Breaching Kingdoms: How A Small Animal Stole Photosynthesis
Since your first steps into the world of science and nature, you were taught about the classification of living beings. You learned that your neighbour's restless cat, the robins chirping out loud in spring, the spider crawling under your bed, and, of course, we, are members of the Kingdom Animalia, organisms that, among other characteristics, have to eat other organisms to survive because they cannot synthesize their own food, organisms that are heterotrophs.
Plants and algae, conversely, are autotrophs. They produce their food from lifeless inorganic substances—such as water, iron, and potassium—found in the soil, rivers, and oceans, and from atmospheric gases such as carbon dioxide. By fixing carbon dioxide from the air, plants and algae weave the fabric of life: they synthesize sugars and amino acids and turn them into proteins, lipids, nucleic acids, and many other essential molecules. Plants and algae, therefore, sit at the top of the supply chain, like royalty, proudly looking down to the dynamics of the world they build, to a world that depends on them. More impressively, they do this by garnering sun's energy to fuel the most important biochemical process on Earth and, perhaps, in the Universe: Photosynthesis.
It was always considered that photosynthesis was an exclusive power of plants, algae, and some bacteria. A faculty that is forbidden to animals. Recently, however, scientists have found that this élite privilege has been challenged. Not by giants of the seas, nor by brainy mammals. But by a tiny slug.
How a minute invertebrate, not larger than a 9V battery, mastered the magic of turning lifeless substance into organic molecules?
Very simply: it stole photosynthesis from algae.
To understand how the sea slug Elysia chlorotica (and others from the same genus) managed to own the capacity to photosynthesize, let's review how, in the first place, plants and algae orchestrate photosynthesis.
From nuclear fusion to red tomatoes: How plants collect sun's energy
Light emerges from the entrails of our star as it is produced from nuclear fusion reactions that occur in a state of matter called plasma. At the center of the sun, protons, nuclei of hydrogen atoms, smash into each other with such an enormous energy that, occasionally, they get tied together—or fuse with each other—forming heavier elements, like Helium. During this process, energy is released in the form of heat and photons, the virtual carriers of light energy.
A minuscule fraction of those photons, which otherwise would travel endlessly (and boringly) across the Universe, is captured by chlorophyll, the green pigment present in the leaves of plants, algae, and other photosynthetic organisms. Chlorophyll works like an antenna, in fact, this is how scientists call the molecular complex formed between chlorophyll and other pigments: antenna complex or light-harvesting complex.
The main job of the antenna complex is to capture the energy of the sun's photons and transform it into chemical energy, which, after a series of reactions, is used to snatch carbon dioxide from the air and stitch carbon atoms to each other, and to hydrogen, and to oxygen and nitrogen to form the molecules of life: sugars, proteins, carbohydrates, and others. This is photosynthesis. The greatest marvel of nature, a process we have been unable to reproduce, but plants and algae have mastered to the point that they have a dedicated compartment in the cells for it; a sun-powered micro-factory equipped with tools and chemicals and assembly lines: the chloroplast.
Microscope picture of chloroplasts in cells.
Diagram of a chloroplast
Scientists found that chloroplasts were, once upon the time, free-living cyanobacteria. Some of these microorganisms were engulfed by others establishing a mutually beneficial symbiosis. Today, chloroplasts still have their own genome (DNA capable of producing proteins), but, after eons of co-evolution, they also rely on numerous proteins supplied by the host (plants or algae cells). The host cell genome supplies close to 90% of the proteins the chloroplast needs for photosynthesis. The exchange of proteins and other molecules between chloroplasts and their host cells is nothing short of the World postage system. They are so connected that pulling a chloroplast out of the cell and keeping it alive would seem impossible, much like removing an organ out of your body. A small animal, however, found how to do just this.
The kidnapping of the sun factory
The larval life of the sea slug Elysia chlorotica is nothing extraordinary. The larvae feed on phytoplankton and digest it completely to obtain its nutrients; much like many other citizens of the blue world do. When the mollusk develops into an adult, however, its diet changes to eat the alga Vaucheria litorea, a coastline algae present in the waters of eastern North America. The slug punctures the alga and sucks the contents as if it was using a drinking straw. This time though it does not digest the contents completely, an important cell component is left in one piece. The slug's digestive system enzymes learned how to preserve the chloroplasts, and only those, intact; what's more impressive, it incorporates them into its own gut cells where they will remain thereafter, for weeks and months, functioning as if in a plant or algal cell.
The slug feeds from the alga until it accumulates a vast amount of chloroplasts that turn the slug into a vivid green and enable it to capture sun's light energy. Studies, such as a research published recently in Nature by an international team of scientists, have shown that the slug can survive for months without eating and, apparently, relying only on photosynthesis-made nutrients. The benefit might be not only nutritional. No having to look for food gives the slug more time for other important activities, like mating. Additionally, the slug's green color endows it with a camouflage that easily confuses predators. The creature also looks like a leaf, maybe both for camouflaging and to improve its light capture efficiency. One might wonder if the leaf-looking slug would have an issue with herbivores. Perhaps, they would be disappointed (and disgusted) at the first bite on the slippery, jelly textured, quivering slug and learn how to avoid the disguised bug.
Other members of the genus Elysia can also photosynthesize.
The mammoth question scientists have asked for a long time is how the slug manages to keep the chloroplasts alive? After all, it is known that chloroplasts depend on proteins made by plant cells thank billions of years of joined evolution. One possibility is that the slug has acquired genes from algae using a process called lateral gene transfer or LGT. As the search goes on for those genes in the slug's genome, contrasting results are not providing yet a definite answer to this question. Could the slug be even more remarkable than that? As a high-profile burglar equipped with custom-made tools, could it be possible that the mollusk is manufacturing its own, slug-style, chloroplast-supporting proteins not known in the plant or algae Kingdoms?
As we continue to be amazed by the extraordinary capacities of this small inhabitant of the seas, the question arises on what are these findings teaching us?
Why is a tiny sea critter relevant to us?
Every day, we discover new facets of life, strange, unfamiliar revelations redefining the very meaning of it.
The classical view of unitary living organisms, their integrity limited by the boundaries of their genome, have only recently been shattered. We now understand that our own well-being depends not only on the part of our body codified by our genome but also on the command of trillions of microorganisms that live in concert with us, the other side of us. How much power do they have? We still don't know. But with new discoveries, their contribution to our persona seems to grow daily.
The bacteria in our guts, skin, and other organs—our microbiome—have shown to engender dramatic influence on many of our own biological processes. For example, when gut bacteria from obese mice were transplanted to normal controls it led to the increase of fat in them. In human medicine, it is already common practice to use fecal transplants loaded with "good" bacteria from healthy donors to cure digestive tract diseases, particularly, illnesses associated with complications from antibiotic therapy. This last, a treatment that kills not only pathogens but also beneficial bacteria.
Moreover, the human genome already contains evidence of lateral gene transfer from bacteria. Recent studies show that these transfers occur often between bacteria and our—or other mammals'—somatic cells (cells not involved in reproduction). How these gene transfers take place is still unknown. Learning their mechanism is enormously important because those transfers can produce harmful mutations and represent an unrecognizable source of diseases like cancer and diabetes.
Gene transfers to human genome are not limited to microorganisms like bacteria. Recently, Cambridge University scientists found that humans evolved with genes acquired not only from single cell microorganisms but also from fungi and plants, and not just a few genes. As much as 1% of our genome has been obtained from other species. What's more, while the process is still ongoing, one thing is clear: we know from very little to nothing about the role of that other side of us.
The human genome
One reason for our ignorance is the lack of traceability of the inherited processes. All organisms on Earth share multiple biochemical functions making it difficult to pinpoint the source of variations in certain metabolic activities. Here is where the discovery of our slug's quirky physiology, comes into play. Animals do not photosynthesize, they do not have chloroplasts either. The slug's traits could enable to track down specific genes not present in other animal species and discover how they got there. Therefore, perhaps Elysia chlorotica, and few other slugs of the same genus, could serve as unequivocal models for the study of LGT and acquired functions. Maybe, in the next few years, this tiny sea critter will help us to answer some of the most intriguing questions of human biology. The journey could not be more exciting.
Will the study of photosynthesizing sea slugs contribute to understand better human biology?
© 2018 Jorge Cruz