Magnetotactic Bacteria-Based Biorefinery: Potential for Generating Multiple Products from a Single Fermentation

The magnetotactic bacteria (MTB) model Magnetospirillum gryphiswaldense MSR-1 (Mgryph) is typically known for its capacity to produce magnetic nanoparticles with unique properties, namely, magnetosomes. However, the magnetosome fraction represents only around 4% of the total cell mass. Therefore, the downstream processing of Mgryph generates a substantial amount of under-utilized microbial biomass waste (MBW) rich in proteins and polyhydroxyalkanoates (PHAs), which can be used, for example, as animal feed and biodegradable bioplastics, respectively. In this work, we have designed an integrated Mgryph-based biorefinery through the utilization of the MBW for the recovery of PHA and soluble proteins using NaClO extraction, revealing that poly­(3-hydroxybutyrate-co-3-hydroxyvalerate) is produced with a relative abundance of 99:1 mol % (3HB/3HV). We have further upgraded PHA into crotonic acid using pyrolysis, which can be used in adhesive and biofuel manufacturing. The effect of the MBW concentration used in the NaClO extraction step (10, 30, and 50 g MBW L^–1) was evaluated to determine PHA recovery yields, purity, and purification factor, as well as the thermal stability and fraction of volatile components. The condition using 10 g MBW L^–1 was the best among those tested with 51.3 ± 0.14% PHA content in the extract, 97.8% extraction yield, and 2.93 purification factor. The thermogravimetric analysis of PHA extracts from Mgryph showed a degradation range between 232 and 292 °C and a purity of up to 73 ± 4%. Under optimal extraction conditions (10 g MBW L^–1), 53.3% of the total cellular protein was recovered. Analysis of products from isothermal pyrolysis of PHA extracts at 300 °C yielded up to 86.0 ± 1.5% of crotonic acid. To the best of our knowledge, our study is the first to extract PHA from Mgryph, thus representing a benchmark for future optimization studies for PHA recovery in this microorganism. Moreover, this work explores the development of an integrated Mgryph-based biorefinery for valorizing MBW into added value biochemicals, which can be used in a wide range of applications, thus representing an opportunity to improve the efficiency of magnetosome production toward the development of sustainable bioprocesses.