How giant tubeworms survive on deep-sea hydrothermal vents

In the depths of the East Pacific Rise, a unique environment of extreme temperatures, intense pressures, and toxic carbon compounds, thrives Riftia pachyptila, a giant hydrothermal vent tubeworm.

Growing up to 6 feet tall with a striking deep-red plume, Riftia tubeworm lacks a digestive system and relies on a symbiotic relationship with bacteria within its body.

These bacteria convert carbon dioxide into sugars, sustaining both themselves and the tubeworm.

Carbon fixation in Riftia tubeworms

Unlike most autotrophic organisms that use a single carbon fixation pathway, Riftia pachyptila’s bacterial symbionts utilize two: the Calvin-Benson-Bassham (CBB) cycle and the reductive tricarboxylic acid (rTCA) cycle.

Until recently, scientists had limited understanding of how these pathways function and integrate with other metabolic processes.

New research from Harvard University’s Department of Organismic and Evolutionary Biology has uncovered how these two pathways are coordinated, revealing a sophisticated adaptation that allows Riftia’s symbionts to thrive.

The study, published in Nature Microbiology, involved collecting tubeworms from the East Pacific Rise and mimicking their natural environment in the lab, including pressures of 3,000 PSI and near-toxic sulfur levels.

“This paper is really a tour de force of going from studying living organisms and measuring their metabolic rates and allying them directly to transcripts in a way that allowed the research team to show that the pathways are very likely being run in parallel,” said senior co-author, professor of Organismic and Evolutionary Biology, Peter Girguis.

“The paper shows that the dual pathways are biased by the environmental conditions, and that there are other metabolic systems in orbit around each of these two.”

Deep-sea carbon fixation mechanics

Carbon fixation, the process of converting carbon dioxide to sugars, is essential for sustaining life.

While photosynthetic organisms like plants use sunlight for this process, Riftia pachyptila’s chemoautotrophic symbionts use energy from hydrogen sulfide.

By varying the experimental conditions for Riftia, the research team identified how environmental changes influence the coordination of the two carbon pathways.

“This is the most in-depth analysis of bacteria that has two carbon fixation pathways, the rTCA and CBB,” said lead author and postdoctoral scholar Jessica Mitchell.

“This is also the first network analysis done on a hydrothermal vent symbiosis and the first network analysis done on a dual carbon fixation pathway system.”

Tubeworms metabolic insights

Network analysis allowed the team to detect patterns in gene expression data, providing a comprehensive view of the system.

They identified metabolic hub genes crucial for maintaining and regulating the complex network of metabolic reactions within cells.

Distinct roles in metabolic function

The study found significant differences in the transcriptional patterns of the rTCA and CBB cycles in response to varying geochemical conditions.

The rTCA cycle is linked with hydrogenases and dissimilatory nitrate reduction, crucial for processing hydrogen and nitrates without oxygen, indicating its key role under low-energy conditions.

In contrast, the CBB cycle is associated with sulfide oxidation and assimilatory nitrate reduction, vital processes in the sulfide-rich environment of hydrothermal vents.

This linkage allows the symbionts to utilize available chemical energy to fix carbon effectively.

Complementary carbon pathways

One of the study’s most intriguing findings is the complementary nature of the two pathways in the tubeworm.

The rTCA cycle is particularly important under conditions where sulfide and oxygen are limited, highlighted by the role of a Group 1e-hydrogenase.

This flexibility gives the tubeworms a significant advantage, enabling them to thrive in the variable conditions of hydrothermal vents.

The high carbon fixation rates measured during the study support Riftia pachyptila’s rapid growth and survival.

The dual carbon fixation pathways, each optimized for different environmental conditions, help maintain metabolic stability during environmental shifts.

Understanding tubeworms and carbon capture

This research opens new avenues for understanding biological carbon capture and basic biochemistry.

The findings could have practical applications in biotechnology, where these pathways might be harnessed to develop more efficient systems for carbon fixation.

“This study really paves the way for future studies, and understanding how these dual pathways are enabling this organism to fix this amount of carbon,” Mitchell said.

By uncovering the mechanisms behind Riftia pachyptila‘s extraordinary ability to thrive in such a hostile environment, this research deepens our understanding of deep-sea ecosystems and has the potential to inform advancements in sustainable technologies and our knowledge of life’s adaptability.

The full study was published in the journal Nature Microbiology.

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