About 15 years ago, researchers discovered asphalt volcanoes in the Gulf of Mexico. The discovery presented ocean scientists with an entirely new playground to explore and led to a fascinating finding: sea life that is able to thrive among the toxic oil and tar-spewing volcanoes by forming a symbiotic relationship with oil-degrading bacteria.
In recent work published in the journal Nature Microbiology and featured in The Atlantic, Dr. Nicole Dubilier from the Max Planck Institute for Marine Microbiology in Bremen, Germany, along with her colleague Dr. Maxim Rubin-Blum and an international research group, focused on one group of oil-degrading bacteria belonging to the genus Cycloclasticus (which means “ringbreaker”). Marine researchers have long known about free-living bacteria that degrade oil in other environments (for example, free-living Cycloclasticus bloomed in large numbers in the Gulf of Mexico after the Deepwater Horizon oil catastrophe), but this is the first deep dive into understanding the oil-eating bacteria that live in symbiosis with animals.
Forming partnerships with microbes
Many animals flourish because of their mutually beneficial relationship with microbes. In harsh environments, such as the asphalt volcanoes, the animals provide the bacteria a more hospitable environment for shelter and in turn, the bacteria provide the animals with nutrients through a process called chemosynthesis, the conversion of chemicals to food.
Dubilier and colleagues sequenced Cycloclasticus genomes to compare and contrast the metabolic strategies that may allow these bacteria to grow in such extreme environments and to describe in more detail the symbiosis between Cycloclasticus and its animal hosts. The team found that Cycloclasticus deploy multiple approaches for growing on oil and gas. And, for the first time, determined that both forms of Cycloclasticus (free-living and symbiotic) focus on easily degradable compounds: short-chain alkanes (such as butane, ethane and propane) found in natural gases released from the vents. The free-living Cycloclasticus also have the ability to break down hard-to-crack ring structures in the oil called polycyclic aromatic hydrocarbons or “PAHs.” These compounds are highly toxic for most organisms and degrading them is an arduous process that costs a lot of energy. In contrast, the symbiotic Cycloclasticus have – unexpectedly – lost the ability to degrade PAHs and instead gain all their energy from the natural gases. “That was surprising as until now it was thought that Cycloclasticus could only live from PAHs,” explains Dubilier.
Short-chain alkanes are mainly found in the early stages of an oil spill and are quickly used up by free-living microbes, whereas the PAHs remain for a longer amount of time. The ability of some free-living Cycloclasticus to grow on both nutrient sources (PAHs and short-chain alkanes) in the water column may be a strategy for long-term persistence in the environment. “This allows them to remain flexible. When the short-chain morsels are gone, they can still degrade the considerably tougher PAHs,” says Dubilier. For the symbiotic Cycloclasticus, there may be less of a need to degrade PAHs; their mussel and sponge hosts concentrate the short-chain alkanes through filter feeding, providing the bacteria with a reliable source of nutrition.
“This discovery hints at strategies for how free-living bacteria and symbiotic bacteria living with an animal host have evolved to make the most out of their environment and the resources available to them,” says Marine Microbiology Initiative program officer, Dr. Sara Bender.
The Moore Foundation’s Marine Microbiology Initiative has supported MMI Investigator Dr. Dubilier in her research efforts since 2013. Her work has focused on understanding symbioses between marine animals and chemosynthetic bacteria, and her pioneering research resonates with the initiative’s 2019 goal of deepening the community’s understanding of microbial interactions and nutrient flow in the ocean.
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