How temperature shapes the biosynthesis of PHAs in mixed microbial cultures

Three sequential batch reactors were operated for the enrichment in microbial communities able to store polyhydroxyalkanoates (PHAs) using activated sludge as inoculum. They ran simultaneously under the same operational conditions (organic loading rate, hydraulic and solids retention time, cycle length, C/N ratio) just with the solely difference of the working temperature: psychrophilic (15°C); mesophilic (30°C); and thermophilic (48°C). The microbial communities enriched showed different behaviours in terms of consumption and production rates. In terms of PHA accumulation, the psychrophilic community was able to accumulate an average amount of 17.7 ± 5.7 wt% poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); the mesophilic 40.3 ± 7.0 wt% PHBV, and the thermophilic 14.8 ± 0.3 wt% PHBV in dry weight over total solids. The average PHBV production yields for each selected community were 0.41 ± 0.12 Cmmol PHBV /Cmmol VFA at 15°C, 0.64 ± 0.05 Cmmol PHBV /Cmmol VFA at 30°C, and 0.39 ± 0.14 Cmmol PHBV /Cmmol VFA at 48°C. The overall performance of the mesophilic reactor was better than the other two, and the copolymers obtained at this temperature contained a higher PHV fraction. The physico-chemical properties of the obtained biopolymers at each temperature were also measured and major differences were found in the molecular weight, following an increasing trend with temperature.