Did Microbes Eat the Methane?

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2nd June 2011.

As if there was not already enough controversy surrounding the April 2010 Deepwater Horizon disaster in the Gulf of Mexico, a vigorous debate is now underway in the pages of Science magazine concerning the fate of methane released in the wake of the fatal drilling rig explosion.

That the explosion discharged a vast volume of hydrocarbons into 1480 m deep water over a period of 84 days is not in doubt. The oil spill, the most visible effect, garnered the lion’s share of the publicity. However, methane researchers quickly grasped the significance of the fact that the explosion on the rig was caused by the high volumes of gas entrained with the oil, and recognised the opportunity for a unique methane experiment. In the months immediately following the initial blast two research ships were mobilised with the principle goal of investigating the amount of methane released and its fate. This work also aimed, more broadly, to contribute to understanding of the fate of methane from all sea floor sources; in particular, the extent to which microbial activity in the overlying water column can intercept and consume the methane, preventing it from reaching the atmosphere and contributing to the greenhouse effect.

First on the scene was the R/V Walton Smith, which, between 25 May – 6 June 2010 measured water column hydrographic and optical signatures at 70 stations within 5 km of the leaking wellhead. The results (Joye et al., 2011a) revealed discrete layers of dissolved gaseous hydrocarbons between 1000 and 1300 m depth, in which concentrations were up to 75,000 times greater than background levels. The strongest oxygen depletion was observed in the waters with lowest hydrocarbon concentration, potentially indicating that microbial hydrocarbon oxidation had already occurred. The research team calculated that if all the available excess methane was oxidised, multiple small-scale anoxic zones could result.

Later in the summer, between 18 August – 4 October 2010, when the hydrocarbon plume had spread out and mixed with the ocean waters, three research expeditions were conducted on the NOAA ship Pisces (Kessler et al., 2011b). The scientists tracked the submerged hydrocarbon plume 500 km southwestward from the wellhead by using fluorescence and oxygen anomalies. They measured the depth distribution of dissolved methane and oxygen at 207 stations and analysed methane oxidation rates and microbial community structure at 7 stations. The key finding was that most of the methane seen by the earlier researchers had disappeared. i.e. the concentrations measured were no greater than ambient. A zone of anomalously low dissolved oxygen, at 1000 – 1200 m below sea level, suggested that a ‘bloom’ of methane consuming microbes had already consumed all the methane. This hypothesis was supported by significant changes observed in the microbial community structure, including a high relative abundance of methylotrophs associated within the low dissolved oxygen zone. One dimensional modelling of the data led to the conclusion that there was “no apparent limitation to the methanotrophic response to a methane intrusion of this magnitude”. The authors suggest that the highly effective ‘methane biofilter’ implied by their findings could have limited the amount of methane from methane hydrates or other natural sources reaching the atmosphere in the past, and may do so again in the future.

However, any notion that the case was now settled in favour of the ‘microbes ate all the methane’ hypothesis was dispelled last week, when Samantha Joye and colleagues publishing a robustly worded comment contesting almost all of the Kessler teams findings, in particular, stating that they considered “the evidence linking the observed oxygen anomalies to methane consumption and extension of these findings to hydrate-derived methane climate forcing premature” (Joye et al., 2011b). John Kessler and colleagues responded to the criticism with equal force, expanding on some of their evidence and citing support for their claims from more recently published work (Kessler et al., 2011b).

One looks to future studies to resolve some of these hotly contested issues, and, importantly, to tell us what did happen to all the methane, if it turns out not to have provided a feast for the Gulf of Mexico methanotrophs.

References

Joye, S. B., MacDonald, I. R., Leifer, I., and Asper, V. (2011a). Magnitude and oxidation potential of hydrocarbon gases released from the BP oil well blowout. Nature Geoscience, 4(3), 160-164.

Kessler, J. D., Valentine, D. L., Redmond, M. C., Du, M., Chan, E. W., Mendes, S. D., Quiroz, E.W., Villanueva, C.J., Shusta, S.S., Werra, L.M., Yvon-Lewis, S.A. and Weber, T.C. (2011a). A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico. Science. 331, 312-315.

Joye, S.B., Leifer, I., MacDonald, I.R., Chanton, J.P., Meile, C.D., Teske, A.P., Kostka, J.E., Chistoserdova, L., Coffin, R., Hollander, D., Kastner, M., Montoya, J.P., Rehder, G., Solomon, E., Treude, T. and Villareal, T.A. (2011b). Comment on “A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico”. Science, 332, (1033).

Kessler, J. D., Valentine, D. L., Redmond, M. C., Du, M. (2011b) Response to comment on “A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico”. Science. 332 (1033)

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