Nutritional Requirements 123 Organisms that use CO2 as the principal carbon source are defined as autotrophic; organisms that use organic compounds as the principal carbon source are defined as heterotrophic. A combination ofthese two criteria leads to the establishment of four principal categories: (i)photoautotrophic,(ii) photoheterotrophic, (iii)chemoautotrophic and (iv) chemoheterotrophic organisms Photoautotrophic organisms are dependent on light as an energy source and employ CO2 as the principal carbon source. This category includes higher plants, eucaryotic algae, blue green algae, and certain photosynthetic bacteria(the purple and green sulfur bacteria) Photoheterotrophic organisms are also dependent on the light as an energy source and employ organic compounds as the principal carbon source The principal representatives of this category are a group of photosynthetic bacteria known as the purple non-sulfur bacteria; a few eucaryotic algae also belong to it Chemoautotrophic organisms depend on chemical energy sources and employ CO2 as a principal carbon source. The use of CO 2 as a principle carbon source by chemotrophs is always associated with the ability to use reduced inorganic compounds as energy sources. This ability is confined to bacteria and occurs in a number of specialized groups that can use reduced nitrogen compounds(NH3, NO,), ferrous iron, reduced sulfur compounds (HS,S,S2O32), or H, as oxidizable energy sources Chemoheterotrophic organisms are also dependent on chemical energ sources and employ organic compounds as the principle carbon source. It is characteristic of this category that both energy and carbon requirements are supplied at the expense of an organic compound. Its members are numerous and diverse, including fungi and the great majority of the bacteria The chemoheterotrophs are of great commercial importance. Thi category may be subdivided into respiratory organisms, which couple the oxidation of organic substrates with the reduction of an inorganic oxidizing agent(electron acceptor, usually O2), and fermentative organisms, in which the energy yielding metabolism of organic substrates is not so coupled. In addition to an energy source and a carbon source, the microorganisms require nutritional factors coupled with essential and trace elements that combine various ways to form cellular material and products Since photosynthetic organisms(and chemoautotrophes)are the only net producers of organic matter on earth, it is they that ultimately provide, either directly or indirectly, the organic forms of energy required by all other organismsNutritional Requirements 123 Organisms that use CO, as the principal carbon source are defined as autotrophic; organisms that use organic compounds as the principal carbon source are defined as heterotrophic. A combination ofthese two criteria leads to the establishment of four principal categories: (i) photoautotrophic, (ii) photoheterotrophic, (iii) chemoautotrophic and (iv) chemoheterotrophic organisms. Photoautotrophic organisms are dependent on light as an energy source and employ CO, as the principal carbon source. This category includes higher plants, eucaryotic algae, blue green algae, and certain photosynthetic bacteria (the purple and green sulfur bacteria). Photoheterotrophic organisms are also dependent on the light as an energy source and employ organic compounds as the principal carbon source. The principal representatives of this category are a group of photosynthetic bacteria known as the purple non-sulfur bacteria; a few eucaryotic algae also belong to it. Chemoautotrophic organisms depend on chemical energy sources and employ CO, as a principal carbon source. The use of CO, as a principle carbon source by chemotrophs is always associated with the ability to use reduced inorganic compounds as energy sources. This ability is confined to bacteria and occurs in a number of specialized groups that can use reduced nitrogen compounds (NH,, NO,), ferrous iron, reduced sulfur compounds @I,S, S, S,03,-), or H, as oxidizable energy sources. Chemoheterotrophic organisms are also dependent on chemical energy sources and employ organic compounds as the principle carbon source. It is characteristic of this category that both energy and carbon requirements are supplied at the expense of an organic compound. Its members are numerous and diverse, including fungi and the great majority of the bacteria. The chemoheterotrophs are of great commercial importance. This category may be subdivided into respiratory organisms, which couple the oxidation of organic substrates with the reduction of an inorganic oxidizing agent (electron acceptor, usually O,), and fermentative organisms, in which the energy yielding metabolism of organic substrates is not so coupled. In addition to an energy source and a carbon source, the microorganisms require nutritional factors coupled with essential and trace elements that combine in various ways to form cellular material and products. Since photosynthetic organisms (and chemoautotrophes) are the only net producers of organic matter on earth, it is they that ultimately provide, either directly or indirectly, the organic forms of energy required by all other organisms. [l]