How Terrestrial Organisms Sense, Signal, and Respond to Carbon Dioxide1

SYNOPSIS. Because of anthropogenic increases in atmospheric CO2 content, there is a need to understand how organisms sense and respond to CO2 variation. An important distinction is whether CO2 responses result from direct effects of CO2 on signal-transduction pathways, enzyme catalysis, or regulatory processes, as opposed to indirect, secondary responses that are a consequence of the direct effects. In plants, direct effects occur because rising CO2 A) increases the activity of Ribulose-1,5-bisphopshate carboxylase/oxygenase (Rubisco) via its role as a substrate for RuBP carboxylation and its inhibition of RuBP oxygenation; B) reduces stomatal aperture; C) alters mitochondrial respiration; and D) possibly reduces transcription of genes for Rubisco activase and carbonic anhydrase. Because of these direct effects, the carbon and water balance of plants is altered leading to secondary effects on growth, resource partitioning and defense com-pound synthesis. Reduced investment in photosynthetic protein is one of the characteristic acclimation re-sponses of plants to high CO2. This is modulated by increased carbohydrate levels, probably in concert with hormone signals from the roots. Roots are hypothesized to be the main control points for CO2 acclimation because they are well situated to integrate the carbohydrate status of the plant. In higher fungi, development of the mushroom fruiting body is inhibited at high CO2, but the mechanism is poorly known. Fungal CO2 sensing may serve to position the spore-bearing tissue above the soil boundary layer to ensure effective spore dispersal. The animals that are most sensitive to anthropogenic CO2 enrichment are insects. Many insects have a well-developed ability to sense CO2 variation as a means of locating food. Unlike plants, insects have CO2 receptors that can detect variation in CO2 as low as 0.5 ppm. However, the sensitivity of these receptors is reduced in atmospheres with double or triple current levels of CO2, indicating some insect species may be threatened by rising atmospheric CO2