Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 71

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds, which are photosynthetic but lack the many distinct organs found in land plants. Microalgae are organisms that are less than 0.4 mm in diameter and include the diatoms and cyanobacteria and are capable of photosynthesis. Macroalgae are organisms such as seaweed.

Algae are not highly differentiated in the way that plants are, and they lack true roots, stems and leaves, and a vascular system to circulate water and nutrients throughout their bodies. They can exist as single, microscopic cells; they can be macroscopic and multicellular; live in colonies; or take on a leafy appearance as in the case of seaweeds.

The general term algae includes prokaryotic organisms cyanobacteria, also known as blue-green algae, as well as eukaryotic organisms (all other algal species). Algae lack the various structures that characterize land plants, such as leaves, roots, and other organs that are found in vascular plants. Many algae are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.

The majority of algae live in aquatic habitats and these organisms can thrive in freshwater lakes or in saltwater oceans. They can also endure a range of temperatures, oxygen or carbon dioxide concentrations, acidity, and turbidity. An important contribution of algae to the environment and well-being is the generation of oxygen through photosynthesis.

Algal biofuels are a promising replacement for fossil fuels. All algae have the ability to produce energy-rich oils, and several microalgal species naturally accumulate high levels of oil in their dry mass. Moreover, algae are found in diverse habitats and can reproduce quickly and also efficiently use carbon dioxide. Algae help to keep atmospheric carbon dioxide levels stable by storing carbon dioxide in organic materials that include crude oil and inorganic carbonate rocks. Green algae, diatoms, and cyanobacteria are just some of the microalgal species that are considered good candidates for the production of biofuel.

See also: Algae Fuel, Aquatic Plants, Biomass.