What is kelp made of?

Excuse me, could you please clarify something for me? I've been curious about the composition of kelp for quite some time. Could you tell me, what exactly is kelp made of? Are there specific components or nutrients that make up this seaweed? I'm particularly interested in understanding its chemical makeup and how it differs from other types of algae. I hope you don't mind my asking, I'm just trying to broaden my knowledge in this area.