Carnegie Faculty Associate Seminar - Arthur Grossman

Chlamydomonas reinhardtii, a unicellular green algal model organism, has adapted to low CO2 conditions through the evolution of a Carbon Concentrating Mechanism (CCM). The CCM facilitates efficient photosynthetic fixation of CO2 and cell growth and division when CO2 levels are low (ambient or below). Well characterized activities/enzymes associated with the CCM include carbonic anhydrases and inorganic carbon (Ci: CO2, HCO3-, CO32-) transporters, with the latter located on the plasma membrane, chloroplast envelope, and photosynthetic or thylakoid membranes. These activities help concentrate CO2 in the algal pyrenoid, the site at which fixation is initiated. The pyrenoid is a liquid-liquid structure in the chloroplast of many algae that is packed with ribulose-1,5-bisphosphate carboxylase (Rubisco), the initial enzyme associated with CO2 fixation. Several laboratories have contributed to our understanding of algal CCMs, although there is still little known about the role of mitochondria in CCM function. Here, we have analyzed changes in the intracellular location of mitochondria and the orientation of the mitochondrial membrane tubules as Chlamydomonas cells transition between high and low levels of Ci. Upon transferring cells from high to low Ci, the mitochondrion relocates from a central to a peripheral subcellular distribution where it appears to be wedged between the plasma membrane and the outer chloroplast envelope. Furthermore, under low Ci conditions, the mitochondrial membrane tubules reorient from an apparent random distribution to one in which they form parallel arrays that extend from the anterior to the posterior of the cell. These dramatic spatial changes require protein and RNA synthesis and are correlated with the induction of CCM-associated genes; both phenomena occur within 90 min of shifting cells to low levels of Ci. Furthermore, Chlamydomonas cells disrupted for CIA5, the gene that encodes the ‘master regulator' of CCM responses in algae, are neither able to relocate the mitochondrion to the cell periphery nor reorient mitochondrial membrane tubules into parallel arrays. We also demonstrate that re-organization into parallel arrays requires cytoskeletal components (e.g. microtubules, MIRO protein), although they have not been found to be required for repositioning the mitochondrion to the cell periphery. Finally, mitochondrial function was found to be required for the development of optimal photosynthetic efficiency in cells maintained under low CO2 conditions, potentially through energy generation (for transporting Ci) and the recapture of CO2 that leaks out of the chloroplast. Finally, our expanding understanding of CCM in multiple organisms is providing us with new insights into the engineering of rudimentary systems that concentrate Ci in non-CCM-containing algae and vascular plants.