Influence of inorganic carbon on purple phototrophic bacteria polyhydroxyalkanoates production under high reductive stress environment

Fermented wastes rich in reduced compounds challenge anaerobic purple phototrophic bacteria (PPB) systems by causing redox imbalances. This study evaluated polyhydroxyalkanoates (PHA) production and CO₂ fixation by PPB mixed cultures as means to balance internal redox, under four inorganic carbon (IC) availability conditions in a sequencing batch reactor. Culture selection under permanent IC presence promoted higher microbial diversity, but, strongly dependent on IC to balance internal redox, even when more oxidized substrates were supplemented. Increasing IC limitation favoured Rhodopseudomonas palustris, revealing its redox balancing capability independently of IC fixation. PHA contents of 20-29% gPHA/gVSS were achieved across all IC availabilities, however, when IC was present, growth was promoted over PHA production, indicating a preferable electron balance through CO₂ fixation. Nevertheless, cultures selected under feast-famine IC exhibited a preference for PHA accumulation under IC limitation, suggesting IC tuning as a potential growth-controlling strategy to boost phototrophic PHA production using reduced feedstocks.