Symbiotic relationships are in nature are quite common. But the partnership struck up between green algae and spotted salamanders is highly bizarre. The algae infiltrate the amphibians' eggs, and then get encompassed into the animals' tissue, and we’re not entirely sure why.

“Science shows us the many ways that life is interconnected, especially on the microscopic level, where we see how many organisms depend on close contact with or internalization of other species for food, defense, or reproduction,” explained John Burns, lead author of the paper published in eLife. “But the relationship between this particular alga and salamander is very unusual.”

The unique relationship between spotted salamanders and green algae was first documented over a century ago, when it was first noticed that the green tinge of the salamanders' eggs was actually due to tiny algae. Until just a few years ago, most assumed that the green algae was simply living on the outside of the eggs, with the developing salamanders providing the algae with nitrogen, and the algae increasing the concentration of oxygen, helping the embryos grow.

^{The developing embryo is surrounded by the green algae. © Roger Hangarter}

It was not until researchers tagged the algae with fluorescent markers that they got conclusive evidence that the organisms were actually contained within the embryo, and thus found in the living cells of the salamanders. “This is really such a strange arrangement to think about, that the salamanders allow the algae to live in their egg cases. It would be like having a bunch of green algae in a womb,” said Ryan Kerney, who co-authored the paper.

But the relationship between the two organisms may not be as simple as we thought. Investigating how both species benefit from the partnership, the researchers found that while the salamander cells are doing well, the green algae are seemingly under masses of stress. It may be asked why a salamander might enter into such a relationship, but what we should really be asking is why does the algae?

This question, it seems, might be a little trickier to answer. The algae are so stressed when they are inside the salamander cells that it actually alters how they produce energy, floundering as they try and adapt to an environment with restricted light. In contrast, the salamander cells seem to flourish, and it even seems that the amphibians' own immune system is dampened to let the relationship unfold.

This imbalance in how the two separate organisms react could indicate that it is not really a symbiotic relationship – in which both species mutually benefit – at all, or it may be that this is a totally new form of symbiosis that has never been seen in anything else.