A solar salamander
Photosynthetic algae have been found inside the cells of a vertebrate for the first time.
Spotted salamander embryos have an unexpectedly close relationship with a single-celled alga.TED LEVIN/photolibrary.com
Occasionally, researchers stumble across something extraordinary in a system that has been studiedfor decades.
Ryan Kerney of Dalhousie University in Halifax, Nova Scotia, Canada, did just that while looking closely at a clutch of emerald-green balls — embryos of the spotted salamander (Ambystoma maculatum). He noticed that their bright green colour comes from within the embryos themselves, as well as from the jelly capsule that encases them.
This viridescence is caused by thesingle-celled alga Oophila amblystomatis. This has long been understood to enjoy a symbiotic relationship with the spotted salamander, which lays its eggs in bodies of water. However, the symbiosis was thought to occur between the salamander embryo and algae living outside it — with the embryo producing nitrogen-rich waste that is useful to algae, and the algae increasing the oxygen content of the water inthe immediate vicinity of the respiring embryos.
At a presentation on 28 July at the Ninth International Congress of Vertebrate Morphology in Punta del Este, Uruguay, Kerney reported that these algae are, in fact, commonly located inside cells all over the spotted salamander's body. Moreover, there are signs that intracellular algae may be directly providing the products of photosynthesis — oxygenand carbohydrate — to the salamander cells that encapsulate them.
Breaking the rules
Such a close co-existence with a photosynthetic organism has previously been found in invertebrates, such as corals, but never in a vertebrate.
Because vertebrate cells have what is known as an adaptive immune system — which destroys biological material not considered 'self' — it was thought to be impossiblefor a symbiont to live stably inside them. But, in this case, the salamander cells have either turned their internal immune system off, or the algae have somehow bypassed it.
"On a lark, I decided to take a long-exposure fluorescent image of a pre-hatchling salamander embryo," says Kerney. When this revealed widely scattered dots of unstained cells fluorescing in the background — an indicatorthat those cells might contain chlorophyll — Kerney switched to transmission electron microscopy (TEM) to take a closer look.
"The surrounding salamander cells that contain the algae often have several mitochrondria bordering the algal symbiont," Kerney says, pointing to a TEM image.
Mitochondria are the powerhouses of animal cells, converting oxygen and a metabolic product of glucose into ATP,a molecule that cells use to store chemical energy. So salamander mitochondria gathered around an algal cell might be there to take advantage of the oxygen and carbohydrate generated by photosynthesis in that particular cell.
How the relationship between the two species originated is unknown. But Kerney is probing how algae enter salamander cells, and some earlier findings areproving helpful.
Lynda Goff, a molecular marine biologist at the University of California, Santa Cruz, worked on this pair of organisms about 30 years ago and demonstrated, among other things, that embryos lacking algae in their surrounding jelly are slow to hatch. "We saw a logarithmic increase in algal cells as the embryo developed," she says. And in those that did contain algae, the community wasnot static.
This logarithmic increase suggests that algae associated with the salamander embryos either divide rapidly as the embryo develops, or quickly enter the jelly or the embryo from outside as it grows.
So how might the algae enter the embryos? A likely moment occurs as the embryos' nervous systems begin to form. A time-lapse video made by Roger Hangarter at Indiana University in...