Diatoms 

    Diatoms are freshwater and marine organisms that are important photosynthesizers and are primary producers in the global ecosystem.  They contain three different kinds of pigments: 
Chl a, Chl c and Fucoxanthin, a carotinoid.  They are a major source of atmospheric oxygen. They are unicellular organisms with cell walls made of silicon dioxide and can reproduce both sexually and asexually.  They have extensive fossil records that go back to the Cretaceous period.  Rocks formed from fossil diatoms are known as diatomaceous earth.  

    Diatoms take up inorganic carbon and use energy from the sun to produce carbohydrates.  Carbon dioxide concentrations are low in marine environments and most inorganic carbon is found in the form of bicarbonate (HCO₃)^-. This eventually leads to carbon limitation for marine primary production.  To compensate for this, many algae has a carbon concentrating mechanism (CCM).  The CCM contains the enzyme carbonic anhydrase which converts bicarbonate to carbon dioxide and make it available for fixation by  the enzyme RubisCO.  See Carbonic Anhydrase

  Two groups of diatoms are: the centric diatoms which have a radial symmetry and the  pennate diatoms that are bilaterally symmetrical.  The centrate forms tend to predominate the marine habitats, while the pennate diatoms are more typical of freshwater environments.

Thalassiosira weissflogii (centric)

Thalassiosira weissflogii culture (centric)

  Diatoms provide 40% of the primary production on earth. Studying diatoms and their responses to carbon dioxide levels is important in understanding our future climatic conditions, since they may play a  major role in regulating atmospheric carbon dioxide.  In addition, two thirds of the earth surface is covered with water and the oceans play an important role in the global carbon cycle.  Deep oceans are a very important sink for carbon as dead organic matter (mainly plankton)  rain down from the mix layers of the ocean.   

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