Biological Hybrids – The diet to end all diets?

It’s easy to tell a plant from an animal. Plants make their food from sunlight and carbon dioxide (CO₂), while animals have to consume plants and other animals in order to get the nutrients and energy they need. In fancy science talk, plants and certain bacteria are autotrophs (“own nourishment”), while animals and most bacteria are called heterotrophs (“different nourishment”).

Algae, plants, and certain bacteria (cyanobacteria) can make their own carbohydrates (sugars) by fixing CO₂. “Fixing” a molecule means to convert it from an unusable form to a usable form. To fix CO₂) from the air, it is incorporated into a solid form, using the energy of sunlight to add carbons to an existing 5-carbon sugar called ribulose bisphosphate. This bigger molecule is then broken into two 3-phosphoglyceric acid molecules before building glucose from these building blocks. We all know this process by another name: photosynthesis.

Animals, including humans - to McDonald’s everlasting delight - can’t perform photosynthesis. They have to get their carbohydrates and other building blocks the old fashioned way - they steal them from something else. By eating and digesting plants and other animals, heterotrophs make use of the plant’s hard work, or piggyback on another animal’s use of plant-produced carbohydrates. Downstream from photosynthesis or eating, the process is the same for both plants and animals; the chemical energy in the bonds that hold the carbons of sugars together are converted into a chemical currency that cells can cash in: mostly ATP.

So, plants are plants and animals are animals, and never the twain shall meet (or meat, in this case)…… EXCEPT for a certain group of sea slugs. Believe it or not, there is a type of sea slug that can perform photosynthesis. Drought-induced food shortages? No problem for these guys. Increasing CO₂ and greenhouse gases? The more the merrier for our little friends. Imagine the possibilities if humans could do this; never again would the sink be piled high with dirty dishes!

Nudibranches are the stars of the sea slug world.

So who is this tree-wanna-be? “Sea slug” is a catchall name for many types of fresh- and saltwater mollusks; underwater snails without the shell (or at least without a visible shell). He’s a homeless gastropod - all wriggly and jelly-filled. Nudibranches are the most numerous and colorful of these groups, but in total there are nearly 4000 species that might be called sea slugs.

E. chlorotica, our sea slug of interest.

Elysia chlorotica is the flavor we are interested in, from the order Sacoglossa, the family Elysiidae. E. chlorotica lives in the shallow saltwater marshes and tidal pools of the U.S. East Coast. Elysdiae are a hearty bunch, being found as far north as Nova Scotia as well as in the warm waters of Florida. They are medium sized for a sea slug, about 2-4 cm in length, but can grow to up to 6 cm.

The key to E. chlorotica’s success as an apprentice plant lies in its ability to make the most of its few meals. It is a picky eater, dining on only two species of algae, Vaucheria litorea and Vaucheria compacta. Using its radula, a sort of serrated tongue, the sea slug punctures the algae and sucks out the contents. But, instead of digesting the chloroplasts, they are taken inside the cells of the slug’s gut cells and they stay there. What’s more, they still work! One organism co-opting another organism for its own gain- the audacity!

However, this is far from first time something like this has occurred. About 1.5 billion years ago, when all life consisted of single celled organisms, one organism ate another organism; just like every other day since time began. But like E. chlorotica with chloroplasts, for some reason the meal wasn’t digested. The two organisms came to an arrangement, and the internal organism stayed and divided when his captor divided. This was called endosymbiosis, and is thought to account for mitochondria, nuclei, and even the chloroplasts of plants themselves.

E. chlorotica’s trick is a little different though, it doesn’t retain the entire algal cell, just the chloroplasts (or plastids as they are sometimes called). This requires a different name be devised for the new process, and I love the one they came up with – kleptoplasty. Literally, the sea slug is a kleptomaniac for the algal plastids; it has a deep-seated need to steal the belongings of another. I picture parental slugs going down to the police station to bail out their teenage offspring after a wild night at the kelp beds.

To me, kleptoplasty is much more amazing than endosymbiosis, because it is selective. Most of the algae is digested, but what is it about V. litorea chloroplasts that allows them to be maintained? No one knows how this occurs, but when they do figure it out, they will still have some issues with which to deal. For instance, after the chloroplasts are separated from the rest of the algae parts, they move inside the gut cells of the sea slug and stay there for the life of the animal! Just how did that come to be?

One matter that can be resolved is how a chloroplast in the gut can help a slug perform photosynthesis. E. chlorotica is a skinny fellow; turn him sideways and have him stick out his radula and he looks like a zipper. So, even though the chloroplasts are in the gut wall, they are still close enough to the surface to receive sunlight. This placement is helpful in another way as well.

Is it animal or vegetable?

Look at the image of E. chlorotica at the right. He is oval shaped, and because of the chloroplasts inside, he is green. Scientists believe this is important for the protection of the animal; a very convincing costume to make him look like a leaf that has fallen into the water. Even his digestive tract helps out, as it fans out to parts of the slug’s body, it looks like the veins in a leaf.

When the slug is born, it is in an immature form called a veliger. It feeds for about a week, and places the chloroplasts in its tissues. From that moment on, the sugary products of photosynthesis in the chloroplasts are exported into the gut of the slug, and distributed through its body to be used for fuel, just as if he had continued to eat and digest food. The animal doesn’t have to feed for the rest of its life, as long as there is sunlight and dissolved CO₂ in the water. Since it lives in water no more than 0.5 m deep, getting sunlight isn’t a problem and seawater is 15.1% dissolved CO₂, as compared to only 0.033% of air (but your soda has 80X more CO₂ than seawater, at least until you open the can). The result of this kleptoplasty is that even in times of food scarcity, E. chlorotica is a happy camper, just laying around, looking like a leaf, avoiding trouble, while at the same time soaking up the sun and turning it into food. That’s the life.

Unfortunately, it isn’t that simple. This sea slug has had to pull some even niftier tricks to hang onto its photosynthetic ability. We’ll talk about another amazing aspect of this animal’s plant envy next time. Stealing a chloroplast is one thing, but apparently E. chlorotica is willing to accept charity as well.

For more information on photosynthesis or endosymbiosis, as well as web based activities and experiments, go to:


Photosynthesis-
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html
www.phschool.com/science/biology_place/biocoach/photosynth/intro.html
www.youtube.com/watch?v=C1_uez5WX1o

www.pbs.org/wgbh/nova/nature/photosynthesis.html

www.science-class.net/Biology/Photosynthesis.htm
www.the-aps.org/education/k12curric/activities/pdfs/carswell.pdf
http://local.brookings.k12.sd.us/biology/teacherlinks/photosynactivities.htm
http://nhscience.lonestar.edu/biol/bio1int.htm


Endosymbiosis –
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html
www.fossilmuseum.net/Evolution/Endosymbiosis.htm
www.sumanasinc.com/webcontent/animations/content/organelles.html
http://evolution.berkeley.edu/evolibrary/article/endosymbiosis_01
www.youtube.com/watch?v=RaAM8qQcs6E
www.astrobio.net/pressrelease/3223/endosymbiosis-timeline
www.accessexcellence.com/AE/AEC/AEF/1995/everson_endosymbiosis.php
http://wps.prenhall.com/esm_biology_belk_1/13/3353/858395.cw/index.html