The nitrifying bacteria are represented by two genera, Nitrosomonas and Nitrobacter. Together these bacteria can accomplish the oxidation of NH₃ to NO₃, known as the process of nitrification. No single organism can carry out the whole oxidative process. Nitrosomonas oxidizes ammonia to NO₂ and Nitrobacter oxidizes NO₂ to NO₃. Most of the nitrifying bacteria are obligate lithoautotrophs, the exception being a few strains of Nitrobacter that will utilize acetate. CO₂ fixation utilizes RUBP carboxylase and the Calvin Cycle. Nitrifying bacteria grow in environments rich in ammonia, where extensive protein decomposition is taking place. Nitrification in soil and aquatic habitats is an essential part of the nitrogen cycle.

Chemoautotrophic sulfur oxidizers include both Bacteria (e.g. Thiobacillus) and Archaea (e.g. Sulfolobus). Sulfur oxidizers oxidize H₂S (sulfide) or S (elemental sulfur) as a source of energy. Similarly, the purple and green sulfur bacteria oxidize H₂S or S as an electron donor for photosynthesis, and use the electrons for CO₂ fixation (the dark reaction of photosynthesis). Obligate autotrophy, which is nearly universal among the nitrifiers, is variable among the sulfur oxidizers. Lithoautotrophic sulfur oxidizers are found in environments rich in H₂S, such as volcanic hot springs and fumaroles, and deep-sea thermal vents. Some are found as symbionts and endosymbionts of higher organisms. Since they can generate energy from an inorganic compound and fix CO₂ as autotrophs, they may play a fundamental role in primary production in environments that lack sunlight. As a result of their lithotrophic oxidations, these organisms produce sulfuric acid (SO₄), and therefore tend to acidify their own environments. Some of the sulfur oxidizers are acidophiles that will grow at a pH of 1 or less. Some are hyperthermophiles that grow at temperatures of 115°C.

Iron bacteria oxidize Fe⁺⁺ (ferrous iron) to Fe⁺⁺⁺ (ferric iron). At least two bacteria probably oxidize Fe⁺⁺ as a source of energy and/or electrons and are capable of chemoautotrophic growth: the stalked bacterium Gallionella, which forms flocculant rust-colored colonies attached to objects in nature, and Thiobacillus ferrooxidans, which is also a sulfur-oxidizing lithotroph.

Fig. 19. Chemoautotrophic or Lithotrophic oxidations. These reactions produce energy for metabolism in the nitrifying and sulfur oxidizing bacteria.

REFERENCES:

Text Books:

1. Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth Edition). Jones and Bartlett Student edition.

2. Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson - Microbiology: An Introduction. Benjamin Cummings.

Reference Books:

1. Lansing M. Prescott, John P. Harley and Donald A. Klein. Microbiology. Mc Graw Hill companies.