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Functional Roles of Biosurfactants in Bacterial and Environmental Processes

  • Author(s): Belcher, Richard Wilson
  • Advisor(s): Crowley, David
  • et al.
Abstract

Biosurfactants are amphipathic molecules exuded by bacteria that play critical roles in a variety of bacterial and environmental processes due to their interfacial interactions. The involvement of biosurfactants in these processes has vast potential to enhance bioremediation and expedite swarming motility, to name a few, and research into this arena is pivotal. Surface tension reduction by surface active agents can induce swarming motility lending competence of plant growth-promoting soil inocula. Bioavailability of inaccessible petroleum hydrocarbons can be promoted by biosurfactant presence by a myriad of mechanisms. Many new biosurfactants are still being found from microbes living in petroleum concentrated soils and novel habitats such as natural asphalt seeps should be considered.

The Crowley lab first revealed bacteria in the Rancho La Brea tar pits where they have adapted degradative enzymes to grow on aromatic hydrocarbons and produce biosurfactants to enhance the bioavailability of these petrochemicals. Extracted bacterial cells were enriched on monoaromatics and a polycyclic aromatic hydrocarbon (phenanthrene). Isolates were screened for biosurfactant production with a variety of techniques using a novel medium. Thirty percent of the isolated bacteria were able to produce biosurfactants and were exclusively enriched on BTEX compounds. Stenotrophomonas maltophilia RB91B, Pseudomonas sp. RB91F, and Shewanella sp. RB91G were shown to produce biosurfactants that could lower surface tension down to 52 ± 0.6, 35 ± 3.1, and 54 ± 0.7 mN/m, respectively.

Leading off from Chapter Two, the most potent biosurfactant was used along with a phenanthrene-degrading bacterium, to assess if mineralization of 14C-phenanthrene was affected and what possible mechanisms are involved. Pseudomonas sp. RB91F was shown to produce a polymeric surface active agent comprised of carbohydrate, protein, and lipid that could lower surface tension down to 36 ± 0.4 mN/m at a CMC of 925 mg L-1 and could emulsify kerosene, n-hexane, toluene, xylene, and gasoline. Degradation by Delftia tsuruhatensis RB91H was enhanced by the polymeric bioemulsifier produced by Pseudomonas sp. RB91F depending on amended CMC levels. At each CMC level, unique mechanisms appear to be dominant based on cell surface hydrophobicity, growth on bioemulsifier, as well as micellar transport and sequestration.

The Orwin lab discovered a Variovorax paradoxus strain capable of swarming motility that may produce a biosurfactant within its wetting agent. Using specially designed extraction and purification techniques, the wetting agent was assayed for surface activities and any biosurfactant present was characterized. The results strongly suggest that V. paradoxus EPS produces a lipopeptide biosurfactant that can lower surface tensions to 30 ± 0.5 mN/m at a CMC of 215 mg L-1. The biosurfactant has a molar mass of 679.3 m/z and is currently being structurally characterized to determine if it's a novel surface active agent. The data seems to suggest that variowettin is related to the serrawettin class of lipopeptide wetting agents produced by members of the Serratia genera. This dissertation, therefore, identifies environmental isolates with biosurfactant-producing capabilities that have a range of critical functions to enhancing bacterial and environmental processes.

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