Acquisition. Acclimation to Changing Carbon Availability

Abstract

Aquatic organisms, including those such as Chlamydomoncas reinhardtii that inhabit the soil water solution, face a variable supply of dissolved inorganic carbon (Ci) for photosynthetic carbon assimilation. Accordingly, C. reinhardtii has the ability to acclimate to the changing (Ci) supply through a variety of responses, which include development of a limiting-CO2-inducible CO2-concentrating mechanism (CCM). The CCM uses active (Ci) transport, probably both at the plasmalemma and the chloroplast envelope, to accumulate a high concentration of bicarbonate in the chloroplast stroma. The initial enzyme of photosynthetic carbon assimilation, Rubisco, is located in, a stromal, structure called the pyrenoid, and one hypothesis, suggests, that dehydration of the accumulated bicarbonate to supply CO2, the substrate for Rubisco, occurs only in the pyrenoid. However, an $α$-type carbonic anhydrase responsible for dehydration of the stromal bicarbonate pool apparently is located in the thylakoid lumen, so it may be restricted to thylakoid membranes encompassed by the pyrenoid. The high CO2 concentration generated at the site of Rubisco both increases the photosynthetic rate and suppresses oxygenation of RuBP, a wasteful side pathway. In addition to the changes demonstrably related to the function of the CCM, C. reinhardtii exhibits several other changes upon acclimation to limiting (Ci), such as induction of three additional carbonic anhydrases (one periplasmic and two mitochondrial), changes in the intracellular location of mitochondria, up-regulation of photorespiratory enzymes and up-regulation of several other genes, of unknown function. Important transient changes in gene regulation also occur during the acclimation period before the CCM becomes functional, including an arrest of the cell division cycle and a decline in Rubisco synthesis. One of the key areas of interest currently under investigation is how the C. reinhardtii cells recognize the change in (Ci) or CO2 concentration and transduce that signal into gene expression changes needed for induction of a functional CCM, as well as all the other changes seen upon acclimation to limiting CO2.