RuBisCO catalyzes two reactions: the carboxylation of D-ribulose 1,5-bisphosphate, the primary event in carbon dioxide fixation, as well as the oxidative fragmentation of the pentose substrate in the photorespiration process. Both reactions occur simultaneously and in competition at the same active site. Although the small subunit is not catalytic it is essential for maximal activity. Heterohexadecamer of 8 large and 8 small subunits. This cyanobacterium makes alpha-type carboxysomes. Cells grown on urea as a nitrogen source have more enzyme activity than those grown on nitrate (at protein level). RuBisCO activity of urea- but not nitrate-grown cells decreases with rising temperature (from 25 to 28 degrees Celsius, present versus predicted future ocean temperatures); there is only one RuBisCO in this cyanobacterium. The basic functional RuBisCO is composed of a large chain homodimer in a 'head-to-tail' conformation. In form I RuBisCO this homodimer is arranged in a barrel-like tetramer with the small subunits forming a tetrameric 'cap' on each end of the 'barrel'. Belongs to the RuBisCO small chain family.