From fruit juice wastewater to biopolymer – How the mixed microbial culture and PHA content develop over time

• Fruit juice wastewater is a suitable substrate for PHA production using MMC. • PHA production over eight hours shows a regular increase with time of enrichment. • Over the course of 14 days a PHA-content > 60 % VSS is achieved after 6 days. • MMC sustains a stable metabolism after 6 days, inducing a regular feeding pattern. • DNA analysis links high PHA-content to high abundance of potential PHA-producers. Biodegradable polymers in the form of polyhydroxyalkanoates (PHA) were produced using residual streams from the fruit juice industry and excess sludge from a municipal sewage treatment plant. Initially, PHA-synthesising bacteria were enriched through a feast-famine regime, supplying the feedstock in a 12 h cycle. The development of the PHA accumulation potential was investigated closely during a 14-day-long experiment, taking hourly samples during several eight-hour-long batch accumulations. Thus, the substrate was provided to the enriched biomass using a feed-on-demand strategy based on the increase of the dissolved oxygen concentration. Hereby, the PHA content of the mixed microbial culture (MMC) increased from 8 % up to 65 % of the dry matter after 6 days. At the same time a stable metabolism of the MMC could be observed, showing a regular feeding pattern. Gas chromatography was used to analyse the monomeric composition of the produced PHA, revealing changes in the composition of hydroxybutyrate (HB), hydroxyvalerate (HV), and hydroxy hexanoate (HH). Additionally, molecular analysis of the microbial community composition showed a shift in the relative abundance of potential PHA-producing bacteria. Despite this shift, the PHA productivity of the biocenosis remained nearly stable and followed the same pattern of the relative abundance of potential PHA producers, demonstrating the system’s resilience and functional redundancy.