The distinction between plants and animals has always seemed fairly clear, but recently the line has been blurred with the discovery of the first animal that can make molecules essential for photosynthesis. The journal Symbiosis last week reported the amazing finding—the fruit of efforts led by Prof. Sidney K. Pierce at the University of South Florida—that the sea slug Elysia chlorotica can make molecules called chlorophylls.
E. chlorotica lives along the eastern coast of the United States and Canada. It has been known for years to have a symbiotic relationship with its food, the alga Vaucheria litorea. It was known that when E. chlorotica eats algae it integrates parts of the algae cells into itself, chloroplasts that are necessary for photosynthesis. This then allows the slug to gain energy from sunlight, like the alga does. Not only that, but the stolen chloroplasts are so efficient that E. chlorotica could live up to nine months without eating anything while still maintaining its normal nutritional rates, making it the longest-lived symbiotic relationship of its kind.
Part of this is accomplished by unique cells that line the slug’s digestive system. In our digestive tracks, food is broken down very finely into small molecules that are absorbed by cells lining the small intestine. But E. chlorotica doesn’t break down its food as well as we do; the slugs have special epithelial cells lining the walls of their digestive system that take in whole parts of the cells they eat in a process called phagocytosis (the intestinal cell forms a pouch around the cells it digests, engulfing parts of the cell whole). This allows E. chlorotica to uptake chloroplasts, which are large, complex structures within plant cells.
These chloroplasts are the site where photosynthesis takes place within the plant cell. Cells contain many types of compartments, called organelles, which serve different roles to ensure the cell machinery functions properly; chloroplasts are organelles that act as the plants’ power stations: They take in sunlight and carbon dioxide to produce energy for the plant cell (and oxygen for us).
It’s thought that long ago chloroplasts were actually a type of bacteria that could carry out photosynthesis, called cyanobacteria. These cyanobacteria, which have existed for billions of years, were absorbed by plants and integrated into their cells permanently; cyanobacteria and the plant cells developed a permanent endosymbiotic relationship (the prefix “endo-” means “within”).
It has long puzzled researchers that E. chlorotica can engulf chloroplasts and consequently carry out photosynthesis. Why? Because it’s just not that easy to “steal” an ability like photosynthesis from a plant—a fact related to the chloroplast’s very long history with plants.
Although the precursors of chloroplasts once roamed free as cyanobacteria, chloroplasts today are not very independent. Each chloroplast still retains its own internal genome, but this genome is very much reduced. Chloroplasts have come to be dependent upon the plant cell’s own nucleus (which holds all of a cell’s DNA). The cell’s nucleus contains the DNA for over 90% of the proteins the chloroplasts need to function. Additionally, photosynthesis proteins are rapidly degraded, meaning the cell must continually make new proteins for photosynthesis to work. That’s why it’s not so easy to use stolen chloroplasts to carry out photosynthesis for a long time in a non-plant cell: A lot of DNA from the plant nucleus is needed too.
But the sea slug E. chlorotica encountered this problem and solved it; in addition to stealing chloroplasts from the alga V. litorea, these thieving slugs also stole many photosynthesis genes from the alga itself. Pierce, who’s been studying the slug for decades, spearheaded studies to unravel the sea slug’s unique methods. In 2001, Pierce’s group first reported that many genes used in photosynthesis, found in the nucleus of V. litorea, are also present in the nuclear DNA of E. chlorotica. .
Such a transfer of genes between two organisms, where one is not the offspring of another, is called horizontal gene transfer (HGT). HGT is common between bacteria, and is actually how bacteria can gain resistance to certain antibiotics. However, HGT is very rare in other organisms; Pierce reported the first discovery of functional, nuclear genes being transferred from one multicellular organism to another. Pierce’s group also found that these genes are not just “borrowed” from the alga, but are permanently a part of the slugs; these genes are passed down to the slugs’ offspring.
Over the last decade, Pierce’s group has been searching for other stolen photosynthesis-related genes living in the slug’s nucleus. While many organisms used in research labs today have had their genomes sequenced and their DNA sequences are available for study in public databases, E. chlorotica has not been one of these lucky ones. Although their research was impended by a lack of available DNA data on the slugs, and while the genome still needs to be sequenced, Pierce’s researchers have nevertheless been able to make much progress. As they discovered more genes in the slug’s nucleus, they discovered more genes in common with the alga V. litorea.
The latest report from Pierce’s group shows that the slugs have within their nuclei the algae genes to make chlorophyll (specifically chlorophyll a). Chlorophyll is a very large pigment molecule that is essential for photosynthesis. Specifically, its role is at the beginning of the photosynthesis process; inside the chloroplast, it absorbs light and transfers the energy, as an electron, that is used to drive the rest of the photosynthetic process. Because of the specific wavelengths of light the chlorophyll absorbs (blue and red), plants (or thieving slugs) with chlorophyll appear green (a wavelength chlorophyll poorly absorbs). Photosynthesis creates energy for the plant (in the form of adenosine triphosphate, or ATP) and oxygen that the plant releases.
This finding helps further explain why E. chlorotica only needs to eat V. litorea at the beginning of its life, but can live without eating it again; the slugs need chloroplasts to put chlorophyll into, but may not need algal help after that.
Amazingly, several close relatives of E. chlorotica can also capture chloroplasts and carry out photosynthesis. E. chlorotica belongs to a group of gastropods called Sacoglossa (“sap-sucking sea slugs”), which includes some sea snails and sea slugs. They too can, like E. chlorotica, uptake entire chloroplasts in specialized epithelial cells lining their intestines. The animals need only direct light and carbon dioxide and have the ability to live healthily for months, often getting most of their energy from photosynthesis.
However, most of these other solar-powered sea slugs have relatively short relationships with the stolen chloroplasts; E. chlorotica can go longer without eating algae than any others known. It remains to be seen how many species, and to what degree, have incorporated plant genes related to photosynthesis in their own genetic arsenal.
These processes occurring in seas slugs highlight the rich biodiversity of the world’s oceans. Several laboratories at the University of California, Santa Barbara, study unusual marine organisms on both the molecular and whole-organism levels. Through better understanding of these unique animals, we can gain a better understanding not only of how they function, but how they are similar to us, how they became what they are, and how we may apply our knowledge of them to ourselves and technologies.
So while solar-powered sea slug cars may not be on the horizon, the knowledge that photosynthesis is possible in some animals means it might be possible to use this truly “green,” energy-generating pathway in other animals.
For more on this amazing sea slug’s photosynthetic abilities, see Prof. Sidney K. Pierce’s “most recent publication in the journal Symbiosis,” ScienceNews’ “Sea Slug Steals Genes…,” Pierce’s“other publications on E. chlorotica,” Wikipedia’s article on “Chloroplasts,” Wikipedia’s article on “Chlorophyll,” Wikipedia’s article on “Elysia chlorotica,” or “Prof. Sidney K. Pierce’s website.”
Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at science@independent.com.