Chlorella

Chlorella is a genus of approximately thirteen species of green algae. These organisms belong to the division Chlorophyta. They can exist as single cells or as small groups called colonies.

Each cell is shaped like a sphere, a subsphere, or an ellipsoid. They are very small, measuring only 2 to 10 micrometers in diameter. These cells lack flagella, which are tiny tail-like structures used for swimming. Instead, they rely on specialized parts to survive and grow.

Inside each cell, a single chloroplast performs the work of photosynthesis. The chloroplast is parietal, meaning it lies against the inner side of the cell membrane. This organelle contains green pigments known as chlorophyll-a and chlorophyll-b. Within the chloroplast, a single pyrenoid is surrounded by grains of starch. To reproduce, Chlorella uses a process called autosporulation. This is a form of asexual reproduction where the cell contents divide into two, four, or eight protoplasts. These new units are called autospores. Eventually, the parent cell wall ruptures to release them.

Researchers have identified four specific phases during asexual reproduction in Chlorella ellipsoides. First is the growth phase, where cells increase in size using photosynthetic products. Next is the ripening phase, where cells mature for division. The third is the post-ripening phase. During this stage, a mature cell divides twice, either in light or in the dark. Cells formed in the dark are called dark-to-light phase cells, and they grow in size again. Finally, the division phase occurs when the parent cell wall ruptures to release the unicells. Sulfur is considered essential for this cell division to happen.

Many Nobel Prize-winning scientists have studied Chlorella to understand life. Martinus Beijerinck first described the genus in 1890. In 1931, Otto Heinrich Warburg won the Nobel Prize in Physiology or Medicine for his research on cell respiration. Melvin Calvin later won the Nobel Prize in Chemistry in 1961. He used Chlorella to study the pathways of carbon dioxide assimilation in plants. These studies helped scientists understand how plants turn gas into food. Chlorella also exists in symbiosis with other living things. Certain strains, like K10, live inside the animal Hydra viridissima.

In the mid-20th century, Chlorella was viewed as a potential solution to world hunger. Following World War II, there were fears of a massive human population boom. In 1946, a report suggested the world would need much more food by 1960. Some scientists thought Chlorella could be a highly efficient primary food source. Initial tests at the Stanford Research Institute showed it could convert 20% of solar energy into biomass. When dried, the algae could contain 50% protein. One scientist even predicted that a 1,000-acre farm could produce 10,000 tons of protein annually using only 20 workers.

However, large-scale production proved to be much more difficult than expected. To reach maximum efficiency, the algae required artificial light or shade. Growing it at high productivity required expensive carbonated water. Scientists also discovered that the cell walls were too tough for humans to digest easily. After a decade of study, researchers found it captured only 2.5% of solar energy in sunlight. This was not much better than conventional crops. Because of these high costs and technical hurdles, Chlorella never became a major global food source like soybeans or grains.

Today, Chlorella is mostly used as a niche dietary supplement.

It is sold in various forms, including pills. Some companies have falsely claimed it can treat diseases like cancer. The American Cancer Society states that studies do not support these claims. The U.S. Food and Drug Administration has issued warnings to companies making these false advertisements. Despite this, Chlorella remains important in other areas. For example, the Russian BIOS-3 experiment found that 8 square meters of Chlorella could replace oxygen for one human in a sealed environment. It remains a vital subject for studying how organisms manage carbon dioxide and oxygen.