Lactic acid from crude glycerol as a renewable resource for the cultivation of Magnetospirillum gryphiswaldense MSR-1

The production of biodiesel generates large amounts of glycerol as a by-product that holds untapped economic value. Crude glycerol can be revalorised into value-added chemicals such as lactic acid via hydrothermal processing. Lactic acid is an important bulk chemical with numerous applications, including its utilisation as a carbon source to grow industrially relevant bacterial strains. Magnetotactic bacteria (MTB), which produce magnetic nanoparticles called magnetosomes with potential applications in biomedicine and bioremediation, are of particular interest. Notably, Magnetospirillum gryphiswaldense MSR-1 is a strain of MTB that grows optimally using lactic acid as a carbon source.

In the present work, lactic acid was produced via catalytic hydrothermal processing of crude glycerol, achieving a maximum lactic acid yield of 80.35%. Further, the feasibility of using this lactic acid to culture magnetosome-producing M. gryphiswaldense MSR-1 cells was investigated. Bacterial growth and magnetosome production were observed with concentrations of 2 g·L-1 glycerol-derived lactic acid in the growth medium, reaching cell densities of up to 0.2 OD565 and high cellular magnetism (Cmag =2.5), which are comparable to control conditions using flask standard medium. Flow cytometry analyses revealed that the viability of the cells grown using glycerol-derived lactic acid was also comparable to control conditions.

This work demonstrates the feasibility of valorising crude glycerol via catalytic hydrothermal conversion for the cultivation of magnetotactic bacteria and production of magnetosomes, which can be used in a wide range of biotechnological applications.