Effects of nitrogen limitation on uptake of inorganic carbon and specific activity of ribulose-1,5-bisphosphate carboxylase/oxygenase in green microalgae

Abstract

The effects of nitrogen limitation on the characteristics of inorganic carbon-dependent O 2 evolution have been examined in the green microalgae Chlorella emersonii and Gloeomonas sp. When cells were grown under 5% CO 2 or air, decreasing the growth rate (increasing nitrogen limitation) caused a decrease in maximum rates of photosynthetic O 2 evolution, although when expressed on a per cell basis such changes were only evident at growth rates below 0.16 day −1 . Severe nitrogen limitation also caused a marked decrease in k 1/2 (CO 2 ) in light- and CO 2 -dependent O 2 evolution. Although values for this parameter were not as low as for low CO 2 grown cells with a fully induced CO 2 -concentrating mechanism, they were less than one-half the corresponding values from cells with the mechanism fully repressed. Nitrogen-limitation also resulted in decreases in the activity and cellular content of ribulose-1,5-bisphosphate carboxylase/oxygenase. The changes to activity and levels of this enzyme were not equivalent so the specific activity decreased dramatically between growth rates of 0.57 and 0.25 day −1 . Similar effects were noted in Gloeomonas and in air-grown Chlorella. The results are discussed in relation to regulation of CO 2 uptake and assimilation in microalgae. Key words: microalgae, CO 2 assimilation, nitrogen limitation, Rubisco, Chlorella.