Screening of Gas Substrate and Medium Effects on 2,3-Butanediol Production with C. ljungdahlii and C. autoethanogenum Aided by Improved Autotrophic Cultivation Technique

Luca Ricci,Valeria Agostino,Debora Fino,Angela Re

doi:10.3390/fermentation7040264

Abstract

Gas fermentation by acetogens of the genus Clostridium is an attractive technology since it affords the production of biochemicals and biofuels from industrial waste gases while contributing to mitigate the carbon cycle alterations. The acetogenic model organisms C. ljungdahlii and C. autoethanogenum have already been used in large scale industrial fermentations. Among the natural products, ethanol production has already attained industrial scale. However, some acetogens are also natural producers of 2,3-butanediol (2,3-BDO), a platform chemical of relevant industrial interest. Here, we have developed a lab-scale screening campaign with the aim of enhancing 2,3-BDO production. Our study generated comparable data on growth and 2,3-BDO production of several batch gas fermentations using C. ljungdahlii and C. autoethanogenum grown on different gas substrates of primary applicative interest (CO2 · H2, CO · CO2, syngas) and on different media featuring different compositions as regards trace metals, mineral elements and vitamins. CO · CO2 fermentation was found to be preferable for the production of 2,3-BDO, and a fair comparison of the strains cultivated in comparable conditions revealed that C. ljungdahlii produced 3.43-fold higher titer of 2,3-BDO compared to C. autoethanogenum. Screening of different medium compositions revealed that mineral elements, Zinc and Iron exert a major positive influence on 2,3-BDO titer and productivity. Moreover, the CO2 influence on CO fermentation was explored by characterizing C. ljungdahlii response with respect to different gas ratios in the CO · CO2 gas mixtures. The screening strategies undertaken in this study led to the production of 2.03 ± 0.05 g/L of 2,3-BDO, which is unprecedented in serum bottle experiments.

Highlights

Gas fermentation mediated by acetogenic bacteria of the genus Clostridium enables carbon capture and conversion of greenhouse gases into useful products [1,2,3,4]
We identified the gas substrate and biocatalyst which promoted the production of 2,3-BDO by screening growth and product profiles of C. autoethanogenum and C. ljungdahlii on three gas substrates—CO2 · H2, CO · CO2 and simulated syngas
Our results neatly indicate that the CO2 · H2 gas mixture is not appropriate for 2,3-BDO production, confirming previous observations [7,8], and point at CO · CO2 4:1 as the preferable gaseous substrate for 2,3-BDO autotrophic production

Summary

Introduction

Gas fermentation mediated by acetogenic bacteria of the genus Clostridium enables carbon capture and conversion of greenhouse gases into useful products [1,2,3,4]. The gaseous feedstock may be the direct waste streams of industrial processes, such as steel manufacturing and oil refineries, or syngas of variable composition generated from gasification of an ample range of carbon-containing wastes [6]. To reflect such a high feedstocks’ flexibility, the synthetic gas mixtures used to grow acetogens in lab-scale gas fermentations are highly diversified and include CO · CO2 [7,8,9], Fermentation 2021, 7, 264. The gas fermentation primary limitations are the poor gas to liquid mass transfer, due to the low solubility of the gas substrates [17], by the slow growth of acetogens that strive at the thermodynamic edge of life [18] and by low product yields [19]

Methods

Results

Conclusion