Hatch-Slack metabolism

Some plants employ a photosynthetic pathway creating at first a three-carbon phosphoglyceric acid (C3 or Calvin-Benson photosynthesis). These plants fractionate isotopes more intensely, and so have more negative values (-33%o to —22%o PDB) than plants which use a photosynthetic pathway creating at first a four-carbon malic and aspartic acid (C4 or Hatch-Slack photosynthesis -16%o to -9%o PDB). Crassulacean acid metabolism (CAM) is yet another photosynthetic pathway, which creates organic matter of intermediate isotopic composition (-35%o to -ll%o PDB). Methanogenic microbes are even more extreme in their fractionation of the light isotope (5 C down to -110%o and typically -60%o PDB ... 

Dark phase (CO2 fixation phase) 0/ photosynthesis. The various aspects of this phase of photosynthesis are described in the following separate articles, which are best read in the following sequence (i) Calvin cycle, which can be regarded as the basic COj fixation process, (ii) Hatch-Slack-Kortschak cycle, which deals with CO2 fixation in C -plants, such as sugar cane, (iii) Crassulacean acid metabolism, which deals with COj fixation in plants, such as cacti, growing in arid climates, and (iv) Reductive citrate cycle, which describes COj fixation in green sulfur bacteria. 

Typically, grasses originally native to hot, arid environments follow the C4 (Hatch-Slack) photosynthetic pathway, and will have 6 C values averaging about -12%o trees, shrubs, and grasses from temperate regions, which follow the C3 (Calvin-Benson) photosynthetic pathway, will have 6 C values averaging about -26%o. Metabolic fractionation in their consumers results in bone collagen values of about -7%o and -21%o and bone apatite values of about 0%o and -14%o, respectively, for pure C4 and pure C3 diets. 

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