Nutritional types of microorganisms:
In addition to Carbon, hydrogen and oxygen all organisms require sources of energy and electrons for growth.
Carbon sources:
Autotrophs - CO2 sole or principal biosynthetic carbon source
Heterotrophs – reduced, preformed organic molecules from other organisms.
Energy sources:
Phototrophs – use light as their energy source.
Chemotrophs – obtain energy from the oxidation of chemical compounds (either organic or in organic)
Electron sources:
Lithotrophs – use reduced inorganic substances as their electron source.
Organotrophs – extract electrons from organic compounds.
Four major nutritional classes based on their primary sources of carbon, energy and electrons is known.
Phtotolithotrophic autotrophs or photoautotrophs or photolithoautotrophs:
Source of energy – light energy
Source of electrons – Inorganic hydrogen/ electron
Carbon source - CO2
Example: Algae, purple and green sulfur bacteria and cyanobacteria.
Photoorganotrophic heterotrophy or photoorganoheterotrophy:
Source of energy – light energy
Source of electrons – organic hydrogen/ electron
Carbon source – organic carbon sources (CO2 may also be used)
Example: Purple and green nonsulfur bacteria (common inhabitants of lakes and streams)
Chemolithotrophic autotrophs or chemolithoautotrophy:
Source of energy – Chemical energy source (inorganic)
Source of electrons – Inorganic hydrogen/ electron donor
Carbon source - CO2
Example: Sulfur-oxidizing bacteria, hydrogen bacteria, nitrifying bacteria, iron-oxidizing bacteria.
Chemoorganotrophic heterotrophs or chemoorganoheterotrophy:
Source of energy – Chemical energy source (organic)
Source of electrons – Inorganic hydrogen/ electron donor
Carbon source – organic carbon source
Example: Protozoan, fungi, most non-photosynthetic bacteria (including most pathogens)
The most common nutritional types are photolithoautotrophs and chemoorganoheterotrophs. Bacteria Beggiatoa rely on inorganic energy sources and organic (or sometimes CO2) carbon sources. These microbes are sometimes called Mixotrophic because they combine chemolithoautotrophic and heterotrophic metabolic processes.
Requirements for nitrogen, phosphorous and sulfur:
Nitrogen is needed for the synthesis of amino acids, purines, pyramidines, some carbohydrates and lipids, enzyme cofactors and other substances. Most phototrophs and many nonphotosynthetic microorganisms reduce nitrate to ammonia and incorporate the ammonia in assimilatory nitrate reduction. A variety of bacteria like many Cyanobacteria and Rhizobiium can reduce and assimilate atmospheric nitrogen using the nitrogenase systems. Phosphorous is present in nucleic acids, phospholipids, ATP, several cofactors, some proteins and other cell components. All microorganisms use inorganic phosphate as their phosphorous source and incorporate it directly. E.coli can use both organic and inorganic phosphate. Organophosphates such as hexose 6- phosphate can be taken up directly by transport proteins. Other organophosphates are often hydrolyzed in the periplasm by the enzyme alkaline phosphatase to produce inorganic phosphate which is then transported across the plasma membrane. When inorganic phosphate is outside the bacterium, it crosses the outer membrane by the use of a porin protein channel. Sulfur is needed for the synthesis of substances like the amino acids cysteine and methionine, some carbohydrates biotin and thiamine. Most of them use sulfate as a source of sulfur and reduce it by assimilatory sulfate reduction; a few require a reduced form of sulfur such as cysteine.