Using Isotopes to Measure Gas Exchange
Tracers have long been used in photosynthesis experiments to provide information that cannot be obtained by net gas exchange techniques. The radioactive tracer 14C, was instrumental in determining the biochemical pathways of CO2 fixation (Bassham and Calvin, 1957). In addition, assimilation of inorganic 14C has become a standard technique for measuring photosynthesis of unicellular aquatic plants, largely replacing net oxygen exchange, particularly under oligotrophic conditions. The stable carbon isotope 13C can substitute for 14C under some conditions and measurements of natural abundance of 13C relative to 12C (the carbon-isotope ratio δ13C) have provided considerable insight into photosynthetic mechanisms and form(s) of inorganic carbon assimilated by plants. Measurements of δ13C have also been used to elucidate the physical and biochemical rate-limiting steps in photosynthetic carbon fixation. The stable oxygen isotope 18O has been useful in evaluating the roles of respiration, photorespiration, and the Mehler reaction in illuminated plants. Tritium (3H) has not proven to be a useful radioisotope for examining gas exchange, but tritiated amino acids and sugars have been used to examine algal osmotrophy (Rivkin and Putt, 1987) and 3H2O has been used in experiments to estimate rates of protein turnover (Richards and Thurston, 1980).
KeywordsInorganic Carbon Particulate Organic Carbon Photosynthesis Rate Thalassiosira Pseudonana Mehler Reaction
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