Dynamics of microbial-induced oil degradation at the microscale

Abstract

Microbial-induced oil degradation (MIOD) has a wide range of applications, such as microbial enhanced oil recovery and bioremediation of oil pollution. However, our understanding of MIOD is still far from complete. Particularly, how is the dynamics of degradation process at the microscale level with a single-cell resolution remains to be disclosed. In this work, using hexadecane droplets in water as a model system, we have studied the dynamics of hexadecane degradation by different strains, including Pseudomonas aeruginosa PAO1, IMP68, O-2-2, and Dietzia sp. DQ12-45-1b, at the microscale. Based on visualization of MIOD, the dynamics of MIOD can be characterized by a three-stage process, including adhesion, adaptation, and degradation stages. Although different strains showed similar three-stage dynamics of MIOD, the effective degradation rate varied and followed an order of PAO1 > O-2-2 > IMP68 > DQ12-45-1b under aerobic conditions. Different oxygen conditions were also tested, and the dynamics of MIOD was slowed down under anaerobic conditions in comparison to under aerobic conditions. Further investigations at the degradation stage revealed that biofilms formed at the oil-water interface enhanced oil degradation, but a prerequisite for such enhanced degradation is proper stimulation of biofilm cells in the course of biofilm formation. The findings in this work provided a detailed picture on the dynamics of MIOD at the microscale and would be beneficial for better applications of MIOD.IMPORTANCEMicrobial-induced oil degradation is environmental friendly and economic and has become a promising technique in the fields of enhanced oil recovery and remediation of crude oil-polluted environments. For better applications of microbial-induced oil degradation, understanding the degradation dynamics particularly at the microscale is crucial. In this study, we investigated the degradation dynamics of hexadecane oil droplets incubated with different strains, including Pseudomonas aeruginosa PAO1, O-2-2, IMP68, and Dietzia sp. DQ12-45-1b at the microscale by employing microdroplet-based methods and bacterial tracking techniques. The findings in this study provided a detailed picture on the dynamics of microbial-induced oil degradation at the microscale, which will deepen our understandings on the biodegradation mechanisms of alkanes and shed insights for developing more effective biodegradation techniques.

Keywords: Dietzia sp.; Pseudomonas aeruginosa; biofilm; bioremediation; oil-water interface.

Fig 1

Characterization of MIOD at the microscale for different strains. (A) Snapshots taken at different time points during the degradation process of a hexadecane droplet by P. aeruginosa O-2-2. (B) The projected area of all cells that were associated with the oil-water interface and the diameter of the oil droplet measured at different time points for P. aeruginosa O-2-2. (C) The cell length measured at different time points for P. aeruginosa O-2-2. (D) Changes in the projected area of all cells associated with the oil-water interface over time, for PAO1, IMP68, and DQ12-45-1b. Different stages of bacterial degradation of alkanes were indicated by dashed lines. (E) Snapshots taken at different time points during the degradation process of a hexadecane droplet for P. aeruginosa PAO1 (top row), IMP68 (middle row), and Dietzia sp. DQ12-45-1b (bottom row). Statistical significances were measured using a one-way analysis of variance set for multiple comparisons with a Dunnett’s post test. *** P is very highly significant at P < 0.001. The analysis of statistical significance was performed between 0 days and other time points. Error bars represent the standard deviations of the mean of three oil droplets. Scale bar, 10 µm.

Fig 2

(A) BATH measurement and (B) bacterial adhesion densities on hexadecane oil droplets for different strains; (C) effective degradation rate. The analysis of statistical significance was performed between PAO1 and other strains using a one-way ANOVA set for multiple comparisons with a Dunnett’s post test. *P < 0.05 and ***P < 0.001. Error bars represent the standard deviations of the mean of three oil droplets.

Fig 3

Oil degradation tests of stimulated planktonic cells and cell aggregates/biofilms. (A) Degradation of hexadecane at three time points by planktonic cells and cell aggregates/biofilms. The oil droplet indicated by the green circle was degraded by cell aggregates/biofilms, while the one indicated by the orange circle was degraded by planktonic cells. (B) Degradation of hexadecane at three time points by cell aggregates/biofilms formed with glucose as the sole carbon source. The oil droplet was indicated by an orange circle. Scale bars, 5 µm.