Photosynthesis, chemosynthesis and nutrient cycle

Photosynthesis
Photosynthesis is the process used by primary producers to manufacture their own food in the presence of light. These organisms possess a green dye, called chlorophyll, which is the molecule that traps sunlight and converts it to chemical energy in chemical bonds of substances called carbohydrates. When these bonds are broken, the energy is released and used in a variety of ways by organisms.

Carbohydrates are assembled from small, simple, low-energy molecules such as water and CO2, to produce large, high-energy molecules (sugar) and oxygen.

6CO2 + 6H2O --> C6H12O6 + 6O2

These large, high-energy molecules are broken down inside living cells during cell respiration to sustain and maintain various organic functions. Photosynthetic marine organisms contribute 92 % to 98% of the oceans total primary productivity.

Chemosynthesis
This is another energy binding process performed by organisms that do not use light to harness energy for living organisms. Instead, because these organisms live in the aphotic zone, they capture energy from breaking down chemical bonds of simple molecules (such as hydrogen sulfide), and use the energy obtained to synthesize carbohydrates from carbon dioxide and water. Chemosynthesis is estimated to contribute 2% to 8% of the ocean's primary productivity.

Biogeochemical  cycle is the  cycling  or flow  of  chemical  elements  (in  various  chemical  forms)  through  the  major  environmental  reservoirs; atmosphere, hydrosphere, lithosphere, and bodies of living organisms. 

v  Nitrogen cycle

v  Silicon cycle

v  Phosphorous cycle

v  Carbon cycle

Nitrogen cycle

Nitrogen is needed by all organisms for the synthesis of protein,  nucleic  acids  and  other  nitrogen  containing  compounds.  Molecular nitrogen makes up almost 80% of the earths atmosphere. For assimilation and use by  plants,  nitrogen  must  be  fixed,  that  is,  taken  up  and  combined  into  organic compounds.  The  activities  of  specific  microorganisms  are  important  to  the conversion  of  nitrogen  to  usable  forms.  The  nitrogen  biogeochemical  cycle  has been shown in figure- 1.

There are three important stages in the nitrogen cycle. These are as follows:

1. Nitrogen fixation

2. Nitrogen assimilation

3. Nitrogen regeneration

3.1. Decomposition of organic nitrogen compounds to yield ammonia

3.2. Nitrification 

3.3. Denitrification 

Nitrogen  fixation

The  conversion  of  molecular  nitrogen  into  ammonia  is known as nitrogen fixation. Certain blue  green  algae (Trichodesmium sp.) have been shown  to  fix nitrogen on a  large scale  in  tropical and  subtropical  waters  using  solar  energy. Other likely nitrogen-fixing genera are Nostoc (through the  marine  forms), Calothrix,  Tolypothrix,  and Rivularia.  The  nitrogen  fixed  by these  plants  is  rapidly  assimilated. The  fixation  is  also  inhibited  if  an  alternative source  of  inorganic  nitrogen is available which the organisms use in preference to molecular nitrogen.

Denitrification

The process of bacterial reduction of nitrate to nitrite, nitrous oxide and molecular nitrogen is called denitrification. Two types of denitrifying bacteria are available in the sea namely: heterotrophs and autotrophs which utilize organic and inorganic  compounds  respectively  as  energy  sources  for  denitrification  in  the absence of oxygen.

Some  denitrifying  bacteria  are  as:  Achromobacter,  Agrobacterium, Bacillus, Pseudomonas, Flavobacterium, Thiobacillus, and Vibrio

The  overall  biochemical  reactions which express  the  process  of  denitrification  is as:

2NO³⁻ ………..>2NO²⁻ ……….>2NO ………>N₂O……….> N₂

Denitrification  is known to occur  in the water column as well as  in the sediments, provided that the dissolved oxygen  concentration  is  below  a  critical  level (<0.1mg/L)  or  is  absent.  Denitrification  leading  to  the  formation  of  molecular nitrogen or nitrous oxide (N₂O) in the sea is effective in balancing deficiency in the nitrogen budget