Document Type

Honors Thesis

Publication Date

Spring 5-2018


Coral reefs around the world are suffering mass bleaching events caused by a combination of stressors, including rising ocean temperatures, acidity, pollution, increased suspended sediments, and increasing nitrogen levels. Corals harbor a complex microbial ecosystem consisting of bacteria, and algal symbionts known as Symbiodinium. This study examines the effects of elevated temperature, a known cause of bleaching, and elevated nitrogen, an increasingly important potential stressor for reefs, on the Symbiodinium of a zoanthid coral, Palythoa toxica. A total of 65 polyps were subjected to 5 different water treatments of 2 levels of elevated nitrogen, and 2 levels of elevated temperature in populations of 12. Samples were removed every four days and at the experiment’s termination. The samples were fixed for histological examination. Symbiodinium size was analyzed using Feret diameter. The results of this experiment confirm the effect of elevated temperature as a bleaching mechanism for P. toxica and provide evidence that elevated nitrate is a potential bleaching trigger in prolonged exposure. Furthermore, there is evidence that combined elevated nitrate and temperature compound bleaching mechanisms, with the highest amount of bleaching occurring in the most stressed treatments. When exposed to low levels of nitrate and low levels of temperature elevation, there is a trend of Symbiodinium Feret diameter increase. With time, however, there is an eventual collapse of the system, resulting in Symbiodinium Feret diameter decrease and an increase of algal mass potentially from cell lysis within bleached Symbiodinium colonies remaining between tentacles and within the coelenteron.