Bacteria are unicellular micro-organisms that are found everywhere on earth. They are classified under prokaryotes. Prokaryotes are organisms which do not have a membrane bound nucleus unlike eukaryotes which possess a well formed nucleus. Though the bacteria do not possess the nucleus, they have genetic material in their DNA. The bacterial DNA is found in their cytoplasm.
Though bacteria can cause diseases, only a small percentage of bacteria do so. The rest of the bacteria are harmless. The pathogenic bacteria are called parasitic bacteria and they thrive on the nutrition of other living matter for their survival. On the other hand, the saprophytic bacteria lead a symbiotic relationship with other living matter and do not cause any harm.
Bacteria can be classified under various categories. They can be classified according to shape, nutritional and oxygen requirements, staining characteristics, motility, presence of cell wall and spore formation. Based on the nutritional requirements, they are classified in to:

Heterotrophic bacteria
Heterotrophic bacteria grow on organic substances formed by animals and plants. They break down complex organic substances from these sources and derive carbon. Heterotrophic bacteria are divided in to –

Photosynthetic heterotrophs- These utilize the energy of light
Chemotrophic heterotrophs – These obtain energy from oxidation reduction reactions.

The heterotrophic bacteria secrete enzymes which act on the complex organic substances and derive energy. Most of the heterotrophic bacteria are aerobic organisms and they need oxygen for their survival. Based on their relation with the energy sources they are again divided in to –

Saprotrophic bacteria- These bacteria derive nutrition from dead tissues. These are the common types of heterotrophs
Saprophytic bacteria – These bacteria live in close association with another species like plants and both of them get benefited. Most of the saprophytic bacteria live in association with the roots of the plants. The bacteria convert the atmospheric nitrogen in to nitrates which is utilized by the plants which in turn supply carbohydrates to the bacteria for their survival.
Parasitic bacteria – These bacteria live in close association with another species like plants and animals and derive their energy in expense of them.

Heterotrophs exhibit two basic strategies for deriving energy from the organic substrates –

Fermentation – In this process, the organic substrate acts as both electron acceptor and electron donor. There is no requirement of oxygen or other electron acceptor. Fermentation yields lesser energy (58 kcal/mole)
Respiration – This process requires external electron acceptor. The most common electron acceptor is molecular oxygen. When oxygen is used, the process is called aerobic respiration. If nitrates and sulphates are used, the process is called anaerobic respiration. Respiration yields more energy (686 Kcal/ mole). Most of the carbon in the atmosphere is derived by the process of respiration. 40%-80% of the respiration occurs under the soil.

Autotrophs
Autotrophic bacteria are organisms whose sole source of nutrition is carbon dioxide. They need only water, inorganic salts and carbon dioxide for growth. They are divided in to –

Photosynthetic autotrophs – These bacteria utilize energy from light and are anaerobic organisms. This common among the two types and are quite diverse. These include cyanobacteria, green sulfur bacteria, purple sulfur bacteria, and purple non-sulfur bacteria. Suphur bacteria utilize hydrogen sulphide whereas others utilize water for nutritional process.
Chemosynthetic autotrophs – These derive energy from oxidation of inorganic materials like iron, sulphur, ammonia and nitrite. These organisms are anaerobic. These are usually found deep under the water and they are responsible for the food chain under the water. They derive the inorganic substrates from the volcanic vents.

Soil biology
Plants are terrestrial organisms and they obtain most of their nutrients from the soil. Animals derive energy indirectly from plants and other animals. This requires utilization of many elements such as inorganic compounds from the environment which are converted in to organic substances before they can be used as energy. The cycling of these elements is catalyzed by the bacteria present in the soil.
Soil is the upper most layer of earth which varies in depth from a few inches to over twenty feet. The type of the soil depends on the four factors including the parent rock, the climate, the age and the biological factors like plants, bacteria and organic substances. Most of the biological activity occurs at the surface. Most of the organic substances are found in the upper layer or the upper horizon. Majority of the bacteria are found in the upper six to twelve inches of soil. Both autotrophic and heterotrophic bacteria are found in the soil. Among the two heterotrophic bacteria are more commonly found in the soil. The heterotrophic bacteria in soil belong to the order Eubacteriales and Actinomycetales. The Actinomycetales are represented by the genera Streptomyces, Nocardia and Micromonospora. These bacteria have an earthy odour.
Maximum bacterial growth and activity in the soil are found around the roots of the plants. This region is called ‘rhizosphere’. Almost all the ecological interactions occur in this region. These interactions could be favorable, unfavorable, indispensable and sometimes lethal. The most important function of the soil microorganisms is to decompose various kinds of organic matter.
As said earlier, cycling of elements is done by bacteria. This is done to make the elements available for reuse. The elements are usually found in the reduced state inside the cell. When they are mineralized, they are in an oxidized state. They serve three functions –

They form the basic components of the cell
They provide energy
They act as electron acceptors during oxidation reactions

Bacteria are involved in three types of cycles in the cycling of the elements necessary for energy –

Carbon cycle
Nitrogen cycle
Sulphur cycle

Carbon cycle
About 50% of the dry weight of all living organisms is composed of carbon. The ultimate source of this organic carbon is the atmospheric carbon dioxide.
Role of heterotrophs

Reduce carbon dioxide by photosynthesis
Fix carbon dioxide from preformed organic compounds
Decompose the dead tissues of animals and plants and release the elements to be used again

Role of autotrophs

Reduce carbon dioxide by photosynthesis
Utilize carbon dioxide from dissolution of carbonates and bicarbonates

Nitrogen Cycle
Nitrogen is the most important structural element of all living organisms. Though it is abundant in nature, it cannot be utilized by the plants and animals. The nitrogen cycle is concerned with incorporation of atmospheric nitrogen and organic nitrogen of dead plants and animals on to forms that are usable by higher organisms. Bacteria, both heterotrophs and autotrophs are involved in this cycle.
There are five processes of nitrogen cycle:

Ammonification – The dead animals and plants are decomposed by the heterotrophs which release protein and other nitrogenous substances. These substances are broken down in to amino acids which are split to release ammonia. This release of ammonia from organic nitrogenous substance is called ammonification
Nitrification – The oxidation of ammonia in to nitrates is called nitrification. This takes place by two steps. The first step called nitrosification results in the formation of nitrites which are toxic to plants. The second step is oxidation in which the toxic nitrite is converted in to the non-toxic nitrates.
Nitrate reduction – The reversal process of nitrification is called nitrate reduction. The nitrate is converted in to nitrate which in turn is converted in to ammonia. Many organisms can assimilate cellular nitrogen from this ammonia.
Denitrification -Certain microorganisms are capable of reducing nitrates in to nitrites and subsequently to gaseous nitrogen. This process is called denitrification.
Nitrogen fixation – Nitrogen fixation is a process by which the bacteria both autotrophs and heterotrophs fix atmospheric nitrogen. It is symbiotic of it is done by the bacteria living in the plant roots and non-symbiotic if it is done by bacteria independently.

Role of heterotrophs in nitrogen cycle

Primarily responsible for ammonification
Cause nitrate reduction
Involved in denitrification
Involved in symbiotic nitrogen fixation

Role of autotrophs

Involved in ammonification under anaerobic conditions. The amine that is formed is oxidized to release ammonia
Exclusively involved in nitrification. The family of autotrophs called Nitrobactericeaa is involved in this reaction
Cause nitrate reduction
Involved in denitrification
Involved in non -symbiotic nitrogen fixation

Sulphur cycle
Sulphur is essential for all living organisms as sulphur containing amino acids are present in all proteins. It occurs in both organic and inorganic combinations. Cycling of sulphur is similar to nitrogen cycle. Transformation between organic and elemental states and between oxidized and reduced state are carried out by bacteria.
Role of heterotrophs
Heterotrophs in the soil degrade the proteins in the soil and liberate sulphur.
Role of autotrophs
Autotrophs oxidize various forms sulphur like hydrogen sulphide. The phototrophic autotrophs oxidize hydrogen sulphide in to elemental sulphur. The chemosynthetic autotrophs oxidize sulphur in to sulphates. Sulphate is the most suitable source of sulphur for plants. This sulphate is assimilated and converted in to proteins.
Fresh water biology
Fresh water is defined as water containing less than 1% of salt. The different fresh water zones include – ponds and lakes and streams and rivers.
Ponds and lakes
Ponds and lakes consists of three zones:

Uppermost littoral zone- This zone absorbs direct sunlight and contains algae, snails, insects, crustaceans and fishes.
Middle limnetic zone – This contains planktons
Lower profundal zone- This zone contains heterotrophic bacteria. Very little enter this zone and hence it is very ideal for growth of the heterotrophs decompose the dead organisms by using oxygen through the process of respiration.

Streams and rivers
These are water bodies which move in one direction. The characters of these water bodies change during their course. There are three parts of these water bodies – source, mid stream and mouth. The place where they reach and join another water source including ocean is called the’ mouth’. The source of these water bodies contains high oxygen levels and hence the heterotrophs thrive here very well. Near the mouth, the light penetration and the oxygen content are very less and autotrophic bacteria are found in this region.
Both heterotrophs and autotrophs exist and survive in the fresh water bodies. The processes are very similar to those that occur in the soil. In the fresh water bodies, these bacteria take care of the inorganic requirements of the algae which is similar to the plants on the soil. These organisms get in to these water bodies from air, soil, sewage, organic wastes, dead plants and animals. Climatic, geographical and biological conditions bring about great variation in the bacterial population. Rivers and stream show their highest count during rainy season. Dust blowing in to the rivers and streams also contributes to many bacteria. Animals contribute to the bacterial population by bathing and dropping their excreta.
Among the various cycling of elements, the nitrogen cycle is most common one occurring in the water bodies.
Role of heterotrophs

They act on the dead animals and plants and other organic wastes and liberate nitrogen
They can cause nitrate reduction resulting in the release of ammonia

Role of autotrophs

The nitrosomonas and nitrobacter species cause nitrification process
Involved in ammonification under anaerobic conditions. The amine that is formed is oxidized to release ammonia