Metaproteomic insights into salinity’s impact on carbon assimilation and polyhydroxyalkanoate production in mixed cultures of purple phototrophic bacteria

Purple phototrophic bacteria (PPB) are metabolically versatile microorganisms capable of adapting to diverse environmental conditions in biotechnological applications, including polyhydroxyalkanoate (PHA) production. This study explores how salinity drives metabolic specialization by comparing non-saline (R-NS) and saline-adapted (R-S) cultures fed with acetic acid. R-S accumulated significantly more PHA (up to 42 dw%) than in R-NS (up to 11%). Proteomic analyses revealed distinct communities and carbon assimilation pathways. R-NS, dominated by Rhodopseudomonas, relied on the glyoxylate cycle. While R-S, dominated by Cereibacter, employed the ethylmalonyl-CoA (EMC) pathway. EMC may provide proton motive force to support osmotic regulation in R-S, with excess reducing power equivalents stored as PHA. This is supported by glutamate synthesis overexpression and detection of osmolyte transporters. These findings underscore the adaptability of PPB to salinity, the role of reducing power in saline stress response, and suggest that the EMC pathway can sustain PHA biosynthesis even with carboxylation steps.