Many of Science's greatest and most shocking discoveries started out as absurd fever dreams. Like the time Albert Einstein came up with the theory of special relativity after imagining himself riding a light wave and looking at another wave traveling adjacent to him. In much the same way, a lunch conversation about plant biology and neuroscience between two German scientists sparked the question of whether it was possible to use photosynthesis to provide the nerve cells of living tadpoles with Oxygen.
The team injected a controlled amount of Chlamydomonas renhardtii (commonly known as green algae) and Synechocystis (a strain of unicellular cyanobacteria typically found in freshwater) into the hearts of Xenopus laevis tadpoles (African Clawed frog). The algae eventually moved through all the blood vessels and reached the brain, turning the originally translucent tadpole bright green.
The tadpole’s head was then isolated and placed in an oxygen bubble bath with nutrients that would keep the somatic cells alive, allowing for the current neural activity and oxygen level to be monitored. As oxygen was gradually extracted from the setup, the functioning of the tadpole's nerves slowed down and eventually stopped completely. However, when the researchers shone light onto the head, they saw oxygen levels rising in the brain, and after 15 to 29 minutes, neural activity had also been completely restored. In fact, the revived nerves actually performed even better than they were before they died of hypoxia.
In nature, many species of microalgae actually maintain symbiotic relationships with organisms like corals, sea sponges and anemones, often supplying them with requisite oxygen and other nutrients. There are also a few animals that have been able to harness the power of microalgae to produce their own energy. For example, the Elysia chlorotica (Eastern emerald elysia) is a sea slug that eats microalgae and 'steals' their genes for photosynthesis through Horizontal Gene Transfer, a process that was previously thought to be exclusive to bacteria, and pass them down to their offspring. The Costasiella kuroshimae (commonly referred to as Leaf sheep) is another species of sea slug that eats microalgae to steal their chloroplasts in a process called kleptoplasty, and store them for up to ten days. There is also the Ambystoma maculatum (Spotted salamander), which has chloroplasts in every one of its somatic cells and is the only vertebrate (so far) that can benefit from photosynthesis. Like the Elysia, these salamanders have a symbiotic relationship with algae (specifically Oophila amblystomatis), right from the time when they're embryos. While the algae do undergo significant stress and oxygen deprivation while living in a non-translucent, complex animal such as this, their symbiotic arrangement has persisted to the point where this algae can only be found in the cells and eggs of these salamanders. This is actually highly unusual, because the immune systems of vertebrates almost always prevent any sort of foreign cells from making such relationships in the first place.
This issue of immune response in more complex animals such as vertebrates is also a concern brought up in the research study above. While the researchers do hope to extend the principle to the potential development of treatments for hypoxia, they have acknowledged that the sophisticated immune response in humans will serve to be a major hurdle. On the bright side, this study has proved that it is realistically possible and hey, if the salamanders could do it all those years ago, what's really stopping us from giving it a go?
Research Paper in question:
Özugur, Suzan, et al. “Green Oxygen Power Plants in the Brain Rescue Neuronal Activity.” IScience, vol. 24, no. 10, 2021, p. 103158., https://doi.org/10.1016/j.isci.2021.103158.
Other references:
Rumpho, Mary E., et al. “Horizontal Gene Transfer of the Algal Nuclear Gene Psbo to the Photosynthetic Sea Slug Elysia Chlorotica.” Proceedings of the National Academy of Sciences, vol. 105, no. 46, 2008, pp. 17867–17871., https://doi.org/10.1073/pnas.0804968105.
Burns, John A, et al. “Transcriptome Analysis Illuminates the Nature of the Intracellular Interaction in a Vertebrate-Algal Symbiosis.” ELife, vol. 6, 2017, https://doi.org/10.7554/elife.22054.
Christa, Gregor, et al. “Functional Kleptoplasty in a Limapontioidean Genus: Phylogeny, Food Preferences and Photosynthesis Incostasiella, with a Focus Onc. Ocellifera(Gastropoda: Sacoglossa).” Journal of Molluscan Studies, vol. 80, no. 5, 2014, pp. 499–507., https://doi.org/10.1093/mollus/eyu026.
Video showing the algae being injected into the tadpole's circulatory system
https://youtu.be/xHtRDHCKX2Y
Images
Cover image: Olena, Abby. “Scientists Use Photosynthesis to Power an Animal’s Brain Mum.” Scientists Use Photosynthesis to Power an Animal's Brain | The Scientist Magazine®, The Scientist Magazine, 13 Oct. 2021, https://www.the-scientist.com/news-opinion/scientists-use-photosynthesis-to-power-an-animal-s-brain-69307/amp.
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