Fuel for Bacteria

During the last two decades, alternative hydrocarbon fuels have gained popularity. These alternative fuels are produced by biological processes such as agriculture and anaerobic digestion. Not to be confused with biodiesel and ethanol biofuels, these alternative fuels are similar to conventional hydrocarbon fuels and can be used in regular vehicles. Despite being more environmentally friendly in production, alternative hydrocarbon fuels can also have significant impacts on the environment. In their paper, Ruiz et al. (2015) investigated how differences in hydrocarbon composition affected bacterial communities. 

Coastal seawater containing sediments was collected from Florida. Samples were set up in tanks and monitored over 90 days. Petroleum types were chosen based on their prevalence in civilian and military transport. The impacts of conventional fuels, F-76 (military petroleum marine diesel) and JP-5 (petroleum jet propellant) as well as camelina-derived renewable jet propellant (HRJ-5) and a 50/50 mixture of JP-5 and HRJ-5 (Blend) were studied. Bacterial isolates were cultured and bacterial growth was monitored using quantitative Real-Time PCR. Individual fuel degradation profiles were analysed by gas chromatography-mass spectrometry (GC-MS). 

Analysis of the unexposed seawater samples showed a diverse and mainly uncharacterized bacterial community. The diversity was higher in the sediment fractions, and sequencing coverage was not sufficient to detect all bacteria present. Proteobacteria were the most abundant group in both liquid and sediment, with Bacteroidetes and an unclassified group coming in second and third. Firmicutes and Lentisphaerae were also detected. Within each group, most of the bacteria were unclassified at the genus level.

Exposure to fuels decreased the biodiversity, however, the impact of the diesel F-76 was less severe than that of the jet fuels. Proteobacteria abundance was enhanced by all fuels, while Bacteroidetes decreased with fuel presence. Exposure to JP-5 and Blend promoted Firmicutes and Lentisphaerae, respectively. In Proteobacteria, JP-5 increased the abundance of the Marinobacter and Dusulfovibrio genera. HRJ enhanced Hyphomonas. F-76 promoted the growth of Rhodovulum and unclassified genera, while the Blend mainly promoted unclassified groups. The differences in communities were presumably connected to compositional differences in the fuels. 

Subsequently, the hydrocarbon-degradation profiles of two bacterial strains were examined. GC-MS analysis showed Marinobacter hydrocarbonoclasticus N19 mainly degraded short chained n-alkanes and specific aromatics. In contrast, Rhodovulum sp. NI22 primarily degraded naphthalene, light branched and n-alkanes as well as aromatics. These differences were attributed to different degradation enzymes such as alkane monooxygenase and naphthalene dioxygenase in M. hydrocarbonoclasticus and Rhodovulum sp., respectively. 

Metagenomic analysis showed that different fuels increased the growth and abundance of specific bacterial groups. In a separate experiment, M. hydrocarbonoclasticus and Rhodovulum sp. were cultured together with Halobacillus sp. and exposed to different fuels. After ten days, Marinobacter dominated all samples. HRJ supported the highest total abundance of bacteria and enhanced Marinobacter as well as Rhodovulum. Presumably, the abundance of light alkanes in HRJ promoted the growth of these bacteria. JP-5 seemed to impact all three groups negatively. Along with metabolic flexibility, competition between hydrocarbon degraders may influence bioremediation. Once the dominant bacteria have degraded their compounds, out-competed bacteria might grow back by consuming different hydrocarbon compounds. The authors suggest that bioremediation might also benefit from the systemic addition of specific bacteria once some compounds have been degraded.

In conclusion, this paper provides useful information for the development of new fuels and to maximise bioremediation. The main finding that different fuels enhance growth and abundance of specific bacteria is perhaps not very surprising. However, the experimental design could only show culturable bacteria of which most were unclassified. Confusingly, different fuels were used in different experiments without giving any justification. In my review, I consciously chose to refer only to the four fuels used in the first experiment.

Reference:

Ruiz, O. N., Brown, L. M., Striebich, R. C., Smart, C. E., Bowen, L. L., Lee, J. S., ... & Gunasekera, T. S. (2015). Effect of Conventional and Alternative Fuels on a Marine Bacterial Community and the Significance to Bioremediation. Energy & Fuels, 30(1), 434-444. http://pubs.acs.org/doi/abs/10.1021/acs.energyfuels.5b02439