The central theme of the passage is the diversity and characteristics of algae. The passage provides an overview of what algae are, highlighting their ability to perform photosynthesis using pigments like chlorophyll, while also noting that they lack the stems, roots, and leaves typical of vascular plants. It discusses their structural variety, ranging from unicellular organisms to multicellular colonies, and their wide range of habitats, including both aquatic and terrestrial environments. The passage also touches on their symbiotic relationships, such as in lichens, and their classification as a multiphyletic group, meaning they do not share a common ancestor. Additionally, it introduces phycology as the scientific study of algae and mentions the debate surrounding the classification of blue-green algae. All these points collectively emphasize the varied nature and defining traits of algae, making their diversity and characteristics the central focus of the passage.
Hence, option 2 is correct.
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