Structure elucidation and proposed de novo synthesis of an unusual mono-rhamnolipid by Pseudomonas guguanensis from Chennai Port area

Sivamurugan Vajiravelu,R. Lakshmi Sundaram,Vidya Vasudevan,K. C. Ramya Devi,Gnanambal K. Mary Elizabeth

doi:10.1038/s41598-019-42045-9

Sivamurugan Vajiravelu, R. Lakshmi Sundaram + Show 3 more

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https://doi.org/10.1038/s41598-019-42045-9

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Abstract

In this paper, we describe the isolation of an unusual type of high molecular weight monorhamnolipid attached to esters of palmitic, stearic, hexa and octadecanoic acids as against the routinely reported di-rhamnolipids linked to hydroxydecanoic acids. The bioemulsifier was column-purified and the chemical nature of the compound was elucidated using FT-IR, GC-MS and 1D [1H and13C] and 2D NMR. This monorhamnolipid is extracted from a bacterium, Pseudomonas guganensis and is not reported to have biological activities, let alone emulsification abilities. The bacterium continually produced rhamnolipids when nourished with n-hexadecane as its lone carbon source. The extracellularly secreted monorhamnolipids are capable of degrading hydrocarbons, with most preference to n-hexadecane [EI24 of 56 ± 1.42% by 2 mL of the spent medium]. Whilst the crude ethyl acetate partitioned extract had an EI24 of 65 ± 1.43%; the purified rhamnolipid product showed 78 ± 1.75% both at 12.5 mg/mL concentration. The used-up n-hexadecane is biotransformed to prepare its own rhamnolipids which in return is utilized to degrade n-alkanes thus creating a circular pathway which is proposed herein. This bacterium can be seen as a new source of bioemulsifier to reduce hydrocarbon in polluted waters.

Highlights

Long-chained aliphatic hydrocarbons, though described as robust substrates are still utilized only by specific bacteria, which are known to show up soon after an oil spill, while the non-hydrocarbon utilizing groups diminish[1]
The strains were customarily cultured on Bushnell and Haas medium supplemented with increasing concentrations of diesel and n-hexadecane [0.5% increment for every 6 months] to achieve maximal acclimatization to that hydrocarbon source
It is astounding to mention that only these two isolates out of the 849 had the capacities to disperse n-hexadecane, diesel, and kerosene as micro-emulsions in water. It is perhaps true for two reasons: (i) most of the heterotrophic non-differentiating marine bacteria those survive in oligotrophic conditions prevail in a state of dormancy/starvation[16,17,18] (ii) uptake and metabolism of substrates like hydrocarbons is tough, especially when the organisms are obliged to use those as the solitary source of carbon when deprived of all other nutrients[19,20]

Summary

Introduction

Long-chained aliphatic hydrocarbons, though described as robust substrates are still utilized only by specific bacteria, which are known to show up soon after an oil spill, while the non-hydrocarbon utilizing groups diminish[1]. Right from 1970s, there are researchers, who continually report significant bacterial populations to have a preferred carbon source as n-hexadecane which is mineralized by the secretion of bioemulsifiers[5,6,7] It has been demonstrated all over these years that rhamnolipids, a well-known class of bioemulsifier is produced and secreted profusely by bacteria to utilize n-alkanes of higher molecular weights (carbon atoms >14), whose dispersion would otherwise be cumbersome because of their high logP values[8,9,10]. There is a high focus on the production of rhamnolipids of various types from microorganisms to degrade dense Non-Aqueous-Phase Liquids [NAPLs] like, n-hexadecane. The spectral data were used to construct a plausible pathway for the de novo synthesis of bioemulsifier

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