Coral reefs provide habitat for more than 9 million marine species which makes them areas of greatest biodiversity. Thanks to the microalgae (dinoflagellates) from the genus Symbiodinium, the growth, development, and health of reef-building corals are successful, which is why certain dinoflagellates are considered to be the basis of such ecosystems. On the other hand, microalgae can be harmful and responsible for fatal mortalities of coral reef species by forming toxic algal blooms. This thesis describes the positive and negative microalgal impacts on coral ecosystems, emphasizing toxic phytoplankton blooms. The biological characteristics of microalgae and certain coral-associated species (Noctiluca scintillans and Karenia brevis) have been described in detail. This paper aims to emphasize that algal blooms are a natural occurrence, however nowadays, due to climate changes, are more frequent and longer which further threatens coral reefs.