Which group of plants fixes carbon dioxide at night?

CAM plants fix carbon dioxide at night. Crassulacean acid metabolism is a photosynthetic adaptation that separates carbon capture and sugar production by time rather than by cell type. At night, CAM plants open their stomata when temperatures are lower and humidity is often higher, allowing them to absorb carbon dioxide while limiting water loss.

The enzyme phosphoenolpyruvate carboxylase initially fixes that carbon dioxide into oxaloacetate, which is converted into malate and stored as malic acid in cell vacuoles. During the day, the stomata close. The stored malate is transported to chloroplasts and decarboxylated, releasing carbon dioxide internally for the Calvin cycle, which uses light-generated ATP and NADPH to make carbohydrates.

Cacti, pineapples, many orchids, bromeliads, and other succulents use CAM. The strategy is especially useful in deserts and other dry habitats, although CAM also occurs in some aquatic plants. C4 plants are a common distractor: they also concentrate carbon dioxide, but they generally fix it during the day and separate the reactions between different cell types. CAM is primarily a time-based separation.