Lichens and mycorrhiza represent two distinct types of symbiotic associations. A key difference in the nature of these relationships lies in:
Correct: D
Option D accurately describes the roles of the partners in both symbiotic relationships. In lichens, the fungus (mycobiont) provides structural support, protection from desiccation, and absorbs water and minerals, while the alga or cyanobacterium (photobiont) performs photosynthesis to produce organic food. In mycorrhiza, the fungal hyphae significantly increase the plant root's surface area for water and mineral (especially phosphorus) absorption, while the plant provides carbohydrates (sugars) produced during photosynthesis to the fungus. Option A is incorrect because the degree of obligate mutualism can vary, and some partners can survive independently in laboratory settings. Option B is incorrect as both are generally considered obligate or highly interdependent mutualisms in nature. Option C is incorrect because lichens are a classic example of mutualism, not commensalism.