Researchers have successfully modified animal cells to perform photosynthesis, a remarkable achievement that pushes the limits of biology. A team led by Professor Sachihiro Matsunaga from the Graduate School of Frontier Sciences at the University of Tokyo has reported that they have integrated chloroplasts from the alga Cyanidioschyzon merolae into cultured mammalian cells through co-cultivation, and they’ve observed electron transfer during photosynthetic processes in these cells.

Photosynthesis is the biochemical process through which green plants, algae, and certain bacteria transform light energy, usually from the sun, into chemical energy stored as glucose. This process mainly occurs in the chloroplasts found in plant cells and involves chlorophyll absorbing light. During photosynthesis, carbon dioxide (CO₂) from the air and water (H₂O) from the ground are utilized to create glucose (C₆H₁₂O₆) and oxygen (O₂) as a byproduct. This process is essential for life on Earth since it serves as the primary source of organic material for nearly all living organisms while releasing oxygen into the atmosphere. Imagine if this process occurred in animal cells; it’s fascinating, right?

Animal cells lack chloroplasts, unlike plants and algae; as a result, it was once believed that it was impossible to integrate chloroplasts (the structures containing chlorophyll found in plant and algal cells) into animal cells and that these chloroplasts would not survive or function properly. Research on endowing animal cells with photosynthetic capabilities by incorporating chloroplasts was undertaken from the 1970s to the 1980s. Still, it was halted due to the deterioration of the added chloroplasts. Nevertheless, this study demonstrated that photosynthetic activity persisted for at least two days.

The research team aimed to create “planimal cells” that can photosynthesize. These cells would consume less oxygen and produce less carbon dioxide. Additionally, they intended to develop miniature oxygen generators for various applications. “If we can even achieve part of the photosynthesis process in animal cells, we can reduce the amount of oxygen and sugar consumed, as well as the amount of carbon dioxide emitted,” says Matsunaga.

Matsunaga’s team searched for chloroplasts that could survive the warmer environment of animal cells, usually cultured at around 37 degrees Celsius, significantly higher than most plant chloroplasts can endure. The team faced another challenge in ensuring that animal cells did not perceive them as foreign substances. To tackle this issue, they explored a creative strategy to stop the animal cells from eliminating them. Various animals eat plants and, while doing so, ingest chloroplasts. When chloroplasts are voluntarily taken up as “food” by animal cells, they can survive for a certain period.

Matsunaga and his team utilized this concept and successfully integrated chloroplasts into animal cells. As a result, the animal cells accepted the chloroplasts and demonstrated an increased growth rate. This suggests that the chloroplasts provided an additional source of energy.

Super-resolution fluorescence microscopy image

Next, they explored a technique to integrate isolated chloroplasts into CHO cells, cultured hamster-derived animal cells used for general research. The research team established the culture medium conditions necessary for effective phagocytosis by CHO cells and devised a method to introduce chloroplasts into intact animal cells through co-cultivation. In the past, chloroplasts were introduced into cells by creating openings in the cell membrane. Additionally, the overall efficiency of phagocytosis by CHO cells was generally low under standard culture conditions.

Maintaining the survival of chloroplasts in animal cells for extended periods is still a significant challenge. If you’re curious about humans being able to photosynthesize, this continues to be a far-off aspiration. Nevertheless, with advancements in this technology, it may become feasible to satisfy the oxygen requirements of particular organs.

Matsunaga said: “We expect planimal cells to be game-changing cells, which in the future can help us achieve a ‘green transformation’ to a more carbon-neutral society. We will continue to develop innovative biotechnologies to realize a sustainable society and the reduction of carbon dioxide emissions.”

Matsunaga’s research signifies a profound shift in our understanding, revealing the incredible potential of engineering animal cells to harness photosynthesis. This groundbreaking approach could lead to transformative innovations, from lab-grown meat to revolutionary medical treatments. While further work is needed to optimise chloroplast function in these cells, Matsunaga’s team has ignited a beacon of possibility, challenging conventional wisdom in biology and inspiring us to rethink the limits of what cells can achieve.