景天酸代谢（Crassulacean acid metabolism，简称 CAM）是部分植物的一种精巧的碳固定方法。代表性的植物有仙人掌、凤梨和长寿花等。

要在干旱热带地区生存下来，CAM植物演化出一套生存机制，二氧化碳的固定将于卡尔文循环在时间上分开。这样就可以避免水分过快的流失，因为气孔只在夜间开放以摄取二氧化碳。

纯粹的C4类植物对二氧化碳固定实行的是空间分离（通过两种细胞类型实现，叶肉细胞和维管束鞘细胞）。而景天酸代谢植物则服从以下昼夜节律：

景天酸代谢植物必须准备足够的磷酸烯醇式丙酮酸以供夜间二氧化碳固定使用。为此植物在日间储存淀粉，晚间它们将通过丙酮酸转变为磷酸烯醇式丙酮酸。

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO₂). The CO₂ is stored as the four-carbon acid malate in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO₂, which is then used during photosynthesis. The pre-collected CO₂ is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This metabolism was first studied in plants of the Crassulaceae family. These mainly include succulents. The first time it was studied, Crassula was used as a model organism.