A mycorrhizal is a symbiotic relationship between a fungus and roots of a vascular plant. The fungus colonizes in the roots of the host plant, either intracellularly or extracellularly. 

In the above question, the roots of pinus are colonised with a fungus rather than an algae. This is a mutually beneficial association where the fungus obtains its food in the form of carbohydrates and glucose from the roots of the pinus plant, and in return provides the plant with nitrogen obtained from the waste products of the fungus. Nitrogen gets absorbed in the roots of the plant and the reaches the leaves.