Crassulacean Acid Metabolism, or CAM, is a process by which plants take up and sequester carbon dioxide at night and release it during the day to conduct photosynthesis. It is commonly associated with plants in arid environments where it allows them to reduce water loss by keeping their stomata closed during the hottest part of the day. However, CAM is also paradoxically found in some lineages of aquatic plants such as Littorella uniflora, Crassula aquatica, and is particularly prevalent in the enigmatic lycophyte genus Isoetes.
So what's going on? These submerged plants clearly aren't trying to conserve water. As it turns out, aquatic plants are faced with a different limitation that CAM helps them overcome, the low availability of carbon dioxide under water. This is primarily due to two factors, the low diffusion rate of CO2 in water and its rapid depletion during the day by photosynthetic algae suspended in the water column. CAM allows submerged plants to store up CO2 at night and "make hay while the sun is shining." This exposes CAM's primary function, not as an enhancer of water use efficiency per se but as a carbon concentrating mechanism.
This makes CAM a particularly interesting example of convergent evolution of a complex trait, one that has evolved independently in two very different environments to cope with what appear (at least superficially) to be very different limitations. This sort of convergence can help us understand how CAM has evolved in so many diverse lineages and tease apart its genetic underpinnings by potentially uncoupling its evolution from the abiotic stresses it's commonly associated with. Thus, ironically, these diminutive aquatic plants may hold the key to understanding a trait that has allowed their cousins to conquer earth's deserts.
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