Salamanders and algae are partners from birth

Although there are many examples of symbiosis between living organisms in nature, such as the protection of acacia trees by fire ants, these relationships typically exist on a more external level in terms of providing protection or shelter. In some more extreme cases, two different species are merged such that one exists within the cells of another.

This type of union is extremely rare among vertebrate animals, which typically possess immune systems that recognize, react to and eliminate foreign bodies.

However, scientists have recently discovered one such relationship between salamanders and algae — the algal cells live within the cells of the salamander and may even be passed down between salamander generations.

In a paper published in Proceedings of the National Academy of Sciences this month, scientists discovered that algal cells could be found inside embryonic cells of the salamanders.

Although the close coexistence of salamanders and algae has been a well-known fact for more than a century, most scientists believed that the algae dwelled extracellularly, in the jelly-like substance surrounding the actual cell.

The benefits to each organism had previously been revealed to be those of environment optimization — the nitrogen-rich egg is ideal for algal growth and the oxygen-production of the algae inhibits the development of deformities in the salamanders.

While this is still true, the researchers of Dalhousie University in Canada used imaging and DNA amplification to detect fluorescently-labeled algae in the embryonic and larval stages of salamander cells. They found that as the embryo develops, the algae tended to be most concentrated along the digestive tract.

This suggests beneficial roles in addition to those found in the embryo stage, such as nutrient processing, but scientists have yet to determine them.

The few other similar symbiotic relationships between phototrophs (organisms that require photosynthesis to produce energy) and vertebrates all involve the transfer of the photosynthesis product to the host.

However, such an exchange of goods is not readily apparent in the salamander-algae duo, since the algal cells can be found in tissues that are opaque and not directly exposed to sunlight, eliminating the feasibility of photosynthesis.

Another existing hypothesis is that genes that are required to carry out essential cellular processes may be transferred between two cell types. This is supported by the fact that most nuclei in amphibian cells readily accept foreign DNA when it is present in the cell’s cytoplasm.

This study also raises the question of the mechanism by which the algae originally enter the salamander cells. A higher concentration of algae DNA was also measured in adult salamander reproductive tracts, which lends support to the theory that the algae can be transmitted from parent to offspring.

However, other studies have observed the triggering of algae blooms in close proximity to salamander embryos at precise moments in their development. Whether these blooms lead to the invasion of the algae or if it is a combination of these two factors is a question that still needs to be answered.

The results of this study are more than simply an interesting phenomenon. While this may lead to future research on the existence of similar relationships in other amphibian species, it also opens the door to investigating the complexities of recognition between cells of drastically different organisms.