PARENT SESSION

Symposium S2D Marine photosynthesis and production
Monday August 30th, 2004 2:40 PM-4:40 PM Room 510B
Chair: John Raven
Co-Chair: Doug Campbell

Regulation of CO₂ concentrating mechanisms in cyanobacteria. Fiona Woodger^*,1, Murray Badger¹, Dean Price¹, ¹ Molecular Plant Physiology, Research School of Biological Sciences, Canberra, ACT, Australia

ABSTRACT- Approximately 50% of global CO₂-based productivity is now attributed to the activity of phytoplankton including ocean-dwelling cyanobacteria. In response to inherent restrictions on the rate of CO₂ supply in the aquatic environment, cyanobacteria have evolved a very efficient means of capturing inorganic carbon (Ci), as either CO₂ or HCO₃^-, for photosynthetic carbon fixation. This capturing mechanism, known as a 'CO₂ concentrating mechanism' (CCM), involves the operation of active CO₂ and HCO₃^- transporters and results in the concentration of CO₂ around Rubisco in a unique microcompartment called the carboxysome. The CCM exhibits two basic physiological states - a constitutive, low affinity state and a high affinity state that is induced in response to Ci limitation. Many of the genetic components of the CCM, including genes encoding Ci transporters, have been identified. It is apparent that the expression of the inducible, high-affinity Ci transporters is particularly sensitive to Ci availability and we are now interested in defining how cyanobacterial cells sense and respond to Ci limitation. Current theories include direct sensing of external Ci, sensing of internal Ci-pool fluctuations or detection of changes in photorespiratory intermediates, carbon metabolites or redox potential. Presently there is no consensus view. We have investigated the physiological and transcriptional response of CCM mutants and wildtype strains to pharmacological treatments and various light, O₂ and Ci regimes. Our data suggests that perception of Ci limitation by a cyanobacterial cell involves either direct sensing of a decline in the internal Ci pool or, more likely, sensing of altered photorespiratory activity.

KEY WORDS: CO₂ concentrating mechanisms, cyanobacteria, inorganic carbon transport