How do marine diatoms fix 10 billion tonnes of inorganic carbon per year?

Diatoms are responsible for at least a quarter of inorganic carbon fixed each year in the ocean. Despite very considerable research over the last 30 years, there are still a number of fundamental unresolved aspects of inorganic carbon assimilation by marine diatoms. It is not clear how the carbon-concentrating mechanism functions and whether it is based on the direct acquisition of inorganic carbon or on a C 4 pathway, or a combination of both. Although evidence for the operation of a C 4 pathway is accumulating, the role(s) of the enzyme(s) responsible for "C 3 + C 1 " inorganic carbon assimilation in the light and dark are still matters of controversy. In this review, we discuss whether diatoms possess the enzymic and structural components necessary for a C 4 -type CO 2 -concentrating mechanism. These are compared and contrasted with other C 4 systems, both single-celled and those in terrestrial plants, which are based on Kranz anatomy. New data are presented on expression of genes that might be involved in C 4 photosynthesis, including phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase.