Impact of saline osmotic stress on halotolerant polyhydroxyalkanoate (PHA)-accumulating mixed microbial cultures: Boosting PHA production by osmotic downshock

Bioconversion of saline organic side-streams into value-added products, such as polyhydroxyalkanoates (PHAs), can reduce treatment costs and contribute to a circular economy. PHA production by Mixed Microbial Cultures (MMC) under saline conditions enables the use of salty side-streams and seawater (∼3.5 % w/v NaCl) as process water, further enhancing the economic and environmental sustainability of the process, particularly for industries located at coastal sites. This study investigates a strategy to further improve the PHA production capacity of PHA-accumulating MMC selected under near seawater salinity. Specifically, the selected culture was evaluated for its PHA production capacity under sudden osmotic imbalances by subjecting it to osmotic downshock (30–0 gNaCl/L) and upshock (30–60 gNaCl/L). The culture was enriched under double selective pressure (F/f regime and uncoupled C/N feeding) at 30 gNaCl/L, leading to the dominance of Neptunomonas phycophila (79.7 %). During PHA accumulation, a maximum PHA content of 42.4 % wt. was achieved, with a resulting P(3HB-co-3HV) copolymer containing 45 % wt. 3HV. Osmotic downshock (<30 gNaCl/L) resulted in a 1.4-fold increase in PHA content (56.9 %) and storage yield compared to the control (30 gNaCl/L), while high osmotic upshock (60 gNaCl/L) had a negative effect. Additionally, varying food-to-microorganism (F/M) ratios and saline concentrations allowed the production of polymers with varying 3HB monomeric content (53 % to 72 %), expanding the potential applications of these polymers. • Saline organic by-products and seawater can serve as viable alternatives in PHA production processes. • Neptunomonas phycophila dominated the halotolerant PHA-accumulating MMC. • Osmotic downshock (<30 gNaCl/L) led to a 1.4-fold increase in PHA content and storage yield. • Adjusting F/M ratios and saline concentrations allowed for tuning the 3HB polymer content, ranging from 53 % to 71.9 %.