Crassulacean acid metabolism (CAM) is a form of photosynthesis in which initial carbon fixation and subsequent assimilation are separated mainly by time. In its characteristic terrestrial expression, a plant takes up carbon dioxide predominantly at night, stores it in organic acids, and releases it internally during the following day. This allows daytime assimilation to continue while stomata remain largely closed, conserving water. CAM occurs in diverse plant lineages, including desert succulents, tropical epiphytes, and some aquatic plants. (pmc.ncbi.nlm.nih.gov)
Biochemical mechanism
At night, stomata typically open, allowing atmospheric carbon dioxide to enter photosynthetic tissues. Carbon dioxide is converted to bicarbonate, which the enzyme phosphoenolpyruvate carboxylase (PEPC) fixes using the three-carbon compound phosphoenolpyruvate. The resulting four-carbon product is converted to malate. Phosphoenolpyruvate is supplied by the breakdown of stored carbohydrates, such as starch or soluble sugars. Malate accumulates as malic acid in the vacuoles of photosynthetic cells, producing a characteristic overnight increase in tissue acidity. (nph.onlinelibrary.wiley.com)
During daylight, malate leaves the vacuole and is decarboxylated, releasing carbon dioxide within the tissue. Different CAM species use different decarboxylating enzymes, including malic enzymes and phosphoenolpyruvate carboxykinase. Rubisco then incorporates this carbon dioxide through the Calvin cycle, which produces the precursors of carbohydrates. The elevated internal carbon dioxide concentration suppresses photorespiration, while stomatal closure limits transpiration. CAM therefore supplements, rather than replaces, the conventional carbon-assimilation machinery. (pmc.ncbi.nlm.nih.gov)
The nocturnal capture of carbon does not mean that the entire photosynthetic process occurs in darkness. Daytime carbon assimilation still depends on the light-driven production of energy and reducing power. The defining innovation is temporary acid storage between initial capture and subsequent assimilation. (pmc.ncbi.nlm.nih.gov)
Daily phases and regulation
The classical CAM cycle is divided into four gas-exchange phases:
- Phase I—night: stomata open, PEPC fixes carbon, and organic acids accumulate.
- Phase II—early morning: stomata may remain open briefly, and both PEPC and Rubisco can contribute to carbon fixation.
- Phase III—daytime: stored acids are decarboxylated; internal carbon dioxide supplies Rubisco while stomata are largely closed.
- Phase IV—late afternoon: after acid reserves decline, stomata may reopen and permit direct atmospheric carbon dioxide assimilation. (pmc.ncbi.nlm.nih.gov)
These phases describe a flexible pattern, not an obligatory timetable. Their duration and relative importance vary with species, developmental stage, water availability, and environmental conditions; individual phases may become greatly reduced or disappear. (pmc.ncbi.nlm.nih.gov)
An internal circadian rhythm helps coordinate enzyme activity and carbohydrate turnover. At night, a specific protein kinase phosphorylates PEPC, making it less sensitive to inhibition by malate. Temporal control helps prevent acid synthesis and acid breakdown from proceeding simultaneously in an energetically wasteful cycle. (doi.org)
Constitutive and facultative expression
Constitutive CAM plants regularly express the pathway as part of normal development, although its intensity remains environmentally responsive. Facultative CAM plants can rely mainly on C3 photosynthesis under favorable conditions and increase CAM expression during water shortage or, in some species, salinity stress. The common ice plant, Mesembryanthemum crystallinum, is a well-studied example. These categories describe differing capacities for regulation rather than completely fixed metabolic states. (nph.onlinelibrary.wiley.com)
Two related modes emphasize carbon recycling. In CAM cycling, stomata remain closed at night while carbon dioxide from cellular respiration is refixed into acids; atmospheric uptake occurs mainly during daylight. In CAM idling, stomata remain closed through both day and night, and internal carbon is recycled without net atmospheric carbon uptake. Idling can reduce carbon and water losses during prolonged drought, but it does not supply the carbon gain associated with active growth. (pmc.ncbi.nlm.nih.gov)
Distribution, ecology, and evolution
The name refers to Crassulaceae, the stonecrop family, but CAM is not restricted to that group. It occurs across dozens of plant families and has evolved independently in multiple lineages. Familiar examples include many cacti, agaves, pineapple, and various orchids and bromeliads. This broad distribution reflects repeated evolution of a similar physiological solution rather than inheritance from one exclusively CAM ancestor. (pmc.ncbi.nlm.nih.gov)
In terrestrial plants, shifting gas exchange toward cooler, often more humid nights generally reduces water loss per unit of carbon acquired. CAM can therefore be advantageous in arid environments and in intermittently dry microhabitats. Tropical epiphytes may experience water shortage despite high regional rainfall because they lack continuous access to soil water. Large vacuoles and succulent tissues can support both acid storage and internal water reserves. Nevertheless, succulence alone does not establish that a plant uses CAM. (pmc.ncbi.nlm.nih.gov)
Aquatic CAM, found in plants such as some Isoëtes, has a different ecological emphasis. Underwater carbon dioxide availability and slow diffusion can constrain photosynthesis; nocturnal carbon storage helps provide carbon for daytime assimilation. Its benefit is therefore not necessarily water conservation. (pmc.ncbi.nlm.nih.gov)
Comparison and identification
Unlike C4 photosynthesis, which usually separates initial carbon capture and Rubisco activity spatially between different cell types, CAM separates them mainly between night and day within the same photosynthetic cells. Its additional transport and metabolic requirements impose costs, while vacuolar storage capacity limits overnight carbon accumulation. Water-saving advantages therefore do not imply universally faster growth. (pmc.ncbi.nlm.nih.gov)
Researchers identify CAM using day–night gas-exchange measurements, changes in titratable acidity, and carbon isotope composition. Strong nocturnal fixation often produces a distinctive isotopic signature, but weak or facultative CAM can overlap with C3 plants. Consequently, isotope measurements alone may miss low-level CAM, making direct measurements of nocturnal acid accumulation or carbon uptake important. (pmc.ncbi.nlm.nih.gov)
References
- Evolution of Crassulacean acid metabolism in response to the environment: past, present, and futurepmc.ncbi.nlm.nih.gov
- Engineering crassulacean acid metabolism to improve water-use efficiencypmc.ncbi.nlm.nih.gov
- Molecular Genetics of Crassulacean Acid Metabolismpmc.ncbi.nlm.nih.gov
- Ecophysiology of Crassulacean Acid Metabolism (CAM)pmc.ncbi.nlm.nih.gov
- Ability of crassulacean acid metabolism plants to overcome interacting stresses in tropical environmentspmc.ncbi.nlm.nih.gov
- New perspectives on crassulacean acid metabolism biologypmc.ncbi.nlm.nih.gov
- Atmospheric CO2 decline and the timing of CAM plant evolutionpmc.ncbi.nlm.nih.gov
- Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a reviewpubmed.ncbi.nlm.nih.gov
- How Closely Do the δ13C Values of Crassulacean Acid Metabolism Plants Reflect the Proportion of CO2 Fixed during Day and Night?pmc.ncbi.nlm.nih.gov