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This is a new story & potentially a very exciting one (& I must thank Grant for drawing this story to my attention!). A Nature News item (Petherick, 2010) describes the discovery of green algae apparently living within the cells of salamander embryos. I’ll wait with interest for the published paper, but if this finding’s confirmed then it will be the first recorded instance of endosymbiosis in a vertebrate.

The story’s based on a conference presentation by Ryan Kerney, who noticed that the salamander embryos he was studying were a bright green – and not because of algae living on the tiny animals’ skin. This was exciting stuff. Apparently scientists have known for a while that spotted salamanders & the unicellular alga Oophila amblystomatis have a symbiotic relationship. (In fact, Oophila doesn’t live anywhere else, apart from in association with the eggs of a few other amphibians.) Like most frogs, salamanders lay their eggs in water. Here the animal’s urine provides nitrogenous compounds that promote algal growth, while photosynthesising algae living on & in the jelly surrounding the embryos raise the levels of oxygen in the water, whcih would support a higher respiration rate in the tiny salamanders.

However. Kerney found that the algal cells were actually growing inside the cells of his embryo salamanders. (I suspect he would have rubbed his eyes & looked again, on first spotting this one.) This is unexpected – what you’d anticipate is that any algae somehow getting into a vertebrate’s cells would be pretty quickly picked off by the animal’s immune system, which is able to distinguish between ‘self’ and ‘non-self’ cells & pick off any intruders. So an interesting question for future research would have to be, what’s the mechanism that’s allowed the algae to penetrate & survive within the amphibian’s cells – how has it overcome/avoided the animal’s immune response

A related question is, how do the algae actually physically get into the salamander cells? The Nature News item suggests this might happen when the cells are releasing nitrogen-rich waste products, but as the release of these compounds from individual cells would be very much on the subcellular scale, it’s hard to visualise how this would provide an opening for the algae.

And of course, do the embryo salamanders gain photosynthates from the algae living within them? Work by Hutchison & Hammen, way back in 1958, showed that salamander eggs that lack algae in their jelly casings hatch more slowly. And it’s been known for a long time (e.g. Cates, 1975) that invertebrates such as corals and jellyfish gain photosynthates from their endosymbionts, as does the sea slug Elysia chlorotica  . But if it turns out that the embryos are actually able to utilise the sugars produced by the photosynthesising algae, this would be a first. There are suggestions in Kerney’s conference paper that this might be happening: micrographs that show salamander mitochondria sitting close to the internalised algae. The next step here would be to demonstrate that sugars released by the algae were indeen being taken up and used by the animal’s mitochondria.

A solar-powered vertebrate? Perhaps – but there’s a lot of work needed here yet.

N.Cates (1975) Productivity and organic consumption in Cassiopea and CondylactusJournal of Experimental Marine Biology and Ecology 18(1): 55-59. doi:10.1016/0022-0981(75)90016-7

V.H.Hutchison & C.S.Hammen (1958) Oxygen utilisation in the symbiosis of embryos of the salamander, Ambystoma maculatum and the alga, Oophila amblystomatisBiological Bulletin 115: 483-489.

A.Petherick (2010) A solar salamanderNature News published online 30 July 2010, doi:10.1038/news.2010.384