In the depths of the East Pacific Rise, where sunlight fails to penetrate and extreme conditions prevail, the giant hydrothermal vent tubeworm, Riftia pachyptila, thrives. Growing up to an impressive 6 feet tall, Riftia lacks a digestive system and instead relies on a symbiotic relationship with billions of bacteria residing within its body. These chemoautotrophic endosymbionts fix carbon dioxide to sugars, sustaining both themselves and their tubeworm host.
A groundbreaking study, published in Nature Microbiology by a team of scientists from Harvard University's Department of Organismic and Evolutionary Biology, has unraveled the mysteries surrounding the carbon fixation pathways employed by Riftia's endosymbionts. Unlike most autotrophic organisms that rely on a single carbon fixation pathway, Riftia's symbionts possess two functional pathways: the Calvin-Benson–Bassham and the reductive tricarboxylic acid cycles. The research team, led by postdoctoral scholar Jessica Mitchell and senior co-author Professor Peter Girguis, collected tubeworms from the East Pacific Rise and incubated them under conditions mimicking their natural environment, including extreme pressures and near-toxic levels of sulfur, to measure net carbon fixation rates and examine transcriptional and metabolic responses.
The study revealed that the transcriptional patterns of the rTCA and CBB cycles varied significantly in response to different geochemical regimes, with each pathway allied to specific metabolic processes. The rTCA cycle was found to be linked with hydrogenases and dissimilatory nitrate reduction, suggesting its key role under lower-energy conditions. In contrast, the CBB cycle was associated with sulfide oxidation and assimilatory nitrate reduction, enabling the symbionts to effectively utilize the abundant chemical energy available in their environment to fix carbon.
One of the most intriguing findings was the complementary nature of these two pathways. The rTCA cycle appears to be particularly important under conditions where sulfide and oxygen are limited, as highlighted by the identification of a Group 1e-hydrogenase that works in tandem with the rTCA cycle to respond to such limitations. This flexibility confers a significant advantage, enabling the tubeworms to thrive in the highly variable conditions of hydrothermal vents.
The net carbon fixation rates measured during the study were exceptionally high, enabling the rapid growth and survival of Riftia pachyptila in their environment. The dual pathways of carbon fixation, each optimized for different environmental conditions, may allow the symbionts to maintain metabolic stability during environmental shifts.
The analysis of these dual carbon fixation pathways and their coordinated regulation in Riftia opens new avenues for research in biological carbon capture and basic biochemistry. This knowledge could have practical applications in biotechnology, where the principles of these pathways might be harnessed to develop more efficient systems for carbon fixation. Moreover, understanding how these pathways are regulated could provide insights into the evolution of metabolic diversity and adaptation in extreme environments.
As lead author Jessica Mitchell states, "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." The findings of this research not only shed light on the remarkable adaptations of Riftia pachyptila and its endosymbionts but also have far-reaching implications for our understanding of life in extreme environments and the potential for harnessing these processes for various applications.
