Functions as an antenna for photosystem I (PSI) under iron-limiting conditions, when phycobilisomes disappear. In the (PSI)3(Isi3)18 complex most of the harvested energy is probably used by PSI; in other PSI-containing supercomplexes a large part of the energy will probably not be used for light harvesting, but rather is dissipated to protect the organism from light damage. Chlorophyll a. Under iron-starvation forms a complex with PSI trimers, where the trimer is surrounded by a ring composed of 18 IsiA subunits. By iron stress. Belongs to the PsbB/PsbC family. IsiA/Pcb subfamily.