Roughly speaking, our bodies use energy from the sun, but we can't use sunlight directly. Instead, plants and algae collect sunlight and store it as chemical energy through the process of photosynthesis. We can access that fuel directly when we eat plants, or indirectly when we eat other animals that eat plants.
However, in some invertebrate animals (those without a backbone) the relationships to algae are more intimate. Tiny single-celled algal "symbionts" can actually live inside the cells of living corals and small animals like hydra that live in water. The algae live in a safe environment inside animal cells and are provided with building block materials to function. They use sunlight to convert the building block materials into larger molecules to store energy and build cellular structures. At the same time some of that stored solar energy is directly transferred to the host animal, allowing it to live in otherwise nutrient poor environments. Thus the algae and their hosts depend on one another to live and thrive. These mutually beneficial relationships are called photosymbioses. [excerpt]
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Burns John and Ryan Kerney. "Algae Living in Salamanders, Friend or Foe?" The Science Breaker, May 22, 2018. https://thesciencebreaker.org/breaks/evolution-behaviour/algae-living-in-salamanders-friend-or-foe
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