Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates.

Light and temperature are major drivers in the ecology and biogeography of symbiotic dinoflagellates living in corals and other cnidarians. We examined variations in physiology among 11 strains comprising five species of clade A Symbiodinium We grew cultures at 26°C (control) and 32°C (high temperature) over a duration of 18 days while measuring growth and photochemical efficiency (Fv /Fm ). Responses to thermal stress ranged from susceptible to tolerant across species and strains. Most strains exhibited a decrease in cell densities and Fv /Fm when grown at 32°C. Tolerance to high temperature (T32) was calculated for all strains, ranging from 0 (unable to survive at high temperature) to 1 (able survive at high temperature). There was substantial variation in thermotolerance across species and among strains. One strain had a T32 close to 1, indicating that growth was not reduced at 32°C for only this one strain. To evaluate the combined effect of temperature and light on physiological stress, we selected three strains with different levels of thermotolerance (tolerant, intermediate and susceptible) and grew them under five different light intensities (65, 80, 100, 240 and 443 µmol quanta m-2 s-1) at 26 and 32°C. High irradiance exacerbated the effect of high temperature, particularly in strains from thermally sensitive species. This work further supports the recognition that broad physiological differences exist not only among species within Symbiodinium clades, but also among strains within species demonstrating that thermotolerance varies widely between species and among strains within species.

Genomes of coral dinoflagellate symbionts highlight evolutionary adaptations conducive to a symbiotic lifestyle.

Despite half a century of research, the biology of dinoflagellates remains enigmatic: they defy many functional and genetic traits attributed to typical eukaryotic cells. Genomic approaches to study dinoflagellates are often stymied due to their large, multi-gigabase genomes. Members of the genus Symbiodinium are photosynthetic endosymbionts of stony corals that provide the foundation of coral reef ecosystems. Their smaller genome sizes provide an opportunity to interrogate evolution and functionality of dinoflagellate genomes and endosymbiosis. We sequenced the genome of the ancestral Symbiodinium microadriaticum and compared it to the genomes of the more derived Symbiodinium minutum and Symbiodinium kawagutii and eukaryote model systems as well as transcriptomes from other dinoflagellates. Comparative analyses of genome and transcriptome protein sets show that all dinoflagellates, not only Symbiodinium, possess significantly more transmembrane transporters involved in the exchange of amino acids, lipids, and glycerol than other eukaryotes. Importantly, we find that only Symbiodinium harbor an extensive transporter repertoire associated with the provisioning of carbon and nitrogen. Analyses of these transporters show species-specific expansions, which provides a genomic basis to explain differential compatibilities to an array of hosts and environments, and highlights the putative importance of gene duplications as an evolutionary mechanism in dinoflagellates and Symbiodinium.

Cohesive molecular genetic data delineate species diversity in the dinoflagellate genus Symbiodinium.

The diversity of symbiotic dinoflagellates (Symbiodinium) in pocilloporid corals originating from various reef habitats surrounding Heron Island, southern Great Barrier Reef, was examined by targeting ribosomal, mitochondrial, and chloroplast genes using six methods that analyse for sequence differences. The ability of each of 13 genetic analyses to characterize eight ecologically distinct Symbiodinium spp. was dependent on the level of conservation of the gene region targeted and the technique used. Other than differences in resolution, phylogenetic reconstructions using nuclear and organelle gene sequences were complementary and when combined produced a well-resolved phylogeny. Analysis of the ribosomal internal transcribed spacers using denaturing gradient gel electrophoresis fingerprinting in combination with sequencing of dominant bands provided a precise method for rapidly resolving and characterizing symbionts into ecologically and evolutionarily distinct units of diversity. Single-stranded conformation polymorphisms of the nuclear ribosomal large subunit (D1/D2 domain) identified the same number of ecologically distinct Symbiodinium spp., but profiles were less distinctive. The repetitive sequencing of bacterially cloned ITS2 polymerase chain reaction amplifications generated numerous sequence variants that clustered together according to the symbiont under analysis. The phylogenetic relationships between these clusters show how intragenomic variation in the ribosomal array diverges among closely related eukaryotic genomes. The strong correlation between phylogenetically independent lineages with different ecological and physiological attributes establishes a clear basis for assigning species designations to members of the genus Symbiodinium.

Different algal symbionts explain the vertical distribution of dominant reef corals in the eastern Pacific.

Symbiotic reef corals occupy the entire photic zone; however, most species have distinct zonation patterns within the light intensity gradient. It is hypothesized that the presence of specific symbionts adapted to different light regimes may determine the vertical distribution of particular hosts. We have tested this hypothesis by genetic and in situ physiological analyses of the algal populations occupying two dominant eastern Pacific corals, over their vertical distribution in the Gulf of California. Our findings indicate that each coral species hosts a distinct algal taxon adapted to a particular light regime. The differential use of light by specific symbiotic dinoflagellates constitutes an important axis for niche diversification and is sufficient to explain the vertical distribution patterns of these two coral species.

Biogeography of two species of Symbiodinium (Freudenthal) inhabiting the intertidal sea anemone Anthopleura elegantissima (Brandt).

We have analyzed the genetic profiles of dinoflagellate populations obtained from the Pacific coast sea anemone Anthopleura elegantissima (Brandt) at collection sites from Washington to California. Genetic differences within the symbiont populations of California anemones have been uncovered by restriction length polymorphism (RFLP) analysis of the small subunit (SSU) and large subunit (LSU) ribosomal RNA genes, and by denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer region 2 (ITS 2). The existence of two Symbiodinium species is substantiated by sequence analysis of the variable regions V1, V2, and V3 of the SSUrDNA, which also establishes their phylogenetic relatedness to other members of the genus Symbiodinium. Anemones from Washington and Oregon harbor a single dinoflagellate species, for which we propose the name S. muscatinei sp. nov. At these northern locations, S. muscatinei either exists alone or co-occurs with the Chlorella-like green algal symbiont. Our results indicate that S. muscatinei co-occurs with a second dinoflagellate, S. californium, in mixed populations in central and southern California. We suggest that the geographic distribution of these dinoflagellates is related to the temperature cline created by latitude.