Final answer:
Algae serve as an important primary producer in coral reef ecosystems, alongside zooxanthellae, contributing to photosynthesis, nutrient cycles, and reef structure.
Step-by-step explanation:
In addition to zooxanthellae, another important primary producer in a coral reef ecosystem is algae. Algae, including dinoflagellates like zooxanthellae, diatoms, and multicellular forms, play a crucial role in carbon fixation and photosynthesis within reef environments. They provide the basis for complex food webs that support a diverse array of marine life. Without the presence of these photosynthetic organisms, coral polyps would not be able to obtain the necessary nutrients for survival, leading to processes like coral bleaching. Furthermore, algae also contribute to the structural integrity of coral reefs by contributing to their physical formation over time. Hence, while zooxanthellae are intimately connected with coral health, other types of algae also serve as fundamental primary producers within these ecosystems.