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. 2021 Oct 26:9:773104.
doi: 10.3389/fbioe.2021.773104. eCollection 2021.

Vitreoscilla Hemoglobin Improves Sophorolipid Production in Starmerella Bombicola O-13-1 Under Oxygen Limited Conditions

Jun-Feng Li  1 , Hong-Fang Li  1 , Shu-Min Yao  2 , Meng-Juan Zhao  1 , Wen-Xun Dong  1 , Sheng-Kang Liang  3 , Xing-Yong Xu  4
Affiliations

Vitreoscilla Hemoglobin Improves Sophorolipid Production in Starmerella Bombicola O-13-1 Under Oxygen Limited Conditions

Jun-Feng Li et al. Front Bioeng Biotechnol. .

Abstract

Sophorolipids (SLs) are homologous microbial secondary metabolites produced by Starmerella bombicola and have been widely applied in many industrial fields. The biosynthesis of SLs is a highly aerobic process and is often limited by low dissolved oxygen (DO) levels. In this study, the Vitreoscilla hemoglobin (VHb) gene was transformed into S. bombicola O-13-1 by homologous recombination to alleviate oxygen limitation. VHb expression improved the intracellular oxygen utilization efficiency under either oxygen-rich or oxygen-limited conditions. In shake flask culture, the production of SLs was higher in the recombinant (VHb+) strain than in the wild-type (VHb-) strain, while the oxygen uptake rate of the recombinant (VHb+) strain was significantly lower than that of the wild-type (VHb-) strain. In a 5 L bioreactor, the production of SLs did not increase significantly, but the DO level in the fermentation broth of the VHb+ strain was 21.8% higher than that of VHb- strain under oxygen-rich conditions. Compared to wide-type strains (VHb-), VHb expression enhanced SLs production by 25.1% in the recombinants (VHb+) under oxygen-limited conditions. In addition, VHb expression raised the transcription levels of key genes involved in the electron transfer chain (NDH, SDH, COX), TCA cycle (CS, ICD, KDG1) and SL synthesis (CYP52M1 and UGTA1) in the recombinant (VHb+) strains. VHb expression in S. bombicola could enhance SLs biosynthesis and intracellular oxygen utilization efficiency by increasing ATP production and cellular respiration. Our findings highlight the potential use of VHb to improve the oxygen utilization efficiency of S. bombicola in the industrial-scale production of SLs using industrial and agricultural by-products like molasses and waste oil as fermentation feedstock.

Keywords: Starmerella bombicola; Vitreoscilla hemoglobin; fermentation; oxygen utilization efficiency; sophorolipids.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
Construction of the plasmid pSBEX-HPT-vgb used for S. bombicola transformation.
FIGURE 2
FIGURE 2
Phenotypic analysis of the S. bombicola vgb strain. (A) Electrophoresis analysis of genomic DNA extracted from wild-type strains Vitreoscilla hemoglobin (VHb−) and transformants (VHb+) of S. bombicola. Lane 1: VHb+ (441 bp); Lane 2: VHb−(wild-type strain); Lane M: DL 2000 DNA marker (2000 bp, 1,000 bp, 750 bp, 500 bp, 250 bp, 100 bp); (B) CO-difference spectra of the vgb-bearing S. bombicola strain (VHb+) and the wild-type strain (VHb−); (C) The color of the VHb+ and VHb− strains.
FIGURE 3
FIGURE 3
Comparison of sophorolipid fermentation between the VHb+ and VHb− strains cultivated in shake flasks (*p < 0.05).
FIGURE 4
FIGURE 4
Effects of VHb expression on DO content and SLs production (A), residual sugar and biomass (B) under oxygen-rich conditions.
FIGURE 5
FIGURE 5
Effects of VHb expression on DO content and sophorolipid production (A), the residual sugar and biomass (B) under oxygen-limited conditions.
FIGURE 6
FIGURE 6
Transcription levels of key genes involved in the TCA cycle, respiratory chain and sophorolipid biosynthesis at 24 and 72 h of fermentation. The transcription levels of genes in the recombinant strains were normalized to the transcription level of genes in the wild-type strains. (A) transcription levels of CS, ICD, and KDG1; (B) transcription levels of NDH, SDH, and COX; (C) transcription levels of CYP52M and UGTA; and (D) expression level of VHb.
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