Final answer:
The controlling idea of 'Read Algae—Nori' is that nori is economically valuable to the Japanese sea farming industry, being a significant food source cultivated in a unique marine farming method.
Step-by-step explanation:
The controlling idea of the section "Read Algae—Nori" appears to be that nori is a valuable product for the Japanese sea farming economy. This is supported by the context which indicates that Porphyra (commonly known as 'nori' or 'laver') is an important component of marine ecosystems and is cultivated for human consumption, especially in Japan. The economic value of nori comes from its popularity in cuisines like sushi, and it represents a distinct form of seaweed harvesting that involves farming in ocean waters on rope networks. Furthermore, alien seaweeds such as Codium fragile, Gracilaria vermiculophylla, and Undaria pinnatifida can have an impact on biodiversity and the economy due to their invasive characteristics.