Inorganic Carbon Acquisition by Diatoms
A few years ago, we reported that diatoms growing under present day atmospheric conditions function as unicellular C4 plants; i.e., that they concentrate carbon by accumulating an intermediate C4 organic compound before CO2fixation in their chloroplast (Reinfelder et al. 2000, 2004; Morel et al. 2002). This work, which implies that CO2, may be a limiting nutrient in the ocean, has been controversial. Our ongoing work shows that specific inhibitors of the two enzymes involved in the formation of the C4 intermediates and the release of CO2 from them --PEPC (phosphoenol pyruvate carboxylase) and PEPCK (phosphoenol pyruvate carboxykinase)-- inhibit photosynthesis in diatoms but not in green algae which operate a well-characterized C3 photosynthetic system (McGinn et al. in prep.). Another key enzyme in inorganic carbon acquisition is carbonic anhydrase, CA, which catalyses the dehydration of bicarbonate to CO2. (CA is a metalloprotein that is the focus of one of our projects; see below.) The inhibition of CA cripples photosynthesis in green algae because it slows down the supply of CO2 to the carboxylating enzyme RubisCO. In contrast, CA inhibition has only a modest effect on C fixation in diatoms. All these observations are consistent with our C4 model of inorganic carbon acquisition in diatoms and imply that the concentration of CO2 may be an important factor in the productivity and ecology of marine phytoplankton. We are now genetically transforming model diatoms to overexpress C4 enzymes of interest to probe their function and establish their cellular localization.
