• Crassulacean acid metabolism;
  • CAM;
  • photosynthetic acid metabolism;
  • aquatic acid metabolism;
  • terrestrial astomatal acid metabolism


The term, photosynthetic acid metabolism (PAM), encompasses processes which involve four-carbon (C4) acids as carbon-carrying intermediates between the carboxylations catalysed by phosphoenolpyruvate (PEP) carboxylase and ribulose biphosphate (RuBP) carboxylase. These carboxylations may be separated either spatially, as in C4 photosynthesis which is not covered here, or temporally. Separation in time is referred to as diel PAM (DPAM) and the variations on this particular theme are the subject of this review. DPAM includes stomatal and astomatal types. The former types, usually described as crassulacean acid metabolism (CAM), are referred to here as stomatal CAM (SCAM). Astomatal PAM includes both aquatic and terrestrial forms; aquatic acid metabolism (AAM) and terrestrial astomatal acid metabolism (TAAM). Physiological, biochemical, structural, ecological and evolutionary aspects of these mechanisms of acquisition of carbon dioxide are reviewed.