromium (Cr) in industrial applications
resulted in large quantities of Cr being discharged into the environment, causing severe
contamination of global soil and water systems. Cr primarily exists in two stable forms,
Cr(III) and Cr(VI). The latter is highly toxic due to its strong oxidizing nature and its high
solubility. The model organism for bioremediation Shewanella oneidensis MR1 has evolved
diverse resistance mechanisms to cope with chromate toxicity.
The first aim of the present thesis was to study the chromate resistance and reduction
mechanisms of this bacterium under semi-aerobic conditions. We showed that chrASO gene is
induced by chromate and its deletion impairs the chromate resistance and reduction capacity
of MR-1 strain, we confirmed that its product functions as an efflux pump to extrude
chromate ions from the cytoplasm protecting cells from chromate toxicity. With cymAdeletion mutants, we revealed the involvement of the c-type cytochrome CymA in chromate
resistance and reduction. We also identified a potential chromate reductase DmsA2, as well
as two other proteins Fdh and DmsA1 that are potentially involved in chromate resistance
and reduction in MR-1 strain.
In the second part of this work, we isolated, identified and characterized two novel
Mediterranean Shewanella sp. strains, S. fidelis H76 and S. algidipiscicola H111. Both
strains are characterized by their great chromate resistance and their ability to reduce it
efficiently even at high concentrations. Although the small size of its genome and the
absence of several genes encoding enzymes known to play a role in chromate resistance and
reduction, the H111 strain is the best chromate resistant strain. Interestingly, the air liquid
interface biofilm (Pellicles) of both strains reduce more efficiently chromate than their freeswimming cells. Moreover, they can accumulate a significant amount of its reduced forms.
These characteristics make those strains, in particular H111 strain, suitable candidates for
chromate bioremediation.
