Deep-sea vent tubeworms grow on carbon dioxide

Deep down on the floor of the Pacific Ocean a species of giant tubeworm – Riftia pachyptila – dwells near hydrothermal vents. Despite living in an environment of extreme temperatures, crushing pressures, and zero sunlight these animals thrive and grow as tall as 1.8 meters.

Lacking its own digestive system, Riftia gets its energy through a symbiotic relationship with bacteria that live in its body. The bacteria use energy from hydrogen sulphide spewed out by hydrothermal vents to fix carbon dioxide to sugars through 2 different metabolic processes.

A new study in Nature Microbiology sheds light on how these poorly understood pathways – the Calvin-Benson–Bassham (CBB) and the reductive tricarboxylic acid (rTCA) cycles – are coordinated. It reveals that the dual pathways are likely being run in parallel, allowing the tubeworms to thrive in the highly variable conditions of hydrothermal vents.

Riftia 850
Riftia pachiyptila. Credit: Peter Girguis

Harvard University researchers in the US collected tubeworms from the East Pacific Rise and incubated them under conditions mimicking their natural environment – including 3,000 PSI pressure and near-toxic levels of sulphur – to identify that each pathway is optimised for different environmental conditions.

They found the rTCA cycle is linked with enzymes crucial for processing hydrogen and nitrates in the absence of oxygen, suggesting that the cycle plays a key role under lower-energy conditions. In contrast, the CBB cycle is associated with assimilatory nitrate reduction and sulphide oxidation –  a vital process in chemically rich environments of hydrothermal vents.

Understanding how these pathways are regulated could provide insights into the evolution of metabolic diversity and adaptation in extreme environments. The findings could also have practical applications in biotechnology for developing more efficient systems for carbon fixation.

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