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. Binds 16 chlorophyll a molecules per subunit. Binds 2 beta-carotene molecules per subunit. Binds 1 echinenone molecule per subunit. Binds 1 zeaxanthin molecule per subunit. Under iron-starvation forms a complex with PSI trimers, where the trimer is surrounded by a ring composed of 18 IsiA subunits. When the PSI subunits PsaF and PsaJ are missing the ring is composed of 17 IsiA subunits, indicating that each IsiA subunit has a different interaction with the trimer. This suggests that the size of the PSI complex determines the number of IsiA units in the surrounding ring. In the absence of PsaL has a tendency to form incomplete rings with PSI monomers, suggesting PsaL helps form the rings. Also forms other aggregates of varying sizes depending on the level of iron-deprivation. By iron stress, salt stress and to a lesser extent by heat shock. Belongs to the PsbB/PsbC family. IsiA/Pcb subfamily.