(a)

(i) Microbes in biogas production

• Biogas is a mixture of gases produced by the microbial activity of methane producing bacteria collectively called methanogens. 
• Methanobacterium is one such common methane producing bacteria found in the anaerobic kludges during sewage treatment. 
• In the first stage aerobic bacteria convert complex organic substances present in the biological wastes in to simple molecules. 
• These molecules are degraded anaerobically to produce organic acids like acetic acids. 
• Methanogens under anaerobic conditions convert acetic acid in to methane and carbon dioxide. 

(ii) Micorrhiza as biofertilizers 

1. It is a symbiotic association of fungus with the roots of higher plants. 

Example : Gomus species form mycorrhiza; Fungal symbiont absorbs phosphorus from soil and made available to plants. 

2. This association also makes the plant to show resistance to root borne pathogens, tolerance to salinity and drought thereby increase in growth and development.

(b) Azotobacter or Azospirillium are free living bacteria fix atmospheric nitrogen.