This article considers molecular biological to global environment work on inorganic carbon acquisition processes in the algae sensu lato (i.e. including the cyanobacteria).
At least 95% of the organic C in photolithotrophic algae has been fixed by RUBISCO, while the remaining 5% or so has involved a range of anaplerotic carboxylases. The catalytic characteristics of RUBISCO show substantial phylogenetic variation; for some algal RUBISCOs (those from cyanobacteria and, possibly, Dinophyta) the kinetics of the carboxylase and oxygenase activity are such that net photosynthetic CO2 fixation could not occur with diffusive CO2 entry from air-equilibrium solution. In many other algae the kinetics of inorganic C assimilation are at variance with in vitro RUBISCO kinetics, and, as in cyanobacteria and Dinophyta, a CO2-concentrating mechanism is hypothesized. In many cases, the operation of such a mechanism can be demonstrated as a higher internal than external CO2 level during photosynthesis. In cases where such a CO2 concentration difference cannot be demonstrated, mechanisms may produce CO2 from HCO3− in an extracellular or intracellular compartment maintained at a lower pH than the cytosol and stroma but, with the exception of the acid zones on the surface of certain characeans, such mechanisms have not been experimentally verified. Reactions downstream of RUBISCO include the polyphyletic metabolic processes which deal with the products of RUBISCO oxygenase activity subsequent to phosphoglycolate phosphatase; even when a CO2-concentrating mechanism is present there is a minor C flux through phosphoglycolate. There are important consequences for bioenergetics, for the allocation among solutes of the low intracellular osmolarity of some freshwater algae, and of the occurrence of CO2-concentrating mechanisms or of diffusive CO2 entry. Some algae today are inorganic C-limited for photosynthesis in their natural environment under otherwise optimal conditions; even in these organisms it is not clear that they are C-limited in nature. The chances of C limitation in the past would have been greater with the low CO2 concentrations of the last glacial maximum 18,000 years ago, but would have been less likely to be C limited in the pre-pleistocene past, and in the anthropogenically CO2-enriched future, than at present. The evaluation of inorganic C acquisition mechanisms must be viewed against the general decrease in CO2, and increase in O2, over the almost 4 billion years since RUBISCO-based CO2 fixation evolved. It is not yet clear whether (polypheletic) CO2 concentrating mechanisms or (polyphyletic) mechanisms of glycolate metabolism evolved first.
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