Photosynthetic utilisation of inorganic carbon in Ulva

The utilisation of inorganic carbon (Ci), and its further photosynthetic fixation and reduction, is summarised for the ubiquitous green marine macroalga Ulva. Possible mechanisms involved in inorganic carbon transport from the bulk seawater medium are further described in a proposed functional, non-quantitative, model. The main form of inorganic carbon in seawater is bicarbonate (HCO₃^-), and this ion can be utilised either by i) extracellular, carbonic anhydrase (CA)-mediated, dehydration to form CO₂ or ii) direct uptake/transport via a recently described anion exchanger (AE). These two ways of bicarbonate utilisation are not mutually exclusive, but both may be present in a given Ulva species, where each may function according to growth conditions. Inside the cells, all inorganic carbon must eventually be converted to CO₂ since ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the principal enzyme of carbon fixation. Consequently, Ulva is a C₃ plant in which CO₂ is both initially fixed and reduced in the photosynthetic carbon reduction (Calvin) cycle. However, photosynthesis of Ulva is not sensitive to high oxygen tensions, and CO₂ compensation points are low. Therefore, this plant possesses an efficient CO₂ concentrating mechanism, the basis of which is not C₄ photosynthesis but rather one or both of the two mentioned bicarbonate utilisation systems.