A coral reef is an animal high-rise hosting an algal co-op.
Each efficiency apartment in the calcium carbonate complex is occupied by both a tentaculate animal related to jellyfish and an alga tucked inside it. The alga uses light to make food which it shares with its roommate. The animal catches food with its stinging tentacles which it uses to fertilize the algae and build the apartment block.
However, corals do not exactly scream “animal”, and most people would be hard pressed to identify them as such. Yet amazingly, there is an animal-that-screams-animal in which an alga also dwells. That would be the spotted salamander Ambystoma maculatum. When it is an embryo, cells of the alga Oophila amblystomatis somehow end up inside it. Technically, the salamander’s now a photosynthetic animal.
This salamander is the sole known example of a vertebrate playing host to a symbiotic microorganism of any kind, photosynthetic or no. And needless to say, something very interesting -- no one is yet sure exactly what -- is going on between the two. Past experiments have clearly shown that the salamander benefits from its unconventional living arrangements. How the algae feel about the situation has been rather less clear.
In coral, the host animals tamp down their own immune systems to admit algae and produce proteins that transfer nutrients to or from their symbionts. The algae likewise alter their metabolism in ways that benefit both themselves and their hosts. Is whatever is happening inside these salamander embryos just like the coral symbiosis? Is it more of a parasitism? Or is it something in between?
Recently a team of scientists from the American Museum of Natural History and Gettysburg College decided to investigate what was going on with the genes of both organisms to try to answer these questions. Which genes were producing more proteins in the symbiotic state, and which fewer? What can that tell us about what is going on? They published their results last year in the journal eLife.
There are actually two forms of symbiosis between the alga and the salamander. In one, algae live within salamander egg capsules containing developing embryos but do not invade cells, as you can see in the photograph above. In the other, the algae enter the cells of the embryo.
Past experiments revealed that in the first association, termed ectosymbiotic, the algae benefit from what amounts to the pee – the nitrogenous waste – of a developing embryo, probably because it acts as fertilizer. In return, the alga provides oxygen and sugar to the embryo.
The second form of symbiosis – the intracellular one – was only recognized in 2011, some 122 years after the first account of green salamander egg masses was first published. Eyebrows, no doubt, were raised.
In this experiment, scientists took cells from wild-caught salamanders and their algae and measured gene usage in all permutations of alga and salamander.
Their data revealed that intracellular algae showed clear signs of stress and oxygen and sulfur deprivation, producing lots more heat shock and autophagy-related proteins in response to finding themselves inexplicably inside a salamander.
Although algae generate oxygen when they photosynthesize, they also require it to generate power from their food, just as animals do, via oxygenic respiration. Algae trapped inside salamander cells do not appear to be able to generate enough oxygen to meet the dual demand from self and salamander. Compounding the trouble, being inside an animal also tends to obscure the sun.
Instead, the algae inside salamander embryos resort to fermentation. This process is a way to make power – hay, really -- from food in the absence of oxygen. Fermentation is the same process turned to by yeast inside rising bread and brewing beer and our own exhausted muscles when demand for oxygen exceeds supply.
The downside is that fermentation offers significantly less bang for your fuel buck than respiration. It's not generally a microbe’s (or any organism’s) first choice. Because fermentation is so inefficient relative to aerobic respiration, the alga also appears to be burning through its starch reserves at a worrying pace. The scientists calculated that algae inside salamander cells had only 42% by volume of the reserves of algae outside in the capsule.
It's not all bad news. The alga does seem to benefit from the abundant nitrogen and phosphorous supplies found in its host cell, so doesn’t have to make as many nutrient transporting proteins as it would on the outside.
The salamander, on the other hand, wasn’t particularly perturbed by the presence of a near-plant inside its cells: only 0.64% of the genes analyzed by the scientists showed any sign of altered usage between salamander cells invaded or not invaded by algae. In coral, that figure is 3%.
In response to resident algae, the embryo bumped up production from genes that suppress immune response to invading organisms, a behavior also shared with coral animals. Salamander cells did not show signs of programmed cell death (apoptosis), cell organelle recycling (autophagy) or other destructive reactions to invaders. Animal cells often react to invasions with suicide. Salamander embryo cells, judging by their gene usage, do not.
The salamander may benefit in other ways from its early algal flirtation. The invasion may help prime the salamander’s immune system without overstimulating it, improving the animal’s ability to fight off hostile invaders later in life. The alga may also be feeding the embryo -- at least a little -- on the products of fermentation or otherwise, although that remains unclear. It is known the salamander grows better with algae inside it.
Interestingly, in coral, it is the animal that appears to be more taxed by the arrangement, which is why corals undergoing environmental stress may evict their algae (a process called bleaching) to improve short term survival. If they don’t find new tenants at some point, however, they’ll die.
So why does the Oophila, the salamander alga, put up with its apparently dreary living conditions inside its host? It’s an intriguing question that lacks a clear answer, but there are clues. The alga is found nowhere else in nature besides salamander egg capsules. Algal cells remain visible inside young salamanders for a long time. Even when they are no longer obvious, algal DNA remains detectable in adult salamanders in the oviducts and the male reproductive tract. Freshly laid eggs contain encysted algal cells. And those algal cells in the capsule don't seem nearly as put-upon as those inside embryos. Where might they come from?
Is it possible that the alga is passed from generation to generation of salamander, a perpetual part of the animal? If so, the salamander has given the algae the ultimate gifts: a free ride, a home, and immortality, at least for the life of the host species.
If that is the case, it was probably a bargain worth making. As suggested by the “selfish gene” hypothesis, the algae’s genome may be delighted by its reproductive luck even as the algae that genome creates are stressed-out and uncomfortable. Sadly for us actual organisms, it seems that the survival of our genes -- and not our personal comfort -- are what matter most to natural selection.
John A Burns, Huanjia Zhang, Elizabeth Hill, Eunsoo Kim, Ryan Kerney. Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis. eLife, 2017; 6 DOI: 10.7554/eLife.22054