Final answer:
Marine harmful algal blooms are caused by the expansion and death of algal populations, leading to 'dead zones'. Corals have a mutualistic relationship with photosynthetic dinoflagellate algae, which is crucial for the health of coral reefs and marine life.
Step-by-step explanation:
The source of marine harmful algal blooms is due to the expansion and subsequent death of algal populations, which provide organic matter to bacteria in the water. This causes an increase in nonphotosynthetic microorganisms, consuming oxygen and creating 'dead zones'. One photosynthetic partner to corals is dinoflagellate algae, specifically zooxanthellae. These algae have a mutualistic relationship with corals, providing them with energy through photosynthesis, while corals provide a protective environment and nutrients. This symbiosis is crucial for the health of coral reefs, which support a significant amount of marine life.