Marine Biosurfactants: Biosynthesis, Structural Diversity and Biotechnological Applications

Abstract

Biosurfactants are amphiphilic secondary metabolites produced by microorganisms. Marine bacteria have recently emerged as a rich source for these natural products which exhibit surface-active properties, making them useful for diverse applications such as detergents, wetting and foaming agents, solubilisers, emulsifiers and dispersants. Although precise structural data are often lacking, the already available information deduced from biochemical analyses and genome sequences of marine microbes indicates a high structural diversity including a broad spectrum of fatty acid derivatives, lipoamino acids, lipopeptides and glycolipids. This review aims to summarise biosyntheses and structures with an emphasis on low molecular weight biosurfactants produced by marine microorganisms and describes various biotechnological applications with special emphasis on their role in the bioremediation of oil-contaminated environments. Furthermore, novel exploitation strategies are suggested in an attempt to extend the existing biosurfactant portfolio.

Keywords: biosynthetic mechanisms; biotechnological application; glycolipids; heterologous expression; lipopeptides; marine biosurfactants; oil degradation; structural diversity; synthetic biology.

Figure 1

Structural diversity of marine low molecular weight (LMW) biosurfactants. 1–4 representative lipoamino acid and lipopeptide biosurfactants: 1 proline lipid (Alcanivorax dieselolei); 2 rhodofactin (Rhodococcus sp.); 3 surfactin (Bacillus subtilis); 4 didemnin B (Tristrella sp.). 5–8 representative glycolipid biosurfactants: 5 di-rhamnolipid (Pseudomonas aeruginosa); 6 glucose lipid (Alcanivorax borkumensis); 7 glucosyl palmitate (Serratia marcescens); 8 tri-glucose-tetraester (Rhodococcus sp.); 9 2-l-quinovose - phenazine ester (Streptomyces sp.).

Figure 2

Biosynthetic pathways leading to low molecular weight biosurfactants. (A) Synthesis of lipoamino acid N-myristoyltyrosine via N-acyl amino acid synthase (NAS). ACP, acyl carrier protein. (B) Lipopeptide surfactin biosynthesis by the non-ribosomal peptide synthetases SrfA-A, SrfA-B, and SrfA-C, showing the principle of the modular non-ribosomal peptide biosynthesis. C, condensation domain; A, adenylation domain; PCP, peptidyl carrier domain; E, epimerisation domain; TE, thioesterase domain. (C) Biosynthesis of 3-(3-hydroxyalkanoyloxy)alkanoic acid, mono-rhamnolipids and di-rhamnolipids, constituting fatty acids and glycolipid biosurfactants, respectively, by the enzymes RhlA-C of P. aeruginosa.

Figure 3

Identification, production and applications of marine biosurfactants. Production of surface-active compounds can be identified in habitats enriched for biosurfactant producers, either by culture-dependent or -independent methods using simple assays like atomised oil assay, grid assay or drop collapsing, and emulsification assay, respectively. NMR-analysis of biosurfactant structures enables the elucidation of new compounds, while sequencing and bioinformatics allows the deciphering of the biosynthetic background. Novel sophisticated strain engineering and expression tools will result in designed next-generation cell factories able to convert renewable substrates into a wealth of desired compounds with high precision and efficiency. Product yields may be further optimized by process engineering. The combination of these methods will provide biosurfactants for highly diverse applications in the future: for environmental remediation, microbially enhanced oil recovery, plant growth promotion or other applications in the food sector as well as in medical and consumer products.