Can animals photosynthesise?

We've all learned in elementary school that plants get their energy from photosynthesis by combining water, CO2 and sunlight. This is why leaves are green and all the CO2 actually make up most of the mass if plants can do it, why not animals? Unfortunately, the science tells us the reality is a bit more complex.

Photosynthesis is an enormously complex process that allows plants, algae, and some types of bacteria to harness energy from sunlight and convert it into glucose (sugar) to fuel their growth and development. Chlorophyll, the green pigment found in leaves and stems, plays a key role by absorbing packets of solar energy called photons. The energy absorbed is used to convert carbon dioxide gas and water into oxygen and energy-rich glucose. This glucose then circulates throughout the plant where it is used as an energy source or converted into other organic compounds like starch.

Plants have evolved specialized chloroplast structures within their cells that are packed with chlorophyll and help drive photosynthesis. The intricate membranes and protein complexes found inside chloroplasts allow plants to carry out the light-dependent and light-independent reactions that make up photosynthesis. Without these specialized cellular structures, photosynthesis simply could not occur in plants.

And therein lies the problem for animals - they lack chloroplasts altogether. Animal cells contain mitochondria which help generate energy through cellular respiration, but they do not have the chloroplast infrastructure required to harness sunlight. Some animals like jellyfish contain algal symbionts that can photosynthesize, but the animals themselves do not carry out the process.

The Elysia slug is a rare example of an animal that has evolved the ability to steal the photosynthetic ability of the algae it eats. It pierces the cells of algae with its specialized teeth, called radula, and sucks out most of its contents, leaving the chloroplasts intact. Chloroplast is a special parts of the plant cell where photosynthesis happens. These chloroplasts are then incorporated into the slug's epithelial cells, which line its digestive system and body, making it look even more like a leaf, and giving it its green color.

While more than 70 species of slug steal chloroplasts from their food, Elysia and a few closely related species in the Mediterranean and Pacific stand out for their ability to hold onto the chloroplasts for longer periods of time. This longevity is due to the survival abilities of both the chloroplasts and the slugs. Certain algae's chloroplasts can repair their own light-harvesting systems, making them able to sustain themselves for longer inside the slug. Meanwhile, the slug adjusts its gene expression to improve its relationship with the chloroplasts and removes damaged plastids to avoid accumulation of potentially damaging chemicals. This symbiosis allows the Elysia chlorotica to go up to a year without eating, replying solely on photosynthesis for its source of energy.

Elysia's photosynthetic abilities are not unique to slugs. Organisms such as corals, giant clams, and sponges have symbiotic algae living inside their cells, providing them with organic compounds through photosynthesis. Without the aid of these algae, filter-feeding corals, clams, and sponges would not get enough nutrition in the nutrient-poor tropical ocean, and the coral reefs they build would not exist.

This fascinating adaptation can also work the other way. The most well known example is the Venus flytraps, where it can consume small insects for energy. Another example is the Tripos furca which is a type of algae that can consume several microscopic animals a day, allowing it to survive in darkness for weeks. Other mixotrophic algae exists in parts of the dark ocean, where plants would normally not be able to inhabit.

Nature's adaptation and survival strategies are truly remarkable. The Elysia slug and other mixotrophic organisms are living examples of how organisms can evolve to survive in challenging environments. The fascinating ability to both consume and produce food has allowed these creatures to thrive and has contributed to the diversity of life on our planet.