Pilot-scale valorisation of salmon peptone into polyhydroxyalkanoates by mixed microbial cultures under conditions of high ammonia concentration

The aquaculture industry produces vast amounts of organic side streams that are often ensilaged for later use in low value applications. However, the potential of converting fish silage, particularly the proteinaceous fraction, into more economically valuable products such as biodegradable polyhydroxyalkanoates (PHA), has not been explored. This study investigates the valorisation of salmon peptone, the proteinaceous fraction obtained from salmon silage, into PHA by mixed microbial cultures (MMC) under conditions without nutrient limitation, where the high concentration of ammonia (up to 2.3 ± 0.2 gN/L) which the culture was subjected to has not been previously reported. Acidogenic fermentation of salmon peptone (SP) was attained in a continuous reactor, resulting in the production of a fermentate rich in acetate and butyrate (16.4 ± 1.6 gCODFP/L). The enriched MMC with PHA-accumulating organisms was selected in a sequential batch reactor (SBR) under nutrient-rich conditions (100 gCOD: 27.6 gN: 0.48 gP at OLR 7.1 gCODFP/(L.d)). The dominant microorganisms in the enriched culture were from the genus Brachymonas, which are described as capable of simultaneous cell growth and PHA accumulation, storing up to 53.0 ± 3.4% wt. of P(3HB-co-3HV) with a 3HV content of 34% wt.. Despite the high ammonia concentration and constant nutrient availability, a good overall PHA yield (189 gPHA/kgSP) and global volumetric PHA productivity (3.10 ± 0.11 gPHA/(L.h)) were achieved. The results demonstrate the technological feasibility of producing PHA from salmon peptone under conditions that are typically considered unfavorable for PHA accumulation, which represents a significant novelty of this work.