Aquatic CAM photosynthesis: A brief history of its discovery

• Aquatic CAM photosynthesis was discovered in study of biochemical anaerobic metabolism.
• CAM is universal in all aquatic species of Isoetes and non-existent in terrestrial Isoetes.
• CAM has a limited occurrence in three other families, including the Crassulaceae.
• Discovery led to studies of terrestrial Isoetes relatives with carbon uptake from sediment.
• CAM in all plants provides an internal source of CO2 for photosynthesis during the day.

Aquatic CAM (Crassulacean Acid Metabolism) photosynthesis was discovered while investigating an unrelated biochemical pathway concerned with anaerobic metabolism. George Bowes was a significant contributor to this project early in its infancy. Not only did he provide me with some valuable perspectives on peer review rejections, but by working with his gas exchange system I was able to take our initial observations of diel fluctuations in malic acid to the next level, showing this aquatic plant exhibited dark CO2 uptake. CAM is universal in all aquatic species of the worldwide Lycophyta genus Isoetes and non-existent in terrestrial Isoetes. Outside of this genus aquatic CAM has a limited occurrence in three other families, including the Crassulaceae. This discovery led to fascinating adventures in the highlands of the Peruvian Andes in search of Stylites, a terrestrial relative of Isoetes. Stylites is a plant that is hermetically sealed from the atmosphere and obtains all of its carbon from terrestrial sources and recycles carbon through CAM. Considering the Mesozoic origin of Isoetes in shallow pools, coupled with the fact that aquatic Isoetes universally possess CAM, suggests the earliest evolution of CAM photosynthesis was most likely not in terrestrial plants.