Degradation of Louisiana sweet crude oil by bacterial communities from 2010

In both analysed sediments (No. 4 from 2010 and No. 8 from 2013), which were compared in this experiment, aerobic microbial activity was observed. Only when deep-sea sediments were added as bacterial source, the oxygen concentrationdecreased and the carbon dioxide concentration and cell density increased, whereas in blank experiments without sediments the values remained constant. This proved that there was no biodegradation of oil without bacteria.

Certain differences were observed between the incubations of the two sediments. On the one hand, the date of sediment collection made a difference. The incubation of the

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sediment, sampled three years after the DWH accident in 2013, showed a longer lag-phase in the oxygen consumption curve than the sediment collected in 2010 at the same site. In conclusion, possibly three years after the spill less and/or less active hydrocarbon-degraders were present in the analysed surface sediment, which needed longer to adapt to the crude oil. A similar result was found in the experiment described in Chapter 3.6.1. Right after the DWH oil spill, the excess supply of crude oil caused a bloom of adapted oil-degrading bacteria, which were able to degrade the hydrocarbons at fast rates (Bælum et al. 2012, Hazen et al. 2010, Kessler et al. 2011, Redmond and Valentine 2012, Valentine et al. 2010 and 2012). The rapid oil degradation by the bacterial community in sediment No. 4, sampled in 2010, confirmed this assumption. Thus, it can be assumed that when the availability of oil decreased, less of these microorganisms were present in the sediment. Moreover, possibly they needed longer to adapt to the oil and thus the bacterial community in the 2013 sediment No. 8 showed a longer lag-phase in the oxygen consumption curve.

Furthermore, different pressure conditions made a difference. In the incubation with the 2010 sediment No. 4, oxygen was consumed faster at 150 bar than at 1 bar, but this difference was only very small. The carbon dioxide production rates matched with this result. In the incubation with sediment No. 8 from 2013 it was vice versa. At 144 bar, bacteria needed longer to start oxygen consumption and consumed oxygen at a lower rate than at 1 bar. For the 2013 as well as for the 2010 sediment incubations, more biomass was produced in the 1 bar incubations than in the 150 bar incubations. Only for the 2013 sediment incubation, tendencies in cell counts agree with the tendencies in oxygen consumption at different pressure conditions.

Only the results from the incubation of sediment No. 8 from 2013 are in accordance to the results of Schwarz et al. (1974, 1975), who found that growth and hydrocarbon degradation of a deep-sea bacterial community was clearly retarded at 506.6 bar in contrast to 1 bar. In 1971, Jannasch et al. found that microbial degradation of organic matter in the deep sea was greatly restricted compared to ambient pressure degradation. In contrast, Turley and Lochte reported in 1990 that deep-sea bacteria degraded organic carbon from detritus at a faster rate at 456 bar than at 1 bar, but the amount of produced biomass was not significantly different. Similarly, ZoBell and Johnson (1949) found that deep-sea bacterial communities grew on glucose faster at 405 to 608 bar than at 1 bar. Thus, like the results

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from the experiments with the 2010 and 2013 sediments, divergent effects of high pressure can be found in literature.

In preliminary analyses of the community composition and succession of the 2010 and 2013 sediment incubations, high pressure was found to change the bacterial consortium in sediments that degrade oil, so that completely different bacterial populations evolved compared to incubations at ambient pressure. The samples cluster by year of sediment collection, pressure and time of sampling. During incubation the diversity declined. In conclusion, at the phylum level, Proteobacteria and Firmicutes dominated at the end of 1 bar incubations and Proteobacteria dominated at the end of 150 bar incubations. Likewise, Kimes et al. (2013), who analysed the metagenome of sediments from the GoM following the DHW spill, found that Proteobacteria and Firmicutes were dominating. Similarly, others reported that bacterial communities, dominating in the deep-water oil plume, belonged to the phylum of Gammaproteobacteria (Hazen et al. 2010, Mason et al. 2012, Redmond and Valentine 2012, Valentine et al. 2010). Members of the Alpha-, Beta-, Gamma- and Deltaproteobacteria and Firmicutes are known to play a major role in aerobic hydrocarbon conversion (Head et al. 2006, Kim and Kwon 2010, Prince et al. 2010). To get more knowledge on how high pressure affects the composition of bacterial communities, further analyses are needed.

Possibly, in addition to the effects of pressure also the inherent heterogeneity of the sediments, which were used for inoculation, played a certain role in the observed differences of the O2 and CO2 curves in the incubations at the two pressure conditions.

Bacteria are not distributed evenly in sediments and grow at different rates. Similarly, Lowit et al. 2000 stated that the reproducibility of an experiment is depending on the variability of the measurement tool as well as on the inhomogeneous distribution of bacterial community members among collected environmental samples.

In summary, for the analysed bacterial communities in sediments, divergent effects of high pressure on their growth and degradation behaviour were found. Also, the date of sampling of the sediments made a difference in bacterial activity. However, from the results it is very clear that high pressure changed the crude-oil degrading bacterial consortium in sediments and that diversity declined during the incubation.

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4.5.3 Degradation of natural gas and crude oil by bacterial communities from a 2010