Comparing poly-3-hydroxybutyrate accumulation in Azospirillum brasilense strains Sp7 and Sp245: The effects of copper(II)

Intracellular accumulation of poly-3-hydroxybutyrate (PHB) was comparatively studied in the plant-growth-promoting bacterium (PGPB) Azospirillum brasilense (epiphytic strain Sp7 and endophytic strain Sp245) grown microaerobically for 2 days under nitrogen deficiency in the standard malate salt medium in the absence (control) or presence of copper (0.1 mM Cu2+). For quantitative determination of PHB content of cells, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of whole-cell biomass samples was used. After 2 days in control cells, PHB accumulation in strain Sp7 reached ca. 24% of dry cell mass (d.c.m.). In strain Sp245, the PHB content in the control was over 1.3-fold higher (32% d.c.m.) than in strain Sp7. In the presence of copper(II), PHB accumulation was notably enhanced in strain Sp7 (over 1.6-fold, up to ca. 39% d.c.m.), whereas in strain Sp245 it changed insignificantly (from 32% up to ca. 35% d.c.m.). The levels of copper(II) uptake were comparable in both strains. These findings are in line with our earlier observations that even in rich NH4+-supplemented medium, some heavy metals, including copper(II), induce PHB biosynthesis in A. brasilense strain Sp7, but not in Sp245. The dissimilarities in the levels of PHB accumulation and in the effects induced by copper(II) in the two strains are attributed to their different adaptive potentials owing to different ecological niches they occupy in the rhizosphere.