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Proliferation of hydrocarbon-degrading microbes at the bottom of the Mariana Trench.

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PubMed ID: 30975208

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Liu J, Zheng Y, Lin H, Wang X, Li M, Liu Y, Yu M, Zhao M, Pedentchouk N, Lea-Smith DJ, Todd JD, Magill CR, Zhang WJ, Zhou S, Song D, Zhong H, Xin Y, Yu M, Tian J, Zhang XH

Microbiome. 04 2019. doi: 10.1186/s40168-019-0652-3

COMMENT: This study sheds new light in the composition and environmental role of the microbial communities in the deepest ocean zones on earth. The results suggest that the bacterial communities found in this environment have a main role in alkane degradation.

Previous studies have tried to address the different microbial communities in deep waters. However, the deepest and perhaps more extreme environment on earth, the hadal zone, has not been extensively studied, and therefore, its role in the ecosystems is basically unknown.

Researchers have analyzed both the microbiome and the environment at different depths in the Mariana Trench. Results show different bacterial niche boundaries depending on depth, with a particular order (Oceanospirillales) being predominant at the deepest, near bottom waters. In fact, this is the first environment where one particular genus belonging to this order, Oleibacter, has been reported as the most abundant.

But what is so special in the Oceanospirillales order? The results show that the species belonging to this order encode genes involved in alkane degradation. Alkanes are simple hydrocarbon chains of varying length. Metagenomics analysis of the species found in this environment shows genes that encode proteins involved in sensing (chemotaxis) and degradation of alkanes, including medium-chain, long –chain and some cyclic hydrocarbons. In fact, the relative abundance of some genes enconding enzymes for sensing and degrading alkanes increases significantly in the deepest zones. This degradation of alkanes by bacterial isolates has been addressed under physiologically relevant conditions.

A further question remains to be answered in this study: the source of the alkanes that support the bacterial populations found in this environment. Novel alkanes have been identified in hadal sediments, but alkanes derived from heterotrophic sources may be involved too. In any case,

These results reveal that hydrocarbon-degrading microorganisms are present in great abundance in the deepest seawater on Earth and shed a new light on potential biological processes in this extreme environment.

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Vicente Díaz