It was a quiet late December day at the National Science Foundation when the call came in. The caller, a high school student from New Hampshire, wanted some help with a science project. When I asked about the project, the answer stunned me, for both for how ambitious the plan the students were proposing, and for how futile and far-fetched I thought it would be: to make a non-photosynthetic organism take up chloroplasts and thus make it photosynthetic. I explained that chloroplasts, the parts of plant cells that evolved from free-living photosynthetic bacteria which allow modern plants to capture energy from the sun, are highly dependent on their host cells, and that it would be unlikely that this experiment would be feasible. Still, I decided to do a little research for them on the topic.

To my great surprise, what I found in the literature was that their experiment, to make a non-photosynthetic organism able to capture energy from the sun for its benefit, has already happened, several times, in nature! And very recently, one group has attempted it in the lab. So when I got back to the students, I was excited to report back that their experiment was not as crazy as I thought.

A brief history of photosynthesis and endosymbiosis

Green with envy? The ancestors of plants made a partnership with free-living cyanobacteria that resulted in these green cells evolving into modern day chloroplasts, much like the ones seen here in these moss cells. Chloroplasts are now dependent on their hosts, and cannot live on their own, while the plant cells are dependent on the chloroplasts to secure energy from sunlight. Credit: Kristian Peters, Wikipedia

It is thought that the first photosynthetic organisms on Earth were single-celled bacteria related to modern day aquatic and photosynthetic bacteria called cyanobacteria. About 1.5 billion years ago, a natural experiment of sorts happened: the non-photosynthetic ancestor of today’s plants took in tiny green bacteria inside their cells, and maintained a partnership with them, the host cell providing a healthy environment for these photosynthetic bacteria to grow, while receiving energy from them that they harvested from sunlight.

Over time, the two types of cells became mutually dependent, so much so, that these green bacteria can no longer can survive on their own and are considered to be parts of the plant cells. We call these remnants of organisms inside cells plastids. The green photosynthetic plastids inside plant cells are known as chloroplasts. The process by which free-living cells are taken up by another to become part of the cell is known as endosymbiosis (from the Greek: endo- meaning inside and -symbiosis meaning cohabiting).

But this natural experiment would not be the last of its kind.

More recent examples of endosymbiosis with photosynthetic organisms

The light that showers down on the Earth from the Sun represents the greatest untapped source of energy on the planet. So many organisms besides plants have repeatedly tried to harness that energy by partnering up with photosynthetic organisms that take residence inside their cells. Amoebas, which are usually predatory cells that devour smaller cells for food, are not usually photosynthetic. But a species exists that contains plastids highly related to free-living cyanobacteria, suggesting the acquisition was relatively recent in evolutionary time.

To support that idea, there is a related species of amoeba that is non-photosynthetic but actively feeds on cyanobacteria. The photosynthetic amoeba species, Paulinella chromatophora, may represent an early stage of endosymbiotic evolution. A recently described unicellular organism, Hatena arenicola, may also represent an intermediate step in the process of endosymbiosis. This tiny ocean organism is usually colorless and must feed on green alga to survive. However, it has a phase during which it takes up the algal cells but instead of digesting them, harbors them and makes use of their photosynthetic capabilities to harvest energy from sunlight.

Green sea slug. This mollusk eats algae, but also steals the chloroplasts from them and insert them into its tissue, giving the slug its green color and enabling it to harvest energy from the sun. Credit: Nicholas E. Curtis and Ray Martinez.

Endosymbiosis can happen multiple times even with the same organism. The above-mentioned Hatena is host to an alga, which is itself a product of an ancient endosymbiotic event. Certain single-celled plants cells, such as green and red algae, have been themselves taken up by other cells to become plastids. These events have led to the modern day brown algae and dinoflagellates (some of which can cause the algal blooms known as red tides). Some dinoflagellate species are in turn symbiotic with coral species.

There is now evidence that even animals have tried to go green. The green sea slug, Elysia chlorotica, like many sea slug species, eats algae. But this species goes further: it extracts the chloroplasts from algae and engulfs them into its cells to harness the energy from the sun. Not only that, the slug contains in its DNA genes which it acquired from the algae, making this organism part animal, part plant.

Naturally fluorescent algal cells can be seen as red dots in this salamander embryo. Credit: Ryan Kerney

Another surprising and recently discovered example is the case of the photosynthetic salamander. For years, scientists had observed green algae in the eggs of a species of spotted salamander, and that the presence of the algae in the eggs appeared to benefit the growth of the eggs.

But more careful analysis has shown these algae are in fact endosymbiotic, living entirely within the salamander cells. “There are many convergent photosymbioses in life, but not in vertebrates,” says Ryan Kerney, the scientist at Dalhousie University in Halifax who made this remarkable discovery. “Our salamanders may be an exception.” The rarity may be attributable to the bioenergetic costs required by vertebrates, or the vertebrate body plan not being very good for hosting sun-loving organisms, but more research on these salamanders may reveal what constraints limit photosynthetic partnerships in this branch of the tree of life.

Before endosymbiosis: symbiosis

The algal cells are entirely enclosed within the salamander embryo cells, much the way chloroplasts are enclosed within plant cells. Credit: Ryan Kerney

Not all partnerships with photosynthetic organisms results in the little green cells being engulfed by the larger cell and becoming an accessory item within. Much more common are the green and non-green cells living side by side in close proximity and sharing and exchanging resources, a relationship simply called symbiosis. Examples of such partnerships include the above mentioned dinoflagellates living among corals, algae partnering up with fungi to form what is known as lichen, and numerous species of sea anemone, sponges and mollusks that allow photosynthetic microbes to live within their bodies (though not inside their cells), sheltering them and providing certain nutrients in exchange for the energy the bacteria harvest for them from the sun.

Green fish

New players in the game of going green are humans. Researchers have recently made a first attempt at artificially creating a chloroplast in a non-photosynthetic organism. Members of Pamela Silver’s lab at Harvard Medical School inserted photosynthetic cyanobacteria into embryonic zebrafish. Surprisingly, the green bacterial cells are maintained in the zebrafish cells, whereas some other control bacteria are not. This experiment may have recapitulated the initial steps that occur during endosymbiosis. Next the researchers want to see if they can engineer a mutualistic relationship between the photosynthesizing bacteria and their fish hosts. “I think that designing communities will allow synthetic biologists to build more complex metabolic pathways,” says Christina Agapakis, lead author of the study. “Engineering plastids, or bacteria that act like plastids, photosynthetic or otherwise, will likely allow for modular engineering of eukaryotes.”

This could have interesting applications in energy capture – if animals could be engineered to capture the energy of the sun, the possibilities for increased food production are fascinating to ponder.

Not that easy being green?

Nathan Battey, Patrick hart and Robert Tardif of Goffstown High School in New Hampshire were not successful in creating photosynthetic amoebas in their experiments as they had aimed to, though they did make some interesting observations about the uptake of chloroplasts, cyanobacteria and algae by these predatory protists, and during their research learned much about unexpected endosymbiosis that has occurred in nature. “Not one of the three of us had ever heard of any animal that photosynthesizes before we started this project,” says Tardif. The students’ study, titled “It’s not easy being green”, won them first prize in the high school science fair in Concord, NH. And their idea was on the right track; nature has shown that the sun’s energy has coaxed numerous unlikely organisms to partner up and be green.

Thinking green. Standing by their First Prize-winning poster, Nathan Battey, Patrick Hart and Robert Tardif, Goffstown High School students from Goffstown, New Hampshire, who used amoeba to try to establish photosynthetic organisms and organelles within them. Credit: Robert Tardiff