Usage of the Fungus Mucor indicus and the Bacterium Rhodovulum adriaticum in a Biorefinery System for Biochemical Production on Grass Hydrolysates

Abstract

Utilization of various biomasses as raw materials in biorefineries represents a promising alternative for the production of valuable chemicals and biofuels. This study investigates the potential of the fungus Mucor indicus DSM 2158, cultivated on media containing the liquid phase of grass hydrolysates (LGH) and various nitrogen sources (yeast extract and corn steep liquor), for the production of valuable metabolites, such as ethanol, chitin, chitosan, and fatty acids. The ethanol yield varied depending on the cultivation media and conditions. The highest substrate-into-ethanol conversion coefficients (0.14-0.2 g g^-1) were achieved during M. indicus cultivation on the LGH medium containing 5 g L^-1 CSL in Erlenmeyer flasks and a bubble column bioreactor. In these cultivations, the highest fungal biomass concentrations (5.61-5.91 g L^-1) were also observed. In flask cultivations, the highest content of total lipids in fungal dry biomass (15.76%) was observed. The obtained fungal biomass contained up to 22 fatty acids, with oleic acid (≈50%) being the most predominant. Chitin and chitosan yields were from 0.1 g g^-1 to 0.3 g g^-1 of dry biomass depending on the cultivation media and conditions. The residual media from the cultivation of M. indicus were used for the growth of the non-sulfur purple bacterium Rhodovulum adriaticum DSM 2781. Cultivations of R. adriaticum DSM 2781 on the residual media, in Erlenmeyer flasks and a stirred-tank bioreactor, resulted in a biomass yield of 0.50 to 2.26 g L^-1. After extraction of bacterial biomass, total pigments (expressed as bacteriochlorophyll-a) were obtained in the range from 1.8 to 48.1 mg g^-1 dry biomass depending on the media and cultivation conditions. The highest titer of bacteriochlorophyll-a was achieved during cultivation on the exhausted LGH medium with 5 g L^-1 yeast extract. The established biorefinery system has to be optimized in order to reach capacity for transfer to a larger scale.

Keywords: Mucor indicus; Rhodovulum adriaticum; biochemicals; biofuels; biorefinery; grass hydrolysate.

Figure 1

Substrates consumption of and products formation during cultivation of M. indicus in Erlenmeyer flasks at 30 °C on media with (a) glucose under aerobic conditions, (b) glucose under anaerobic conditions, (c) xylose under aerobic conditions, (d) xylose under anaerobic conditions, (e) glucose and xylose under aerobic conditions, and (f) glucose and xylose under anaerobic conditions.

Figure 2

Substrates consumption of and products formation during cultivation of M. indicus DSM 2185 in Erlenmeyer flasks on LGH medium without nitrogen source addition.

Figure 3

Substrates consumption of and products formation during cultivation of M. indicus DSM 2185 in Erlenmeyer flasks on LGH media with addition of different nitrogen sources: (a) YE =1 g L⁻¹; (b) YE =5 g L⁻¹; (c), YE =10 g L⁻¹; (d) CSL = 1 g L⁻¹; (e) CSL = 5 g L⁻¹; (f) CSL = 10 g L⁻¹.

Figure 4

Chitin and chitosan yields from dry fungal biomass after cultivations in Erlenmeyer flasks on LGH media (H) containing various concentrations of yeast extract (YE) and corn steep liquor (CSL).

Figure 5

Changes in OD₆₀₀ and absorbance during the cultivation of R. adriaticum in Erlenmeyer flasks on exhausted LGH medium without the addition of a nitrogen source.

Figure 6

Changes in OD₆₀₀ and absorbance during the cultivation of Rhodovulum adriaticum in Erlenmeyer flasks on exhausted LGH media which previously contained the following: (a) 1 g L⁻¹ yeast extract (YE); (b) 5 g l⁻¹ YE; (c) 10 g L⁻¹ YE; (d) 1 g L⁻¹ CSL; (e) 5 g L⁻¹ CSL; (f) 10 g L⁻¹ CSL.

Figure 7

Changes in concentrations of substrates and products during M. indicus DSM 2185 cultivation on LGH media with the addition of a nitrogen source in a bubble column bioreactor.

Figure 8

Substrates consumption of and products formation during cultivation of R. adriaticum DSM 2781 in a stirred-tank bioreactor.