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Dinoflagellates in the genus Symbiodinium are among the most abundant and important group of eukaryotic microbes found in coral reef ecosystems. Recent analyses conducted on various host cnidarians indicated that Symbiodinium assemblages in the Caribbean Sea are genetically and ecologically diverse. In order to further characterize this diversity and identify processes important to its origins, samples from six orders of Cnidaria comprising 45 genera were collected from reef habitats around Barbados (eastern Caribbean) and from the Mesoamerican barrier reef off the coast of Belize (western Caribbean). Fingerprinting of the ribosomal internal transcribed spacer 2 identified 62 genetically different Symbiodinium. Additional analyses of clade B Symbiodinium using microsatellite flanker sequences unequivocally characterized divergent lineages, or “species,” within what was previously thought to be a single entity ( B1 or B184). In contrast to the Indo-Pacific where host-generalist symbionts dominate many coral communities, partner specificity in the Caribbean is relatively high and is influenced little by the host’s apparent mode of symbiont acquisition. Habitat depth (ambient light) and geographic isolation appeared to influence the bathymetric zonation and regional distribution for most of the Symbiodinium spp. characterized. Approximately 80% of Symbiodinium types were endemic to either the eastern or western Caribbean and 40–50% were distributed to compatible hosts living in shallow, high-irradiance, or deep, low-irradiance environments. These ecologic, geographic, and phylogenetic patterns indicate that most of the present Symbiodinium diversity probably originated from adaptive radiations driven by ecological specialization in separate Caribbean regions during the Pliocene and Pleistocene periods. [ABSTRACT FROM AUTHOR]

The photobiology and distribution of dinoflagellates in the genus Symbiodinium was investigated for eight common reef coral species over a depth range of 1-25 m on a coral reef in Belize. The genetic identification of symbionts using polymerase chain reaction-denaturing gradient electrophoresis of the internal transcribed spacer 2 region revealed marked differences in host specificity and depth zonation for certain symbiont types. Each host taxon was found to associate with a limited subset of symbionts that exist in the region. Intraspecific variation was greatest at the shallower sites (1-8 m), where as many as five distinctive symbionts were distributed among a single host population (e.g., Montastraea faveolata). At depth (15-25 m), variation from colony to colony was minimal, where one algal type associated with most or all the colonies of a species. The maximal photochemical efficiency and light-acclimated efficiency of photosystem II (PSII) were determined by active chlorophyll fluorescence and used to assess potential differences in photosynthetic potential. Under normal ambient conditions, little or no physiological differences were noted among different symbionts occurring in the same species of coral at a particular depth, yet interspecific differences in PSII efficiency were noted between coral species at the same depth. Short-term bleaching experiments showed that symbionts B1 and C7 within M. faveolata experienced a higher degree of thermally induced photoinhibition relative to A4a symbionts in Porites astreoides. The differential patterns of PSII inactivation observed within M. faveolata could be explained by the presence of different symbiont populations within this coral. Differences in in situ maximum excitation pressure on PSII between symbionts within some corals may provide a predictive measure of how different species of coral or individual colonies with different symbionts would respond to natural thermal stress events. [ABSTRACT FROM AUTHOR]