Unraveling PHA production from urban organic waste with purple phototrophic bacteria via organic overload

Abstract

The production of polyhydroxyalkanoates (PHA) with purple phototrophic bacteria (PPB) has been limited due to low yields and limited knowledge regarding the diverse routes used for carbon biosynthesis. The present study increases PHA accumulation yields using urban organic waste pretreated by steam explosion and acidogenic fermentation as substrate. Throughout the PPB-based photoheterotrophic process in an anaerobic membrane photobioreactor, the organic loading rate (OLR) was modified to increase the amount of PHA and biomass in the reactor. A maximum PHA accumulation of 42% (g(PHA gBiomass)(-1)) on a dry basis was achieved and maintained for 10 d for an OLR of 1 gCOD L-1 d(-1), and hydraulic and sludge retention times of 2 and 6 d, respectively. This PHA accumulation capacity is the maximum obtained using a mixed culture of PPB fed with waste. Also, a medium-chain PHA (polyhydroxyhexanoate) has been quantified, enhancing the physicochemical properties and diversifying their industrial applications. Furthermore, we show novel alternatives to PHA accumulation: carbon storage as glycogen and extracellular polymers while deriving the excess electrons into hydrogen. Finally, a statistical study of microbial communities has settled the environmental variables with the most significant influence on these communities' variability. This work demonstrates the importance of acquiring a thorough understanding of carbon accumulation and electron allocation strategies of PPB under stressful conditions and shows promising results for a larger scale implementation of a PPB-based photobiorefinery, which could valorize urban organic waste to produce different high added-value products within the context of the circular bioeconomy.