1. Photoautotrophic bacteria : These bacteria are capable of entrapping solar energy and utilizing it for the synthesis of complex food materials due to the presence of pigments like bacteriochlorophyll (bacteriopurpurin) and bacterioviridin.
Purple sulphur bacteria (e.g., Thiospirillum) and green sulphur bacteria (Chlorobium limicola) are the most familiar examples containing pigment bacteriochlorophyll, bacteriopurpurin and bacterioviridin respectively. Bacterial photosynthesis, however, differs from photosynthesis of higher plants in not liberating oxygen. This type of photosynthesis, characteristic of bacteria , is termed as anoxygenic. Normal photosynthesis , occurring in higher plants, is termed as oxygenic.
In bacterial photosynthesis water is not the source of electron that acts as reducing power to convert CO2 into glucose. The bacteria obtain reducing power from various compounds such as hydrogen sulphide thiosulphate or even some organic compounds. No oxygen is evolved as it does not involve splitting of water.
Hydrogen released by various compounds mentioned above is picked up by NAD+ which gets reduced to NADH2 acting as reducing power. NADH2 along with ATP, produced generally by entrapping solar energy are used to reduce CO2 to glucose. Simple equation for anoxygenic photosynthesis may be written as follows :
ii) Chemoautotrophic Bacteria : Bacteria belonging to this category obtain energy for the synthesis of food by oxidizing certain inorganic substances like ammonia, nitrates, nitrites, ferrous ions etc. thus they do not utilize light as energy source. The chemical energy thus obtained, is trapped in ATP molecules. This energy is then used in carbon assimilation with the help of hydrogen from some source other than water, e.g., hydrogen bacteria, nitrifying bacteria, sulphur bacteria , etc. they play a great role in recycling nutrients like nitrogen, phosphorous, iron, sulphur.
II) Heterotrophic bacteria : These bacteria are most abundant in nature and are incapable of synthesizing their own food from simple raw materials. They obtain nourishment either from dead and decaying organic matter or directly from a living host. All heterotrophic bacteria are segregated into three main categories, i.e., saprophytic, symbiotic and parasitic forms.
i)Saprophytic bacteria : they are free living bacteria, obtaining nourishment from organic remains such as dead animals, animal excreta, fallen leaves, decaying vegetables, fruits, bread and other products of animal and plant origin.
These bacteria secrete digestive enzymes into the substrate and the complex insoluble substances are converted into simple souble compounds like water, hydrogen sulphide, ammonia, CO2 etc. some of the simpler substances are absorbed and assimilated by the bacteria, whereas the others are added to the soil and atmosphere to complete the nature’s material cycle. Anaerobic breakdown of carbohydrates and proteins is termed fermentation and putrefaction respectively. Aerobic breakdown of organic compounds is called decay.
ii) Symbiotic bacteria : They are mainly Gram negative type. A familiar example of symbiotic bacteria is Rhizobium leguminosarum, associated with roots of leguminous plants. They are capable of fixing atmospheric nitrogen as ammonia, inside the nodule only and not in free state. However, some bacteria like Azotobacter, Beijerinckia, Klebsiella are free living, aerobic and capable of nitrogen fixation in free state, enriching the soil. Clostridium pasteurianum is anaerobic N2 fixing bacteria.
iii) Parasitic bacteria : These bacteria draw nourishment and obtain special organic compounds required for growth from living organism, wither plants or animals, called hosts. The disease causing bacteria are termed pathogenic and the ones not causing any disease are termed as non-pathogenic.