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

Abstract

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 C4pathway, or a combination of both. Although evidence for the operation of a C4pathway is accumulating, the role(s) of the enzyme(s) responsible for "C3+ C1" 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 C4-type CO2-concentrating mechanism. These are compared and contrasted with other C4systems, 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 C4photosynthesis, including phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase.Key words: CO2-concentrating mechanism, C4photosynthesis, marine diatoms, phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase.