Many species of algae have been identified and used as a food and food additives. Algae are a very nutritious food source, which have a long list of health benefits. Species of algae such as Spirulina, Chlorella, Aphanizomenon and Scenedesmus are some of the species of microalgae that have very high nutritional and health benefits.

Spirulina are the most nutritious of all the other species of algae. They are also the most widely grown algae by food industries. Spirulina are blue green microalgae. They can grow in both fresh waters and salt waters. Spirulina contain as high as 70% of proteins, all the spectrum of the vitamin B complexes including vitamin B-12, vitamin A, and vitamin C, antioxidants such as phenolic acids, alpha-carotene and beta-carotene. They also contain different mineral elements such as calcium, magnesium, iron, phosphorus and potassium. Eating spirulina regularly has a lot of health benefits such as prevention and healing of protein malnutrition, healing and immunity from anemia, alleviation of indigestion, glaucoma, cataract and night blindness. They benefit the health by increasing blood hemoglobin, by controlling and prevention of diabetes, pancreatitis, cirrhosis, hepatitis and cancerous cells. Because of their richness in vitamin E, use of cosmetic products made from Spirulina, will improve skin health and appearance. It also heals wounds when applied on cuts. It increases lactation in nursing mothers. It reduces blood sugar and cholesterol levels too.

Chlorella are also microalgae. They are considered the second most nutritious species of algae. Chlorella are green algae, they are almost comparable in their nutritious content with Spirulina, but with slightly less protein content than Spirulina. Chlorella have very high nucleic acid and chlorophyll content. Nucleic acid will help to reduce aging, facilitates cell renewal and growth. Chlorophyll has health benefits by causing cell renewal and growth, wound healing, prevention of infections, removal of bad breath and body odor, anti-inflammation and increases the availability of vitamins A, E, and K. Chlorella are rich with antioxidants, which act as antiaging and wound healer. They are very effective to reduce cholesterol and body fat.

Aphanizomenon are also blue green microalgae like Spirulina, having both nutritional and health benefits. Even though Aphanizomenon are very nutritious and have a lot of health benefits they should be grown or cultured with upmost care by supplying all the optimal growth conditions such as light, water and mineral nutrients, because if they grew under stress conditions they can be extremely lethal poison.   

Scenedesmus are green microalgae. They have up to 60% of protein content, more than that of soybeans. They also contain the whole set of vitamin B complex including vitamin B 12 and vitamin C, more than that of an egg. Due to their high nutritional content they are used to restore to health malnourished children. But daily intake of Scenedesmus should be below 20 to 35 g of dry weight since they have the ability to raise uric acid level enormously.

Microalgae the potential source for biodiesel production

Based on their size algae are divided in to two broad groups, these are Macroalgae and Microalgae. Macroalgae, which commonly called sea weeds are multi cellular algae; some of its species could grow up to 50 meters in diameters. On the other hand Microalgae are microscopic algae. Depending on the particular species both macroalgae and microalgae can grow in fresh water, sea water and seawages. Difference in size is not the only difference between these two groups, they also differ in the form of the compound they store in their cells. Macroalgae mainly store carbohydrates in their cells for energy source, but microalgae store mainly Triacylglycerol in their cells for energy source. Triacylglycerol is the major feedstock in biodiesel production. Therefore for their higher Triacylglycerol content in their cells, microalgae are considered the potential source for biodiesel production.

Microalgae also have other qualities, to be named the number one choice for biodiesel production:

1. They have very fast rate of growth and reproduction than macroalgae and other oil crops. They can be ready to be harvested with in just a few days.
2. They are microscopic and lives in aggregates so they allow different designs of photobioreacors and ponds.
3.  They can grow easily on seawages and utilizes their nutrients from the seawage water.
4. Two types of products can be produced from microalgae these are ethanol and biodiesel.
5. Carbon dioxide from coal power plants and flue gases can be utilised. Flue gases contain in addition to carbon dioxide,  hydrogen nitride, sulphides, sulphur dioxide and other gases. Microalgae utilises the CO2 for photosynthesis, and the other gases as nutrients.
6. Microalgae purifies waste water which comes out from factories and farms such as fertilizers, iron, manganese, zinc, chromium, nickel, cadmium and cobalt. One famous microalgae used for such purpose is Chlorella vulgaris which purifies wastewater from metals.
7. And most of all microalgae have very high Triacylglycerol content which could reach up to 54% in dry weight basis.

Microalgae are classified in to four groups these are Diatoms, Green algae, Golden brown algae and Blue green algae. In total there are more than 200,000 species of microalgae. Although the number of spices of microalgae is so vast, only a few are used for biodiesel purpose. Those microalgae produced for biodiesel production are highly oleogenic producing more than 40% of oil per dry weight basis. Some of the species of microalgae which are growen for biodiesel purpose are Nannochloropsis spp (54%), Botryococcus spp (54%), Nitzschia spp (47%), Botryococcus braunii, Dunaliella salina, Spirulina spp, Dunaliella spp, Scenedesmus spp and Chlorella spp.

For optimum production of biodiesel from microalgae, artificial creation of stress such as nitrogen starvation and environmental stresses such as very high temperature and light intensity will tremendously increase the quantity and quality of the oil produced. And it is the number one strategy to increase the oil yield.

There are two major sources of CO2 these are, compressed CO2 and CO2 from power plants. Compressed CO2 is very expensive to use for large scale biodiesel production. And Injection of CO2 directly from power plants to photobioreactors and ponds have two problems, first it has very high temperature which many species of microalgae can't resist except those of Chlorella spp, and second the gas mixture in addition to CO2 contains other gases such as NO2 and SO2 in very high concentrations which is toxic for the algae. To avoid the high temprature problem the gases need to cool down and reduced to 2% concentration level before injection.