Magnesium is a micro nutrient, although it is not limiting ,it is required for the plan growth.In certain case if Macro nutrient are enough then phytoplankton depend upon the micro nutrients as like iron, Mg etc.So depend upon the availability of nutrients and the environmental condition required.Sometime it is act as co-limiting with light and temperature.
Thanks Dr. Mishra. Its true. Generally, the micro nutrients are being utilized for the growth of algal cell like Mg, Fe and many more. Still, it is required in micro level.