During periods when combined nitrogen (e.g. ammonia or nitrate) is unavailable, "A. circinalis" form heterocysts, nitrogen-fixing cells with the ability to convert environmental nitrogen (N 2) into compounds such as ammonia or nitrate.
During periods when combined nitrogen (e.g. ammonia or nitrate) is unavailable, "A. circinalis" form, larger, round, nitrogen-fixing cells found every ten to twenty cells or so on the filament. Heterocysts function to convert environmental nitrogen (N 2) into compounds such as ammonia or nitrate.