Curiosity and context revisited: crassulacean acid metabolism in the Anthropocene
Having gained some understanding of the consequences of the CO2-concentrating mechanisms in crassulacean acid metabolism (CAM) that internalize the photosynthetic environment of the Cretaceous on a daily basis, it may be time to consider potential long-term effects of the planetary CO2-concentrating mechanism on growth and ecology of these plants in the Anthropocene. This paper emphasizes our limited understanding of the carbohydrate economy of CAM in relation to growth processes and briefly reviews recent studies of the diel cycles of growth in these plants. An inadvertent long-term, regional-scale experiment from the past is revisited in which an Opuntia monoculture grew to occupy >25 million hectares of farmland in central eastern Australia, producing a total biomass of about 1.5 billion tonnes in about 80 years. Although at the time it does not seem to have been recognized that this invasion involved CAM, a botanist from the University of Melbourne, Jean White-Haney emerges as a heroic pioneer in the control of the invader by poison and pioneered its biological control. The Opuntia population was expanding at 10–100 ha h−1 when it was brought to a halt within a decade by the voracious appetite of Cactoblastis cactorum larvae. It is now known that the female parent moth of this predator detects CAM in O. stricta prior to oviposition by deploying the most sensitive CO2 detector system yet found in the Lepidoptera. The O. stricta invasion is a dramatic demonstration of the capacity of CAM plants to attain and sustain high biomass; to sequester and retain atmospheric CO2. In conclusion, experiments are reviewed that show stimulation of CO2 assimilation, growth, and biomass of CAM plants by elevated atmospheric [CO2], and the proposition that these plants may have a role in atmospheric CO2 sequestration is re-examined. This role may be compromised by predators such as Cactoblastis. However the moth CO2 sensors are adapted to pre-industrial atmospheric [CO2] and FACE (free-air CO2 enrichment) experiments show this exquisite system of biological control is also compromised by rising global [CO2] in the Anthropocene.