Étude des Archaea halophiles extrêmes isolées d’environnements hypersalins algériens et caractérisation de leurs caroténoïdes.

Abstract

Hypersaline lakes constitute a typical example of extreme environments harboring a particular microbial flora, which defies the laws of biology and thrives at the limits of life. Extremely halophilic archaea or haloarchaea dominate the microbial communities found in these environments. These microorganisms represent a deposit of natural resources in biomolecules of industrial interest, in particular pigments. The main objective of this thesis is to isolate extremely halophilic, pigment producers archaeal strains from brine and salt samples collected from 7 different Algerian hypersaline environments. In total, 110 isolates were selected on the basis of their macroscopic morphological characters. Of this total, 43 isolates producing high levels of carotenoids and/or showing highest growth in wide salinity, temperature and pH ranges were selected for phenotypic and molecular characterization. 16S rRNA genes sequencing and analysis revealed that theses strains belong to 7 different archaeal genera of the class Halobaceria: Halorubrum, Haloracula, Haloferax, Natrinema, Halogeometricum, Haloterrigena and Halopiger. Strains Halorubrum sp. BS2 ; Halogeometricum sp. ME3 ; Haloarcula sp. BT9 and Haloferax sp. ME16 were then selected for the study of their carotenoids. Chromatographic and spectrometric analysis of carotenoid extracts revealed a variation in the composition depending on the strain with a predominance of bacterioruberin. The evaluation of antioxidant capacity using in vitro ABTS and DPPH assays showed that these extracts have a strong antioxidant potential, in particular those of Halorubrum sp. BS2 and Haloferax sp. ME16 strains, revealing thus a good potential for biotechnological applications. Antibacterial activity of carotenoid extracts against 4 human‐pathogenic strains and 4 fish pathogenic strains was also evaluated by agar disk diffusion method. The obtained results showed a good antibacterial activity. Lastly, the strain Halorubrum sp. BS2 which showed the highest carotenoid content and interesting biological activities was selected to study the effect of the variation in cultural conditions on its growth and carotenoid production. The results showed that the highest growth and carotenoid contents were obtained at a salinity of 25 % (w/v), under an agitation speed of 300 tpm, at a temperature of 37°C and with a light intensity of 260 µE m-2 s-1.