Rhodococcus as Biofactories for Microbial Oil Production

Abstract

Bacteria belonging to the Rhodococcus genus are frequent components of microbial communities in diverse natural environments. Some rhodococcal species exhibit the outstanding ability to produce significant amounts of triacylglycerols (TAG) (>20% of cellular dry weight) in the presence of an excess of the carbon source and limitation of the nitrogen source. For this reason, they can be considered as oleaginous microorganisms. As occurs as well in eukaryotic single-cell oil (SCO) producers, these bacteria possess specific physiological properties and molecular mechanisms that differentiate them from other microorganisms unable to synthesize TAG. In this review, we summarized several of the well-characterized molecular mechanisms that enable oleaginous rhodococci to produce significant amounts of SCO. Furthermore, we highlighted the ability of these microorganisms to degrade a wide range of carbon sources coupled to lipogenesis. The qualitative and quantitative oil production by rhodococci from diverse industrial wastes has also been included. Finally, we summarized the genetic and metabolic approaches applied to oleaginous rhodococci to improve SCO production. This review provides a comprehensive and integrating vision on the potential of oleaginous rhodococci to be considered as microbial biofactories for microbial oil production.

Keywords: Rhodococcus; biofactory; single-cell oil.

Figure 1

Phylogenomic tree inferred using the Type Strain Genome Server (https://tygs.dsmz.de/ accessed on 13 February 2021) by Genome BLAST Distance Phylogeny approach (GBDP), using distances calculated from genome sequences. The branch lengths are scaled in terms of GBDP distance formula d5. The numbers below branches are GBDP pseudo-bootstrap support values >60% from 100 replications, with an average branch support of 79.8%. The tree was rooted at the midpoint. The light blue box highlights Rhodococcus species (including oleaginous strains RHA1 and PD630) belonging to the group C, according to the classification proposed by Sangal et al. [10]. Leaf labels are annotated by affiliation to ① species and ② subspecies clusters, predicted by the digital DNA:DNA hybridization (dDDH) calculator service; ③ genomic G+C content (Min. 61.7% , Max. 70.7% ); ④ δ values (Min. 0.1 , Max. 0.3 ); ⑤ overall genome sequence length (Min. 3.9 Mb , Max. 10.4 ); and ⑥ number of proteins (Min. 3611 , Max. 9472 ).

Figure 2

Biotransformation of multiple carbon and energy sources by oleaginous Rhodococcus strains to industrially relevant TAG.

Figure 3

Overview of neutral lipid biosynthesis in bacteria. WE pathways are shown in blue while a scheme of the Kennedy route for TAG is shown in pink. Circles indicate compounds participating in each route. To give the fatty alcohol in WE synthesis, Acinetobacter calcoaceticus (dotted line) employs two steps. A second pathway is present in Marinobacter aquaeolei (continuous line) that in one step through a FAR gives the fatty alcohol. The final reaction involves the bifunctional WS/DGAT that utilizes a fatty acyl-CoA and diacylglycerol to form TAG.