High ribulose-1,5-bisphosphate carboxylase/oxygenase content in northern diatom species

Abstract

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is a fundamental enzyme in CO2-fixation in photoautotrophic organisms. Nonetheless, it has been recently suggested that the contribution of this enzyme to total cellular protein is low in phytoplankton, including diatoms (< 6%). Here we show that RuBisCO content is high in some diatom species isolated from northern waters (> 69°N). Two species contained the highest RuBisCO levels ever reported for phytoplankton (36% of total protein). These high RuBisCO requirements do not increase these species’ requirements for nitrogen and do not impart a fitness disadvantage in terms of growth rate. On the contrary, high RuBisCO levels in psychrophilic diatoms may be a necessary mechanism to maintain high growth rates at low temperature at which enzymatic rates are low.