Carbon Concentrating Mechanisms
Carbon Concentrating Mechanisms (CCMs) are found in nearly all algae, as well as many higher land plants. CCMs act to overcome the limitations to land plant growth imposed by low atmospheric CO2 concentrations and high O2 concentrations. For algae, CCMs act to overcome the challenges presented by trying to attain carbon from an aqueous environment (see here). Although CCMs are diverse in their exact mechanisms, all share the common feature of concentrating carbon dioxide around the carbon-fixing enzyme, RuBisCO, such that RuBisCO can operate at a higher efficiency. Carbon fixation is often a limiting factor determining plant growth, so increasing the efficiency of this process could directly increase crop yields.
In land plants, there are two broad types of carbon concentrating mechanism, known as C4, and CAM (Crassulacean Acid Metabolism). Both separate the processes of drawing down CO2 from the atmosphere and of fixing it. In the absence of a CCM, CO2 is drawn down by the fixation step of the light-independent reactions, in which RuBisCO fixes it as 3-phosphoglycerate (3PGA). This is a stable three carbon (3C) compound, and as such this “normal” photosynthesis is called C3 photosynthesis. In both C4 and CAM photosynthesis however, CO2 is drawn down not by the RuBisCO-mediated fixation step, but by initial fixation as a 4C acid, such as malate, by the enzyme phosphoenolpyruvate carboxylase (PEPC). PEPC has a higher affinity for CO2 than does RuBisCO, so can draw down CO2 with greater efficiency than RuBisCO. The 4C acid can then be decarboxylated to CO2 in the presence of RuBisCO, and normal C3 fixation by RuBisCO can therefore take place in a higher CO2 concentration, and therefore more efficiently.
The difference between C4 and CAM is that the former is a spatial separation of the C4 and C3 fixation, facilitated by Krantz anatomy, while CAM entails a temporal separation of the two processes, facilitated by a specific regime of stomatal opening and closure. More details on these processes can be found in the teaching resources section. Succulent plants such as Aloe and Agave are among those that use CAM photosynthesis, while maize and sorghum, for example are C4 plants.
The CCM that we are researching in green algae is mediated by a cellular microcompartment called the pyrenoid. Unlike CAM and C4 in higher plants, which are biochemical means of concentrating carbon around RuBisCO, the algal pyrenoid is a biophysical CCM – this distinction is detailed on the page linked to above. It is also worth noting that a similar biophysical CCM, facilitated by a microcompartment called the carboxysome, exists in cyanobacteria (photosynthetic prokaryotes).