Studier developed the autoinduction media, that allows a high density medium, he also demonstrated that common mediums like LB and 2YT are deficient in Mg, were an increase in biomass was obtained after adding 1 or 2 mM MgSO4. Here is the link for the paper
Magnesium is a macronutrient and acts as a cofactor for some enzymes and is present in cell walls and membrane. Ribosomes specifically require magnesium ions. A cell will not copy its DNA in preparation for cell division unless it knows that it has enough ribosomes to make new proteins, so it can affect cell division also. Magnesium is usually MgSO4.7H2O. 7 H2O supplied as MgSO4.7H2O or MgCl2.