briefly describe autotrophic modes of nutrition in bacteria?

Autotrophic Bacteria
These are bacteria which are able to synthesize their own organic food from inorganic substances. They use carbon dioxide for obtaining carbon and utilise hydrogen sulphide (H₂S) or ammonia (NH₃) or hydrogen (H₂) as the source of hydrogen to reduce carbon. These bacteria can be distinguished further into two types as follows

Photoautotrophic bacteria

The photoautotrophic bacteria possess photosynthetic pigments in membrane bound lamellae (or thylakoids) and utilise solar energy. The bacterial photosynthesis is different from that of green plants since here water is not used as a hydrogen donor. Hence oxygen is not released as a byproduct. For this reason, the process is described as anoxygenic photosynthesis.

Chemosynthetic Bacteria

These are bacteria which manufacture organic compounds from inorganic raw materials utilising energy liberated from the oxidation of inorganic substances. Following are the common types of chemo autotrophic bacteria.

1. Nitrifying bacteria which derive energy by oxidizing ammonia into nitrates. Eg: Nitrosomonas, Nitrobacter.

2. Sulphur bacteria which derive energy by oxidising hydrogen sulphide to sulphur. Eg: Thiobacillus, Beggiatoa.

3. Iron Bacteria which derive energy by oxidising ferrous ions into ferric form. Eg: Ferrobacillus, Gallionella.

Symbiotic Bacteria

1. The most familiar example of symbiotic bacteria are the nitrogen fixing bacteria found in the root nodules of leguminous plants. Bacteria such as Rhizobium and Pseudomonas reside in the root nodules and reduce atmospheric nitrogen directly to ammonia. This becomes the source of nitrogen for the host plants. The plants in return provide bacteria with nutrients and protection.

2. The bacteria found in the human alimentary canal Escherichia coli are nonpathogenic. These bacteria check the growth of harmful putrefying bacteria. In addition, these bacteria release vitamins K and B12 which are necessary for blood components. The human host provides shelter and food for these bacteria

Parasitic Bacteria

These are bacteria which occur in the body of animals and plants, obtaining their organic food from there. Most of these bacteria are pathogenic, causing serious diseases in the host organisms either by exploiting them or by releasing poisonous secretions called toxins.