Microorganisms- The catalysts of aquatic chemical reactions…

Microorganisms, bacteria, fungi and algae are living catalysts, which help a large number of chemical processes to occur in water and soil. A large number of important chemical reactions in aqueous medium, particularly those involving organic matter and redox processor, occur through bacterial intermediates, algae are the primary producers of biological organic matter (biomass) in water. Microorganisms lead to the formation of many sediment and mineral deposits.

 

Fungi and bacteria are classified as reducers, which decompose chemical compounds to simpler species and thereby, drive the energy requirements for their growth and metabolism. On the other hand, algae are classified as producers. They utilize sunlight; they are the stored chemical energy for metabolic requirements, thus algae serves as aquatic solar fuel cells.

 

Microbially Mediated Redox Reactions

 

Bacteria mediate a variety of redox reactions and thereby drive the energy needed for their metabolic processes and reproduction. Some environmentally important redox reactions of this category are given below:

 

Oxidation: -

 

HCHO + H₂O ® CO₂ ­+ 4H⁺ +4e^-

HCOO^- ® CO₂ +H⁺+2e^-

HS^- + 4H₂O ® SO₂^- +9H⁺ + 8e^-

NH₃^- + 3H₂O® NO₃^- +10H⁺+8e^-

FeSO_3(s) +2H₂O ® FeOOH_(s) +HCO₃^-+2H⁺+e^-

 

Reduction: -

 

 

O₂­+ 4H⁺ + 4e^- ® 2H₂O

NO₃^- +6H⁺ +5e^- ® ½ N₂­ + 3H₂O

        CO₂­ + 8H⁺ +8e^- ® CH₄­ + 2H₂O                                           

           

               Bacteria are involved in many biogeochemical processes in water and soil and in many important elemental cycles in nature, including those of nitrogen, carbon and sulphur. They help in the formation of many mineral deposits including some of those of iron and manganese. Bacteria are also instrumental in the formation of iron and manganese deposits in natural water systems and pipes used to transport water.

 

Nitrogen Transformations by Bacteria   

              

                      The nitrogen cycle is one of the most important microorganisms mediated chemical reactions in aquatic and soil environments. It is based on four important chemical transformations:

(a) Nitrogen fixation whereby molecular N₂ is fixed as organic nitrogen (mainly by Rhizobium bacteria):

 

3(HCHO) + 2N₂ + 3H₂O + 4H ® 3CO₂ +4NH₄⁺

 

(b) Nitrification which is the process of oxidizing NH₃ to NO₃^- (by Nitrosomonas and Nitrobactor bacteria):

              

               3NH₃ + 2O₂  ® H⁺ +NO₂^- +H₂0

               NO₂^- + ½ O₂ ® NO₃^-    

 

(c) Nitrate Reduction:

 

2NO₃^- +4HCHO ® 2NO₂^- +4H₂O +4CO₂

 

(d) Denitrification which involves the reduction of NO₃ and NO₃^-followed by recycling of N₂ to the atmosphere:

 

4NO₃ ^-+ 5(HCHO) +4H⁺ ® 2N₂­ + 5CO₂ + 7H₂O

 

Iron and Manganese Bacteria

 

Some bacteria e.g. Ferrobacillus, Gallionella, Sphaerotilus, utilize iron compounds to drive energy for their metabolic processes. These bacteria serve as catalysts for the oxidation of Fe (||) to Fe (|||) by O_2.

 

4Fe²⁺ + 4H⁺ +O₂ ®  4Fe³⁺ +2H₂O

 

The carbon source for some of these bacteria is CO₂ as they do not depend on organic matter for carbon and draw their energy from the oxidation of organic matter. It is found that large deposits of hydrates iron (|||) oxide from in area where iron oxidizing bacteria flourish.

Some of the iron bacteria, specially Gallionella, secrete large quantities of hydrated Fe₂O i.e. Fe₂O .3H₂O or 2Fe(OH), of branched structures. The bacterial cell grows at the end of a twisted stalk of Fe(OH)₃.

 

The manganese cycle in the ocean is strongly influenced by bacteria. The well known manganese nodules, an important source of Mn, Cu,  and Co, give different species of bacteria which mediate both the oxidation  and reduction of Mn. These reactions are governed by enzymes and sea water cations e.g. Ca⁺ and Mg²⁺.