Abstract

Symbiodinium, a unicellular dinoflagellate, is crucial to the survival of symbiotic invertebrate hosts like corals. In some cases, the type of Symbiodinium a host associates with can affect host thermal tolerance and physiological processes. Some species of hosts may be able to switch their symbiont type to acclimate to changing environmental conditions. New, diverse types of Symbiodinium might be available for symbiotic hosts to uptake from free-living Symbiodinium populations. Thus the diversity of free-living Symbiodinium is of particular interest in the face of increasingly frequent and widespread coral bleaching events, primarily caused by increases in sea temperature. This thesis compared the genetic diversity of free-living Symbiodinium in the water column and sediments of coral reefs of Hawai'i, the Florida Keys, and Okinawa Japan. 41 Symbiodinium cp23S-HVR sequence variants were identified and grouped into clades A, B, C, D, F, and G; 24 of these sequence variants were novel. Results indicate that free-living Symbiodinium communities are diverse and have complex overlaps between habitats and regions and with symbiotic communities. In addition, tank experiments were conducted to test whether asymbiotic larvae of the coral Acropora monticulosa preferentially acquired free-living Symbiodinium from the water column or sediments to establish their symbiosis. Results from this experiment clearly showed free-living Symbiodinium were acquired by asymbiotic larvae earlier, in greater proportion and with greater in hospite Symbiodinium densities in sediment-containing treatments when compared to seawater-only treatments. These two studies revealed that free-living Symbiodinium communities are diverse with intricate patterns of distribution and that sediments may represent an important source of free-living Symbiodinium for establishing symbiosis with invertebrate hosts. The data obtained in this thesis can be used in future studies to help determine interactions of free-living Symbiodinium community with their symbiotic counterpart and with symbiotic hosts to help predict the response of Symbiodinium and the symbiotic hosts to climate change.