Microbial oil-degradation under mild hydrostatic pressure (10 MPa): which pathways are impacted in piezosensitive hydrocarbonoclastic bacteria?

Abstract

Oil spills represent an overwhelming carbon input to the marine environment that immediately impacts the sea surface ecosystem. Microbial communities degrading the oil fraction that eventually sinks to the seafloor must also deal with hydrostatic pressure, which linearly increases with depth. Piezosensitive hydrocarbonoclastic bacteria are ideal candidates to elucidate impaired pathways following oil spills at low depth. In the present paper, we tested two strains of the ubiquitous Alcanivorax genus, namely A. jadensis KS_339 and A. dieselolei KS_293, which is known to rapidly grow after oil spills. Strains were subjected to atmospheric and mild pressure (0.1, 5 and 10 MPa, corresponding to a depth of 0, 500 and 1000 m, respectively) providing n-dodecane as sole carbon source. Pressures equal to 5 and 10 MPa significantly lowered growth yields of both strains. However, in strain KS_293 grown at 10 MPa CO2 production per cell was not affected, cell integrity was preserved and PO4(3-) uptake increased. Analysis of its transcriptome revealed that 95% of its genes were downregulated. Increased transcription involved protein synthesis, energy generation and respiration pathways. Interplay between these factors may play a key role in shaping the structure of microbial communities developed after oil spills at low depth and limit their bioremediation potential.

Figure 1. Growth of A. jadensis KS_339 (purple) and A. dieselolei KS_293 (green) under atmospheric (0.1 MPa) and mild pressure (5 and 10 MPa).

Bars indicate 95% confidence intervals. A: Net optical density increase; asterisks indicate that values at 10 MPa were significantly lower (P < 0.05) than those observed at 0.1 and 5 MPa. B: Final cell number; asterisks indicate that values at 0.1 MPa were significantly higher (P < 0.05) than those observed at 5 and 10 MPa.

Figure 2. Decrease in pH value after incubation of A. jadensis KS_339 (purple) and A. dieselolei KS_293 (green) under atmospheric (0.1 MPa) and mild pressure (5 and 10 MPa).

Bars indicate 95% confidence intervals. Black crosses represent sterile controls.

Figure 3. Physiological response in A. jadensis KS_339 (purple) and A. dieselolei KS_293 (green) under atmospheric (0.1 MPa) and mild pressure (5 and 10 MPa).

Bars indicate 95% confidence intervals. All values refer to the final cell number and amounts detected at the end of the incubation. (A): CO₂ production per cell; (B): O₂ respiration per cell; (C): PO₄³⁻ uptake per cell.

Figure 4. Cell integrity in A. jadensis KS_339 (purple) and A. dieselolei KS_293 (green) under atmospheric (0.1 MPa) and mild pressure (5 and 10 MPa) expressed as percentage of intact cells over total number of cells.

Bars indicate 95% confidence intervals. Asterisk indicates that A. dieselolei KS_293 cells were always significantly higher (P < 0.05) than A. jadensis KS_339 cells at any tested pressure.

Figure 5. Effect of 4-day incubation at 10 MPa on the gene expression of A. dieselolei KS_293 cells with respect to 0.1 MPa.

Figure 6. Upregulated COG in A. dieselolei KS_293 cells incubated at 10 MPa with respect to expression levels at 0.1 MPa.

Percentage of expression is normalized on the total number of upregulated gene clusters, as shown in Fig. 5, green bar.