Rhamnolipid Adsorption in Soil: Factors, Unique Features, and Considerations for Use as Green Antizoosporic Agents

Abstract

In aqueous solutions, rhamnolipids effectively kill the motile zoospores responsible for spreading many pathogens, including soy-infecting Phytophthora sojae. For use in soil, adsorption properties need to be considered. Having low critical micelle concentrations, rhamnolipids tend to form micelles/aggregates with unknown effects on soil adsorption. Effects of soil pH, rhamnolipid congener structure, and concentration were examined. Congeners were identified and each quantitated for adsorptive partitioning. The adsorption isotherm at pH 6.5 showed a multi-stage profile plateauing at 1700 μg/g of soil. Less hydrophilic congeners adsorbed preferentially: R-C10-C12 > R-C10-C12:1 > RR-C10-C12:1 > RR-C10-C12 > R-C10-C10 > RR-C10-C10 > R-C8-C10 > RR-C8-C10 (where R is rhamnose and C# is the carbon number of β-hydroxy fatty acid). Adsorptive selectivity among congeners was very clear in dilute solutions but diminished with increasing concentrations. Results were interpreted with aggregate formation in solutions and on the soil surface. The cost estimate made accordingly supported the economic feasibility of rhamnolipid antizoosporic uses in soil.

Biolytic Effect of Rhamnolipid Biosurfactant and Dodecyl Sulfate Against Phagotrophic Alga Ochromonas danica

“…A similar phenomenon of preferred rhamnolipid partitioning to a nonaqueous phase at concentrations above CMC was reported in a recent study of rhamnolipid adsorption in soil [26]. In the soil-rhamnolipid adsorption study, it was reported that at pH 6.5, the adsorption isotherm was a profile with 4 stages; adsorption coefficient increased rapidly with increasing aqueous-phase concentrations in 2 of the stages but slowly in the other 2 stages [26]. One of the rapid increase stages took place at the aqueous-phase concentration of 190-250 mg/L.…”

“…At these concentrations, rhamnolipid solubility as individual molecules has been saturated and a large number of micelles have formed; accommodating more rhamnolipid molecules in the aqueous phase is less favorable and partitioning to cells, such as in the lipid bilayer of cell membrane, may become more favorable. A similar phenomenon of preferred rhamnolipid partitioning to a nonaqueous phase at concentrations above CMC was reported in a recent study of rhamnolipid adsorption in soil [26]. In the soil-rhamnolipid adsorption study, it was reported that at pH 6.5, the adsorption isotherm was a profile with 4 stages; adsorption coefficient increased rapidly with increasing aqueous-phase concentrations in 2 of the stages but slowly in the other 2 stages [26].…”

“…The bacterium Pseudomonas aeruginosa E03-40 used in this study was originally isolated from soil samples taken near a biodiesel plant [24]. The detailed protocol for production and purification of rhamnolipid can be found elsewhere [25,26]. The rhamnolipid mixture contained eight major congeners, which were identified using high-pressure liquid chromatography coupled with mass spectrometry (HPLC-MS) and quantitated using HPLC with an evaporative light scattering detector (ELSD) [19].…”

Increased Rhamnolipid Concentration and Productivity Achieved with Advanced Process Design

“…The nitrate and nitrite concentrations were monitored using the HACH Nitrate and Nitrite test strips (Catalog # 2745425, HACH Company, Loveland, CO, USA). Rhamnolipid analysis was done by the standard anthrone method (Chayabutra et al, 2001) and by using the HPLC with an evaporative light scattering detector (ELSD, Model 300, SofTa Corporation, Thornton, CO, USA) (Dashtbozorg et al, 2016). For the anthrone analysis, the measured rhamnose concentrations were multiplied by 2.38 to estimate the rhamnolipid concentrations.…”

Eco‐friendly rhamnolipid based fungicides for protection of soybeans from Phytophthora sojae

“…The fermentation broth produced was processed to isolate and purify rhamnolipid for use in this study. The composition of rhamnolipid used in this study is given in Table , determined by high performance liquid chromatography‐mass spectrometry (HPLC–MS) and HPLC‐evaporative light scattering detection (ELSD) according to the methods described elsewhere . Sodium hydroxide (NaOH) used in the study was obtained from VWR (Radnor, PA).…”