Within the intertidal salt marsh of Martha’s Vineyard, Massachusetts, a peculiar creature swims among the algae. The Elysia chlorotica, or the Emerald Green Sea Slug, is no more than two inches long at maturation and only lives for about eleven months, but this tiny sea slug has offered some major contributions to studies of symbiosis, horizontal gene transfer, and kleptoplasty.
As a miniscule brown larva, the sea slug feeds on Vaucheria litorea, a species of filamentous algae. After enjoying the only meal it will ever eat, the sea slug harvests plastids from its algal prey and incorporates them into its digestive system. Algal genes within the slug’s gut are then expressed, providing the necessary proteins for the slug to maintain these plastids for the rest of its life. The process of harvesting and keeping plastids from the algae is called kleptoplasty, and the slug performs this process only once in its lifetime. Once the slug absorbs these plastids, it turns bright green and unfolds into a flat, leaf shape which helps it to more effectively absorb sunlight. It is able to use the plastids to perform photosynthesis, which provides the slug with sufficient nutrients to stay alive. This means that the sea slug can survive on photosynthesis for the majority of its life and never has to consume another meal again. The Emerald Green Sea Slug is one of the very first examples of a complex multicellular organism performing photosynthesis successfully over a sustained period of time.
This situation is incredibly rare and has been astonishing scientists for several decades. Animals typically require oxygen in order to survive, but these sea slugs are able to produce it instead. There are more than 100 other species of sea slugs which perform this process as well, and scientists have named this group “sacoglossans,” which means “sap-sucking,” and involves the process of grazing algae to harvest and absorb plastids. Sacoglassans suck the chloroplasts directly from the filaments of the algae and incorporate the plant organelles into their digestive tract, where there are cells ready to absorb them. Scientists believe that this process is facilitated by algal genes that are already present within the slugs’ bodies and allow the animals’ cells to integrate DNA from the algal cells. Once the slugs contain the necessary DNA, they are able to reproduce chloroplasts within their own bodies, an ability that no other photosynthesizing animal possesses. Most sacoglossans are able to consume and absorb chloroplasts. However, once they expose their chlorophyll to the sun, they photosynthesize briefly then the chlorophyll bleaches and cannot be reproduced by the animal itself. This means that E. chlorotica have the unique ability to maintain their photosynthetic properties for their entire lifespan whereas other species can only maintain theirs for short periods of time.
The transfer of genetic information between plants and animals can be potentially groundbreaking for the development of new medicines and other scientific discoveries. Horizontal gene transfer (HGT) is known in the medical field to cause antibiotic resistance in bacteria. Bacteria can transfer genetic information to each other to increase their resistance to antibiotics and antibacterial disinfectants. These resilient bacteria grow stronger and can mutate to cause more serious infections which are harder to treat. This process happens regularly in hospitals, where antibiotics and disinfectants are used very frequently, and becomes a pressing threat to human health and safety. In order to combat the issue of antibiotic-resistant bacteria, it is necessary for scientists to understand how horizontal gene transfer works. This is partially why the processes conducted by E. chlorotica are so intriguing to researchers.
Horizontal gene transfer, whether it takes place between bacteria or between algae and a slug, is similar to a symbiotic relationship in which one organism benefits or becomes stronger from the relationship. Bacteria performing HGT creates problems for humans in the form of more severe illnesses or infections, but sea slugs performing HGT can open the door to new medical solutions. If humans could incorporate specific foreign DNA into their own nuclei, even temporarily, they could use nuclear solutions to solve complex medical problems. For example, cancer is a disease which involves the uncontrollable regeneration of cells, which increases the chance of mistakes or mutations within the replicated DNA. When left unchecked, mutations can accumulate and rapidly deteriorate the health of the affected person. If scientists could harness the power of horizontal gene transfer and produce a treatment to induce the process, they could inject an affected person with healthy DNA encoded to slow the proliferation of cells and replace mutated DNA. This process is far more complex than it sounds, which is why it is so important for scientists to get the opportunity to study the HGT in E. chlorotica.
Furthermore, the ability of animals to complete photosynthesis has even more potential implications for the future of science. Imagine that there is a cow that can complete the same biological processes of this species of sea slug. After eating one meal in its life, this cow would be able to receive its energy directly from the sun, which would reduce grazing and greenhouse gas emissions. The cow would absorb more carbon dioxide than it emits, completely revolutionizing the livestock industry. This would also significantly increase the efficiency of food chains and food production. Animals such as humans, which are high on the trophic scale, need to consume a large amount of organic matter in order to absorb enough energy to survive. If humans could photosynthesize, even for a short period of time, the amount of resources needed to maintain the human population would decrease drastically. Along with benefiting the environment and preventing the overuse of natural resources, this could even generate solutions to socio-political issues such as poverty and world hunger.
The biological processes of the Emerald Green Sea Slug are still mystifying scientists, but with more research they could potentially revolutionize the way that humans live and interact with the Earth. The rare ability of this sea slug to perform photosynthesis over the span of its lifetime indicates that there is an incredibly complex biological process occurring which is not characteristic of other members of the Animalia Kingdom. More investigation of this species has revealed that the process could potentially be horizontal gene transfer, which is far more common in bacteria. If scientists can sustain their research of E. chlorotica and HGT, they could possibly come up with revolutionary solutions to long-standing problems such as antibiotic resistance, genetic infirmities, and even climate change. Advancements in biotechnology, coupled with the powers of a two-inch sea slug, have the potential to change the world.
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