Poster Presentation Australian Marine Sciences Association 2026 Conference

Living in pH purgatory: The impact of symbiosome pH on endosymbiont function. (139239)

Sebastian Bland 1 , Michael Ellwood 1 , Sam Eggins 1
  1. ANU, Acton, ACT, Australia

It has often been suggested that the coral symbiosome, which houses the endosymbiotic dinoflagellates, has a pH lower than that of seawater1. This is proposed as a mechanism to shift the carbonate system towards increased CO2 availability, ensuring a constant carbon source for photosynthesis. In vitro measurements on coral species A. yongei and S. pistillata suggested that these symbiosome could have a pH as low as 42,3. While these authors were confident in these results, the dye utilised has low precision, and the scale of change so large that it would be interesting to investigate this further. By shifting the focus away from the host to the symbionts we aim to illuminate the extent of their pH tolerance. Here isolated dinoflagellate grown in known buffered pH solutions will allow for a robust investigation into the ability of these symbionts to survive at a range of pH conditions, and interrogate the influence of pH on photosynthetic function. These experiments offer new insights on the nature of the symbiosis itself. Breaking from decades of mainstream thought, new research suggests that the symbiosis isn’t as mutualistic as one thought4. Instead, the corals ‘farm’ the symbionts for their photosynthetic products, while limiting nitrogen abundance to ensure compliance5. Could low pH be another way to extend control?

  1. Brading, Patrick, Mark E. Warner, Phillip Davey, David J. Smith, Eric P. Achterberg, and David J. Suggett. 2011. “Differential Effects of Ocean Acidification on Growth and Photosynthesis among Phylotypes of Symbiodinium (Dinophyceae).” Limnology and Oceanography 56 (3): 927–38. https://doi.org/10.4319/lo.2011.56.3.0927.
  2. Barott, Katie L., Alexander A. Venn, Sidney O. Perez, Sylvie Tambutté, and Martin Tresguerres. 2015. “Coral Host Cells Acidify Symbiotic Algal Microenvironment to Promote Photosynthesis.” Proceedings of the National Academy of Sciences 112 (2): 607–12. https://doi.org/10.1073/pnas.1413483112.
  3. Venn, Alexander A., Eric Tambutté, Lucas Crovetto, and Sylvie Tambutté. 2025. “pH Regulation in Coral Photosymbiosis and Calcification: A Compartmental Perspective.” New Phytologist 247 (2): 487–503. https://doi.org/10.1111/nph.70200.
  4. Matz, Mikhail V. 2024. “Not-so-Mutually Beneficial Coral Symbiosis.” Current Biology 34 (17): 798-801. https://doi.org/10.1016/j.cub.2024.07.047
  5. Cui, Guoxin, Jianing Mi, Alessandro Moret, et al. 2023. “A Carbon-Nitrogen Negative Feedback Loop Underlies the Repeated Evolution of Cnidarian–Symbiodiniaceae Symbioses.” Nature Communications 14 (1): 6949. https://doi.org/10.1038/s41467-023-42582-y.