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carbon cycle
    Photolysis of dissolved organic carbon
  Arctic ponds  
  Physicochemical portrait of a pond  
  Microbial life
  Photolysis of dissolved organic carbon  
  Implications for climate change  

Photolysis is the process of chemical decomposition by light of dissolved organic carbon into smaller molecules (including CO2). Consequently, photolysis is of greater importance in shallow ponds with little turbidity, as they have greater exposition to sunlight.

The absorption coefficient of light by water at a wavelength of 320 nm ((1 nm = 1 thousandth of a micrometer), named a320, is used to measure the coloured fraction of dissolved organic carbon (that which is broken down by light).

Photolysis is measured with the help of a spectrophotometer, by examining the decrease of a320 over time as sunlight is having its effect.

Water samples taken in a pond in July 2007 were filtered to remove microorganisms and thus observe only the effect of light. Then these samples were exposed to light or darkness (controlled conditions) before their examination with the spectrophotometer. The results demonstrate how the breakdown of organic molecules by light is expressed by a decrease of water colour. It would seem that part of the molecules that give to water its colour are transformed into CO2, thus contributing to the production of greenhouse gaz. This fraction remains unknown. Other than CO2, photo-products can be smaller organic molecules, which microorganisms can use more easily (increasingly labile molecules). Thus, one indirect effect of dissolved organic carbon photolysis is to stimulate microbial activity, which in turn frees up carbon by respiration.