Effects of Photolysis

Photochemical processes with ecological relevance are well documented for marine and freshwater systems.

They can influence many aspects of carbon cycling in these systems and

• decrease the average molecular weight of the chromo-phoric organic substances;

• change the optical properties of the water as a result of the changes in chromophoric organic substances;

• release a complex mixture of ROS, which can hurt organisms and initiate indirect phototransformation reactions;

• produce oxidized C compounds such as CO, CO2, or short-chain fatty acids, which provide energy and carbon to heterotrophic processes;

• release P- and N-rich products: phosphate ions, amino acids, dissolved primary amines, and finally ammonia, NH4.

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Figure 5 Wavelength dependence 1O2 production rates for chromophoric dissolved organic carbon compounds in a whole water column, exemplified for a midday, summer, sunlight at latitude 40° N. ■, Hietajarvi fall natural organic matter; • , Suwannee River natural organic matter; n, Suwannee River humic acid; □, Birkenes spring natural organic matter; •, Birkenes fall natural organic matter; $, Svartberget spring natural organic matter; O, peat humic acid; ■, soil fulvic acid. Reproduced from Paul A, Hackbarth S, Vogt RD, etal. (2004) Photogeneration of singlet oxygen by humic substances: Comparison of humic substances of aquatic and terrestrial origin. Photochemical and Photobiological Sciences 3: 273-280 -Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology.

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