Microbes mend oil spills? Investigating how nitrogen impacts oil degradation

Background/Question/Methods

The Deepwater Horizon oil spill prompted research on bioremediation processes to determine the best methods to handle future oil spills.  Nitrogen availability is important for micro-organism metabolism.  An increase of nitrogen concentrations can change microbial carbon processing and affect breakdown of carbon compounds in oil.  The goal of our research was to determine how nitrogen availability alters microbial degradation of oil in salt marsh ecosystems.  Because managers can control nitrogen availability in the Gulf Region marshes, these practices may accelerate oil degradation.

We performed incubations of weathered oil from the Deepwater Horizon spill, varying both nitrogen and oxygen availability. We measured carbon dioxide (CO₂) efflux, ¹³C:¹²C ratio of respired CO_2, and oil biomarker abundance using a gas chromatograph mass spectrometer.   In a greenhouse experiment, we planted mesocosms with Spartina alterniflora and Spartina patens, Gulf Coast marsh grasses, in order to assess oil degradation in the presence of plants.  Weathered oil obtained from the BP Deepwater Horizon spill was added to half of the mesocosms and one of three nitrogen concentrations (control, low, and high) was added to each mesoscosm. By measuring carbon isotopic concentrations, we could distinguish CO₂originating from microbial oil degradation from that resulting from plant root respiration.

Results/Conclusions

Increased nitrogen concentrations increased microbial respiration rates in the oil incubations.  A concentration of 4 mg NH₄NO₃/L H₂O (low amount of nitrogen) and a concentration of 40 mg NH₄NO₃/L H₂O (high amount of nitrogen) increased microbial processing by 38.74 percent and 169.85 percent respectively under aerobic conditions.  Under anaerobic conditions, low nitrogen and high nitrogen concentrations increased microbial processing by 65.42 percent and 331.94 percent respectively. 

Using greenhouse mesocosms, we determined that the effects of nitrogen addition on oil degradation depend on plant species.  Using stable isotopic partitioning of CO₂, we found oil degradation in S. patens mesocosms increases with nitrogen concentration.  However, at high nitrogen concentrations, oil in S. alterniflora mesocosms degrades slower than at ambient levels of nitrogen, likely due to reduced root growth and thus decreased soil oxygenation. The recent Deepwater Horizon oil spill has drawn attention to the fact that very little research has been conducted examining the use of plants (in addition to microbes) in breaking down crude oil.  We hope that the conclusions of our research will affect how society manages and responds to future oil spills.