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Nat Microbiol. 2017 Jun 19;2:17093. doi: 10.1038/nmicrobiol.2017.93.

Short-chain alkanes fuel mussel and sponge Cycloclasticus symbionts from deep-sea gas and oil seeps.

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Max-Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany.
MARUM, Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany.
Department of Geoscience, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada.
Department of Biological Sciences, University of North Carolina at Charlotte, 9201 University City Blvd, Charlotte, North Carolina 28223, USA.
Department of Earth Science, University of California at Santa Barbara, 1006 Webb Hall, Santa Barbara, California 93106, USA.


Cycloclasticus bacteria are ubiquitous in oil-rich regions of the ocean and are known for their ability to degrade polycyclic aromatic hydrocarbons (PAHs). In this study, we describe Cycloclasticus that have established a symbiosis with Bathymodiolus heckerae mussels and poecilosclerid sponges from asphalt-rich, deep-sea oil seeps at Campeche Knolls in the southern Gulf of Mexico. Genomic and transcriptomic analyses revealed that, in contrast to all previously known Cycloclasticus, the symbiotic Cycloclasticus appears to lack the genes needed for PAH degradation. Instead, these symbionts use propane and other short-chain alkanes such as ethane and butane as carbon and energy sources, thus expanding the limited range of substrates known to power chemosynthetic symbioses. Analyses of short-chain alkanes in the environment of the Campeche Knolls symbioses revealed that these are present at high concentrations (in the μM to mM range). Comparative genomic analyses revealed high similarities between the genes used by the symbiotic Cycloclasticus to degrade short-chain alkanes and those of free-living Cycloclasticus that bloomed during the Deepwater Horizon oil spill. Our results indicate that the metabolic versatility of bacteria within the Cycloclasticus clade is higher than previously assumed, and highlight the expanded role of these keystone species in the degradation of marine hydrocarbons.

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