Chlorella (Green alga), as food supplement is rich in protein and carbohydrates. It yields about 30% proteins, 15% lipids, 30% carbohydrates and 5% ash. Spirulina platensis (blue-green alga) is one of the richest sources of protein and vitamin $B _{2}$. In long range space exploration, scientist have turned to Chlorella and Spirulina to use them as portable oxygen generators and food sources.

A 250 kg cow produces 200 g of proteins per day. In the same period, 250 g of a microorganisms like Methylophilus methylotrophus because of its high rate of biomass production and growth, can be expected to produce 25 tonnes of protein.

Large quantities of single cell proteins like Spirulina can be grown easily on materials like waste water from potato processing plants, straw, molasses, animal manure and even sewage.

Single cell protein can be obtained from cells of microorganisms such as yeast, fungi, algae and bacteria which are grown on carbon sources for synthesis. SCP is a protein supplement for animals and humans. Example ; Spirulina produces large quantity of food which are rich in protein, carbohydrates, fats, minerals and vitamins. It also reduces environmental pollution. 

Single cell protein means the protein which can be obtained by the culture of single-celled organisms and these proteins are generally used as dietary supplements. Bacteria, algae, fungus all of them are either single-celled structures or the groups have one or more forms which are single-celled. Thus, all of them form sources of single-celled protein.

Spirulina is a cyanobacterium widely used as a source of supplementary protein. It is single-celled and is widely used as an SCP or Single cell protein supplement. 

A- Single-cell proteins (SCP) are produced by a range of microorganisms and substrate. They are dried cells of microorganism. When SCP is produced, the microbial products produce sufficient proteinaceous substances that can be utilized as protein supplements for animal feeds or human foods. It is estimated that algae grown in ponds can produce 20 tons (dry weight) of protein, per acre, per year.

SCPs are alternate sources of proteins for animal and human nutrition produced by a mix culture of microorganisms and cells.

Chlorella is a type of algae that grows in fresh water. The whole plant is used to make nutritional supplements and medicine. It is processed and made into tablets and liquid extracts. These extracts contain "chlorella growth factor," which is described as a water-soluble extract of chlorella containing chemicals including amino acids, peptides, proteins, vitamins, sugars, and nucleic acids. As a medicine, chlorella is used for preventing cancer, reducing radiation treatment side effects, stimulating the immune system, improving response to flu vaccine, increasing white blood cell counts (especially in people with HIV infection or cancer), preventing colds, protecting the body against toxic metals such as lead and mercury, and slowing the aging process.
Chlorella is also used to increase good bacteria in the intestine in order to improve digestion; and to help treat ulcers, colitis, Crohn's disease, and diverticulosis.
Some people also use chlorella for the prevention of stress-related ulcers; treatment of constipation, bad breath, and hypertension; as an antioxidant; to reduce cholesterol; to increase energy; to detoxify the body; and as a source of magnesium to promote mental health, relieve premenstrual syndrome (PMS), and reduce asthma attacks. It is also used for fibromyalgia.
Chlorella is applied to the skin for treating skin ulcers, rashes caused by radiation treatment, and a sexually transmitted disease called trichomoniasis. Chlorella is a good source of protein, fats, carbohydrates, fiber, chlorophyll, vitamins, and minerals. The cell wall of chlorella must be broken down before people can digest it.