Solar-mander
Last week we discussed the essential functions of vernal pools and their importance to human and environmental health. I promised that we would continue talking about a salamander species which frequents vernal pools to mate and lay their eggs— the Spotted Salamander (Ambystoma maculatum). Spotted Salamanders are my favorite salamander species not only because of their vibrant spotting, robust figures, and seemingly smiling faces, but also because of their remarkable relationship with green algae (Oophila amblystomatis).
Spotted Salamanders being processed by students of a Population Biology course (Top photo by Faith Forry; bottom photo by Rachel Dressler).
Spotted Salamander eggs can be identified in vernal pools as thick, gelatinous masses which are anchored to tree branches and debris. Upon closer inspection, individual egg capsules appear green due to the accumulation of green algae around the embryo. The discovery of this symbiotic relationship was made more than 120 years ago. In the past, this relationship was considered an ectosymbiotic mutualism; this is just a fancy way of saying the algae lives on the surface of, or is physically separate from, the embryo, and both species benefit from this relationship. However, more recent research determined that the algae can also invade the tissues and cells of the salamander embryos during embryonic development.
This is an exceptional discovery, making Spotted Salamanders the only known vertebrate with an intracellular symbiotic relationship with a photosynthetic organism. Most animals have an adaptive immune response that protects them from intracellular invasions; when invaded by a foreign microbe, our cells are programmed to self-destruct— this is called apoptosis. Incredibly, salamander cells are tolerant of the algae, and they are found to be structurally similar to alga-free cells. During development, the salamander’s immune system is rather inefficient, which may be why the algae is accepted by the embryonic cells. As the salamander continues to develop, the abundance of algal cells decreases; this may coincide with the development of the salamander’s adaptive immune system, or the algae may simply stop producing chlorophyll, which is necessary for photosynthesis. Either way, algal DNA can still be found even in adult Spotted Salamanders
You might be wondering why this occurs in the first place. The salamander embryos, encapsulated by the jelly-like matrix, benefit from the increased concentration of oxygen produced by the algae. This facultative, symbiotic relationship essentially makes these adorable salamanders “photosynthetic”! Further, there is a correlation between the presence of algae and decreased embryo mortality, earlier and more synchronous hatching, and achieving a larger size and later stage of development upon hatching. In turn, the algae is thought to benefit from the nitrogenous wastes produced by the embryos. Although, research has found that the opaque embryonic tissues make it difficult for enough sunlight to reach the algae and allow them to sufficiently photosynthesize. When the demand for oxygen— from the salamander embryo and for the algae’s own use— exceeds production, the algae can become stressed. When this happens, the algae will switch from photosynthesis to fermentation in order to produce enough oxygen. Fermentation is a less ideal oxygen-producing process.
While the algae can become stressed from this relationship, the salamander embryos show no sign of stress. So why would this “symbiotic” relationship continue? Scientists aren’t entirely sure yet. Though research is not yet conclusive, there is evidence which suggests that the algae can be maternally transmitted from generation to generation. This would allow the algae’s genes to continue to be passed on. It is also thought that the algae could be transmitted through the environment. Or, it could be a combination of the two. Only further research will tell. Either way— I told you Spotted Salamanders were cool.
[1] Burns, John & Zhang, Huanjia & Hill, Elizabeth & Kim, Eunsoo & Kerney, Ryan. (2017). Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis. eLife. 6. 10.7554/eLife.22054.
[2] Kerney R, Kim E, Hangarter RP, et al. Intracellular invasion of green algae in a salamander host. Proceedings of the National Academy of Sciences of the United States of America. 2011 Apr;108(16):6497-6502. DOI: 10.1073/pnas.1018259108.
Written by Faith Forry — May 19, 2021
