Many corals host thermally resistant symbionts in high-temperature habitat

Physiologically distinct lines of dinoflagellate symbionts, Symbiodinium spp., may confer distinct thermal tolerance thresholds on their host corals. Therefore, if a coral can alternately host distinct symbionts, changes in their Symbiodinium communities might allow corals to better tolerate increasing environmental temperatures. However, researchers are currently debating how commonly coral species can host different symbiont types. We sequenced chloroplast 23 s rDNA from the Symbiodinium communities of nine reef-building coral species across two thermally distinct lagoon pools separated by ~500 m. The hotter of these pools reaches 35°C in the summer months, while the other pool’s maximum temperature is 1.5°C cooler. Across 217 samples from nine species, we found a single haplotype in both Symbiodinium clades A and D, but four haplotypes in Symbiodinium clade C. Eight of nine species hosted a putatively thermally resistant member of clade D Symbiodinium at least once, one of which hosted this clade D symbiont exclusively. Of the remaining seven that hosted multiple Symbiodinium types, six species showed higher proportions of the clade D symbiont in the hotter pool. Average percentage rise in the frequency of the clade D symbiont from the hotter to cooler pool was 52% across these six species. Even though corals hosted members of both the genetically divergent clades D and C Symbiodinium, some showed patterns of host–symbiont specificity within clade C. Both Acropora species that hosted clade C exclusively hosted a member of sub-clade C2, while all three Pocillopora species hosted a member of sub-clade C1 (sensu van Oppen et al. 2001). Our results suggest that coral–algal symbioses often conform to particular temperature environments through changes in the identity of the algal symbiont.     

Discordant coral–symbiont structuring: factors shaping geographical variation of Symbiodinium communities in a facultative zooxanthellate coral genus,Oculina

Understanding the factors that help shape the association between corals and their algal symbionts, zooxanthellae (Symbiodinium), is necessary to better understand the functional diversity and acclimatization potential of the coral host. However, most studies focus on tropical zooxanthellate corals and their obligate algal symbionts, thus limiting our full comprehension of coral–algal symbiont associations. Here, we examine algal associations in a facultative zooxanthellate coral. We survey the Symbiodinium communities associated with Oculina corals in the western North Atlantic and the Mediterranean using one clade-level marker (psbA coding region) and three fine-scale markers (cp23S–rDNA, b7sym15 flanking region, and b2sym17). We ask whether Oculina spp. harbor geographically different Symbiodinium communities across their geographic range and, if so, whether the host’s genetics or habitat differences are correlated with this geographical variation. We found that Oculina corals harbor different Symbiodinium communities across their geographical range. Of the habitat differences (including chlorophyll a concentration and depth), sea surface temperature is better correlated with this geographical variation than the host’s genetics, a pattern most evident in the Mediterranean. Our results suggest that although facultative zooxanthellate corals may be less dependent on their algal partners compared to obligate zooxanthellate corals, the Symbiodinium communities that they harbor may nevertheless reflect acclimatization to environmental variation among habitats.

     

Phototrophic microendoliths bloom during coral “white syndrome”

Following rapid lesion progression of white syndrome in tabular Acropora spp., the white bare skeleton gradually changes to green, a result of endolithic algae blooms (primarily Ostreobium spp.). Endolithic algal biomass and chlorophyll concentration were found to be an order of magnitude higher in the green zone compared with healthy appearing parts of each colony. Chl b to Chl a ratio increased from 1:1.6 in the healthy area to 1:2 and 1:3.5 in the white exposed skeleton and green zones, respectively. These observations together with pulse amplitude modulated (PAM) fluorometry suggest photoacclimation of the endoliths in the green zone. Histopathological microscopy revealed that the endolithic algal filaments penetrate the coral tissue. This study highlights the interaction of endolithic algae with both the skeleton and host tissue. This may have a critical role in the processes that accompany the post-disease state in reef-building corals.     

Functional significance of dinitrogen fixation in sustaining coral productivity under oligotrophic conditions

Functional traits define species by their ecological role in the ecosystem. Animals themselves are host–microbe ecosystems (holobionts), and the application of ecophysiological approaches can help to understand their functioning. In hard coral holobionts, communities of dinitrogen (N₂)-fixing prokaryotes (diazotrophs) may contribute a functional trait by providing bioavailable nitrogen (N) that could sustain coral productivity under oligotrophic conditions. This study quantified N₂ fixation by diazotrophs associated with four genera of hermatypic corals on a northern Red Sea fringing reef exposed to high seasonality. We found N₂ fixation activity to be 5- to 10-fold higher in summer, when inorganic nutrient concentrations were lowest and water temperature and light availability highest. Concurrently, coral gross primary productivity remained stable despite lower Symbiodinium densities and tissue chlorophyll a contents. In contrast, chlorophyll a content per Symbiodinium cell increased from spring to summer, suggesting that algal cells overcame limitation of N, an essential element for chlorophyll synthesis. In fact, N₂ fixation was positively correlated with coral productivity in summer, when its contribution was estimated to meet 11% of the Symbiodinium N requirements. These results provide evidence of an important functional role of diazotrophs in sustaining coral productivity when alternative external N sources are scarce.     

Latent virus‐like infections are present in a diverse range of Symbiodinium spp. (Dinophyta)

Coral reefs are increasingly threatened by disease outbreaks, which affect the coral animal and/or its algal symbionts (Symbiodinium spp.) and can cause mass mortalities. Currently around half of the recognized coral diseases have unknown causative agents. While many of the diseases are thought to be bacterial in origin, there is growing evidence that viruses may play a role. In particular, it appears that viruses may infect the algal symbionts, causing breakdown of the coral‐algal mutualism. In this study, we screened a wide range of Symbiodinium cultures in vitro for the presence of latent viral infections. Using flow cytometry and electron microscopy, we found that many types of Symbiodinium apparently harbor such infections, and that the type of putative virus varied within and among host types. Furthermore, the putative viral infections could be induced via abiotic stress and cause host cell lysis and population decline. If similar processes occur in Symbiodinium cells in hospite, they may provide an explanation for some of the diseases affecting corals and other organisms forming symbioses with these algae.     

An automatic device for in vivo absorption spectra acquisition and chlorophyll estimation in phytoplankton cultures

In order to aid the study of photoacclimation, a new programmable deviceis described which provides automatic on-line acquisition of in vivo cellabsorption in phytoplankton cultures. The system was used for a long-termstudy of Rhodomonas salina grown at constant photon flux density ina nitrate-limited continuous culture with different dilution rates. Particulate absorption measured at the red chlorophyll a (Chl a)maximum was not a good proxy of biomass, because of the large variabilityof cellular chlorophyll induced by nitrogen limitation. However, thedevice is well suited to automatic assessment of Chl a andphycoerythrin (PE) concentrations in phytoplankton cultures, if algal cellsize and concentration are measured in parallel to correct the packagingeffect. The effects of nitrogen limitation on Chl a and PE contentsand particle absorbance are discussed.     

<Emphasis Type="Italic">Symbiodinium</Emphasis> associations with diseased and healthy scleractinian corals

Despite recent advances in identifying the causative agents of disease in corals and understanding the impact of epizootics on reef communities, little is known regarding the interactions among diseases, corals, and their dinoflagellate endosymbionts (Symbiodinium spp.). Since the genotypes of both corals and their resident Symbiodinium contribute to colony-level phenotypes, such as thermotolerance, symbiont genotypes might also contribute to the resistance or susceptibility of coral colonies to disease. To explore this, Symbiodinium were identified using the internal transcribed spacer-2 region of ribosomal DNA from diseased and healthy tissues within individual coral colonies infected with black band disease (BB), dark spot syndrome (DSS), white plague disease (WP), or yellow blotch disease (YB) in the Florida Keys (USA) and the US Virgin Islands. Most of the diseased colonies sampled contained B1, B5a, or C1 (depending on host species), while apparently healthy colonies of the same coral species frequently hosted these types and/or additional symbiont diversity. No potentially “parasitic” Symbiodinium types, uniquely associated with diseased coral tissue, were detected. Within most individual colonies, the same dominant Symbiodinium type was detected in diseased and visually healthy tissues. These data indicate that specific Symbiodinium types are not correlated with the infected tissues of diseased colonies and that DSS and WP onset do not trigger symbiont shuffling within infected tissues. However, few diseased colonies contained clade D symbionts suggesting a negative correlation between hosting Symbiodinium clade D and disease incidence in scleractinian corals. Understanding the influence of Symbiodinium diversity on colony phenotypes may play a critical role in predicting disease resistance and susceptibility in scleractinian corals.     

Exploring Symbiodinium diversity and host specificity in Acropora corals from geographical extremes of Western Australia with 454 amplicon pyrosequencing

Scleractinian corals have demonstrated the ability to shuffle their endosymbiotic dinoflagellate communities (genus Symbiodinium) during periods of acute environmental stress. This has been proposed as a mechanism of acclimation, which would be increased by a diverse and flexible association with Symbiodinium. Conventional molecular techniques used to evaluate Symbiodinium diversity are unable to identify genetic lineages present at background levels below 10%. Next generation sequencing (NGS) offers a solution to this problem and can resolve microorganism diversity at much finer scales. Here we apply NGS to evaluate Symbiodinium diversity and host specificity in Acropora corals from contrasting regions of Western Australia. The application of 454 pyrosequencing allowed for detection of Symbiodinium operational taxonomic units (OTUs) occurring at frequencies as low as 0.001%, offering a 10 000‐fold increase in sensitivity compared to traditional methods. All coral species from both regions were overwhelmingly dominated by a single clade C OTU (accounting for 98% of all recovered sequences). Only 8.5% of colonies associated with multiple clades (clades C and D, or C and G), suggesting a high level of symbiont specificity in Acropora assemblages in Western Australia. While only 40% of the OTUs were shared between regions, the dominance of a single OTU resulted in no significant difference in Symbiodinium community structure, demonstrating that the coral‐algal symbiosis can remain stable across more than 15° of latitude and a range of sea surface temperature profiles. This study validates the use of NGS platforms as tools for providing fine‐scale estimates of Symbiodinium diversity and can offer critical insight into the flexibility of the coral‐algal symbiosis.     

DIFFERENTIAL IMPACTS OF PHOTOACCLIMATION AND THERMAL STRESS ON THE PHOTOBIOLOGY OF FOUR DIFFERENT PHYLOTYPES OF SYMBIODINIUM (PYRRHOPHYTA)1

The capacity for photoacclimation to light at 100 or 600 μmol photons·m^?2·s^?1 and the subsequent response to thermal stress was examined in four genetically distinct cultures of symbiotic dinoflagellates in the genus Symbiodinium with the ITS2 designations A1, A1.1, B1, and F2. While all algal types showed typical signs of photoacclimation to high light via a reduction in chl a, there was a differential response in cellular growth, photosystem II (PSII) activity, and the chl a‐specific absorption coefficient between cultures. When maintained at 32°C for up to 10 days, significant variation in the susceptibility to thermal stress was observed in the rate of loss in PSII activity and electron transport, PSII reaction center degradation, and cellular growth. The order of thermal tolerance did not change between the two light levels. However, as expected, loss in photosynthetic function was exacerbated in the thermally sensitive phylotypes (B1 and A1.1) when acclimated to the higher light intensity. There was no consistent relationship between thermal tolerance and changes in light energy dissipation via non‐photochemical pathways. Phylotypes F2 and A1 showed a high degree of thermal tolerance, yet the cellular responses to light and temperature were markedly different between these algae. The F2 isolate showed the greatest capacity for photoacclimation and growth at high light and temperature, while the A1 isolate appeared to adjust to thermal stress by a slight decline in PSII activity and a significant decline in growth, possibly at the expense of increased photosystem and cellular repair rates.     

Coral disease physiology: the impact of Acroporid white syndrome on <Emphasis Type="Italic">Symbiodinium</Emphasis>

Acroporid white syndrome, a disease-like syndrome from the Great Barrier Reef, results from degenerative host tissue at lesion borders. Tissue preceding lesion borders appears visually healthy, but it is currently unclear whether the endosymbiotic zooxanthellae (Symbiodinium) are physiologically impacted. Compared to healthy colonies, this study found no significant differences in symbiont density, mitotic index or chlorophyll a content in tissue bordering (0 cm), and 8 cm away from white syndrome lesions. Using chlorophyll a fluorescence techniques, the border tissue did not appear to be photosynthetically compromised, and Symbiodinium extracted from this area were photosynthetically competent. Transmission electron microscopy revealed extensive degeneration of host tissues surrounding symbionts in affected areas, however, Symbiodinium cells were structurally intact with no sign of in situ degradation. Collectively, these results suggest that Symbiodinium at white syndrome lesion borders exist in a dynamic intra-cellular state during active host tissue loss, yet remain physiologically uncompromised.     

A microsampling method for genotyping coral symbionts

Genotypic characterization of Symbiodinium symbionts in hard corals has routinely involved coring, or the removal of branches or a piece of the coral colony. These methods can potentially underestimate the complexity of the Symbiodinium community structure and may produce lesions. This study demonstrates that microscale sampling of individual coral polyps provided sufficient DNA for identifying zooxanthellae clades by RFLP analyses, and subclades through the use of PCR amplification of the ITS-2 region of rDNA and denaturing-gradient gel electrophoresis. Using this technique it was possible to detect distinct ITS-2 types of Symbiodinium from two or three adjacent coral polyps. These methods can be used to intensely sample coral-symbiont population/communities while causing minimal damage. The effectiveness and fine scale capabilities of these methods were demonstrated by sampling and identifying phylotypes of Symbiodinium clades A, B, and C that co-reside within a single Montastraea faveolata colony.     

Symbiont acquisition strategy drives host–symbiont associations in the southern Great Barrier Reef

Coral larvae acquire populations of the symbiotic dinoflagellate Symbiodinium from the external environment (horizontal acquisition) or inherit their symbionts from the parent colony (maternal or vertical acquisition). The effect of the symbiont acquisition strategy on Symbiodinium-host associations has not been fully resolved. Previous studies have provided mixed results, probably due to factors such as low sample replication of Symbiodinium from a single coral host, biogeographic differences in Symbiodinium diversity, and the presence of some apparently host-specific symbiont lineages in coral with either symbiont acquisition strategies. This study set out to assess the effect of the symbiont acquisition strategy by sampling Symbiodinium from 10 coral species (five with a horizontal and five with a vertical symbiont acquisition strategy) across two adjacent reefs in the southern Great Barrier Reef. Symbiodinium diversity was assessed using single-stranded conformational polymorphism of partial nuclear large subunit rDNA and denaturing gradient gel electrophoresis of the internal transcribed spacer 2 region. The Symbiodinium population in hosts with a vertical symbiont acquisition strategy partitioned according to coral species, while hosts with a horizontal symbiont acquisition strategy shared a common symbiont type across the two reef environments. Comparative analysis of existing data from the southern Great Barrier Reef found that the majority of corals with a vertical symbiont acquisition strategy associated with distinct species- or genus-specific Symbiodinium lineages, but some could also associate with symbiont types that were more commonly found in hosts with a horizontal symbiont acquisition strategy.     

The effect of temperature stress on coral–<Emphasis Type="Italic">Symbiodinium</Emphasis> associations containing distinct symbiont types

Several studies have demonstrated that the temperature tolerance of scleractinian reef-building corals is controlled, in part, by hosting physiologically distinct symbiotic algae. We investigated the thermal tolerance of coral–algal associations within seven common species of reef-building corals hosting distinct Symbiodinium sub-clades collected from Heron Island during experimentally induced bleaching conditions. During experimental heating, photosynthetic fitness was assessed by the dark-adapted yield of PSII (F _v/F _m), and excitation pressure across PSII (Q _m) of each coral–algal association using pulse amplitude modulation fluorometry. The onset of bleaching was determined by the measurement of Symbiodinium cell density. Using the ribosomal internal transcribed spacer 2 (ITS-2) region, we showed that Symbiodinium type–coral host associations were temporally and spatially conserved in a high proportion of the colonies sampled within each species. Generally, the species Acropora millepora, Platygyra daedalea, Acropora aspera and Acropora formosa contained Symbiodinium ITS-2 type C3, whereas the species Montipora digitata, Porites cylindrica and Porites lutea contained Symbiodinium type C15. Bleaching susceptibility showed some association with Symbiodinium type, but further research is required to confirm this. Corals hosting C3 Symbiodinium displayed higher reductions in F _v/F _m during heating compared to their C15 counterparts, irrespective of host species. However, a corresponding reduction in Symbiodinium density was not observed. Nonetheless, A. aspera and A. formosa showed significant reductions in Symbiodinium density relative to controls. This correlated with large increases in Q _m and decreases in F _v/F _m in heated explants. Our results suggest a range of bleaching susceptibilities for the coral species investigated, with A. aspera and A. formosa showing the greatest susceptibility to bleaching and M. digitata showing the lowest bleaching susceptibility. The data provide strong evidence for distinct differences in temperature tolerance between C3 and C15 Symbiodinium types when in-hospite; however, future studies addressing the confounding effect of host species would help to confirm this.     

The distribution of the thermally tolerant symbiont lineage (Symbiodinium clade D) in corals from Hawaii: correlations with host and the history of ocean thermal stress

Spatially intimate symbioses, such as those between scleractinian corals and unicellular algae belonging to the genus Symbiodinium, can potentially adapt to changes in the environment by altering the taxonomic composition of their endosymbiont communities. We quantified the spatial relationship between the cumulative frequency of thermal stress anomalies (TSAs) and the taxonomic composition of Symbiodinium in the corals Montipora capitata, Porites lobata, and Porites compressa across the Hawaiian archipelago. Specifically, we investigated whether thermally tolerant clade D Symbiodinium was in greater abundance in corals from sites with high frequencies of TSAs. We recovered 2305 Symbiodinium ITS2 sequences from 242 coral colonies in lagoonal reef habitats at Pearl and Hermes Atoll, French Frigate Shoals, and Kaneohe Bay, Oahu in 2007. Sequences were grouped into 26 operational taxonomic units (OTUs) with 12 OTUs associated with Montipora and 21 with Porites. Both coral genera associated with Symbiodinium in clade C, and these co‐occurred with clade D in M. capitata and clade G in P. lobata. The latter represents the first report of clade G Symbiodinium in P. lobata. In M. capitata (but not Porites spp.), there was a significant correlation between the presence of Symbiodinium in clade D and a thermal history characterized by high cumulative frequency of TSAs. The endogenous community composition of Symbiodinium and an association with clade D symbionts after long‐term thermal disturbance appear strongly dependent on the taxa of the coral host.     

Do introduced endosymbiotic dinoflagellates ‘take’ to new hosts?

In a recent communication by Stat and Gates (Biol Invasions 10: 579–583, 2008), discovery of a symbiotic combination involving the coral Acropora cytherea and the dinoflagellate endosymbiont, Symbiodinium A1 (Symbiodinium microadriaticum, Freudenthal sensu stricto) in the Northwest Hawaiian Islands was interpreted to be the result of a ‘recent’ introduction. While introductions of symbiotic dinoflagellates have occurred and are occurring, the authors’ conclusion was made without sufficient information about the geographic range and host specificity exhibited by A1. The only direct genetic analysis of symbionts from the putative host vector, a jellyfish in the genus Cassiopeia sp., from Kaneohe Bay on the Island of Oahu, found that it contained a different symbiont species, A3. Furthermore, Stat and Gates (Biol Invasions 10: 579–583, 2008) did not consider the importance of host-symbiont specificity in preventing the establishment of a foreign symbiont species. In comparison to A. cytherea, A. longicyathus on the southern most Great Barrier Reef also hosts Symbiodinium A1 and a closely related endemic, A1a. Instead of assuming that A. cytherea has an unnatural association, a practical explanation is that long-term ecological and evolutionary processes influenced by local environments underlie the unusual, but not unprecedented finding of a Pacific acroporid associating with Clade A Symbiodinium spp.     

Host tolerance,not symbiont tolerance,determines the distribution of coral species in relation to their environment at a Central Pacific atoll

Tolerance of environmental variables differs between corals and their dinoflagellate symbionts (Symbiodinium spp.), controlling the holobiont’s (host and symbiont combined) resilience to environmental stress. However, the ecological role that environmental variables play in holobiont distribution remains poorly understood. We compared the drivers of symbiont and coral species distributions at Palmyra Atoll, a location with a range of reef environments from low to high sediment concentrations (1–52 g dry weight m⁻² day⁻¹). We observed uniform holobiont partnerships across the atoll (e.g. Montipora spp. with Symbiodinium type C15 at all sites). Multivariate analysis revealed that field-based estimates of settling sediment predominantly explained the spatial variation of coral species among sites (P < 0.01). However, none of the environmental variables measured (sedimentation, temperature, chlorophyll concentration, salinity) affected symbiont distribution. The discord between environmental variables and symbiont distributions suggests that the symbionts are physiologically tolerant of the variable environmental regime across this location and that the distribution of different host–symbiont combinations present is largely dependent on coral rather than Symbiodinium physiology. The data highlight the importance of host tolerance to environmental stressors, which should be considered simultaneously with symbiont sensitivity when considering the impact of variations in environmental conditions on coral communities.     

Stress response of two coral species in the Kavaratti atoll of the Lakshadweep Archipelago, India

Frequent occurrences of coral bleaching and the ensuing damage to coral reefs have generated interest in documenting stress responses that precede bleaching. The objective of this study was to assess and compare physiological changes in healthy, semi-bleached and totally bleached colonies of two coral species, Porites lutea and Acropora formosa, during a natural bleaching event in the Lakshadweep Archipelago in the Arabian Sea to determine the traits that will be useful in the diagnosis of coral health. In April 2002, three “health conditions” were observed as “appearing healthy,” “semi-bleached” and “bleached” specimens for two dominant and co-occurring coral species in these islands. Changes in the pigment composition, zooxanthellae density (ZD), mitotic index (MI) of zooxanthellae, RNA/DNA ratios and protein profile in the two coral species showing different levels of bleaching in the field were compared to address the hypothesis of no difference in health condition between species and bleaching status. The loss in chlorophyll (chl) a, chl c and ZD in the transitional stage of semi-bleaching in the branched coral A. formosa was 80, 75 and 80%, respectively. The losses were much less in the massive coral P. lutea, being 20, 50 and 25%, respectively. The decrease in zooxanthellar density and chl a was accompanied by an increased MI of zooxanthellae and RNA/DNA ratios in both the species. There was an increase in accumulation of lipofuscin granules in partially bleached P. lutea tissue, which is an indication of cellular senescence. Multivariate statistical analyses showed that colonies of P. lutea ranked in different health conditions differed significantly in chl a, chl c, ZD, RNA/DNA ratios, and protein concentrations, whereas in A. formosa chl a, chl c, chl a/c, phaeopigments and MI contributed to the variance between health conditions.     

Occurrence of the putatively heat-tolerant <Emphasis Type="Italic">Symbiodinium</Emphasis> phylotype D in high-latitudinal outlying coral communities

Biogeographic investigations have suggested that coral-symbiont associations can adapt to higher temperatures by hosting a heat-tolerant Symbiodinium, phylotype D. It is hypothesized that phylotype D is absent in high latitudes due to its heat-tolerant characteristics. In this study, this hypothesis was tested by examining the symbiont diversity in a scleractinian coral, Oulastrea crispata, throughout its entire latitudinal distribution range in the West Pacific. Molecular phylotyping of the 5′-end of the nuclear large subunit of ribosomal DNA (lsu rDNA) indicated that phylotype D was the dominant Symbiodinium in O. crispata from the tropical reefs to the marginal non-reefal coral communities. Several colonies of tropical populations were associated with phylotype C, either alone or simultaneously with phylotype D. Analysis of the polymerase chain reaction products using single-strand conformation polymorphism (SSCP) detected relatively low densities of phylotype C in most of the O. crispata colonies surveyed. These results provide evidence for the occurrence of phylotype D in cold-water outlying coral communities. The dominant occurrence of phylotype C in some O. crispata colonies on tropical reefs and the relatively low densities of phylotype C identified by SSCP in subtropical and temperate populations show that the dominant symbiont type can vary in this coral species and that multiple symbionts can co-occur in the same host.     

Estimating the Potential for Adaptation of Corals to Climate Warming

The persistence of tropical coral reefs is threatened by rapidly increasing climate warming, causing a functional breakdown of the obligate symbiosis between corals and their algal photosymbionts (Symbiodinium) through a process known as coral bleaching. Yet the potential of the coral-algal symbiosis to genetically adapt in an evolutionary sense to warming oceans is unknown. Using a quantitative genetics approach, we estimated the proportion of the variance in thermal tolerance traits that has a genetic basis (i.e. heritability) as a proxy for their adaptive potential in the widespread Indo-Pacific reef-building coral Acropora millepora. We chose two physiologically different populations that associate respectively with one thermo-tolerant (Symbiodinium clade D) and one less tolerant symbiont type (Symbiodinium C2). In both symbiont types, pulse amplitude modulated (PAM) fluorometry and high performance liquid chromatography (HPLC) analysis revealed significant heritabilities for traits related to both photosynthesis and photoprotective pigment profile. However, quantitative real-time polymerase chain reaction (qRT-PCR) assays showed a lack of heritability in both coral host populations for their own expression of fundamental stress genes. Coral colony growth, contributed to by both symbiotic partners, displayed heritability. High heritabilities for functional key traits of algal symbionts, along with their short clonal generation time and high population sizes allow for their rapid thermal adaptation. However, the low overall heritability of coral host traits, along with the corals'' long generation time, raise concern about the timely adaptation of the coral-algal symbiosis in the face of continued rapid climate warming.     

Response of Tetraselmis suecica to nutrient and grazer manipulation

Three methods of algal quantification (direct cell counts, chlorophyll a extraction, in vivo fluorescence) were used to evaluate the response of the unicellular green flagellate Tetraselmis suecica to nutrients and grazers. Nutrient enrichment enhanced total cell counts, chlorophyll a concentration and in vivo and DCMU-fluorescence. Photosynthetic efficiency was reduced in the complete F2 medium as indicated by the high level of in vivo fluorescence, whereas photosynthetic efficiency was increased by the introduction of mussels to the F2 medium. The addition of mussels significantly increased the proportion of non-motile cells, but did not reduce the total cell count. The effect of mussel grazing on algae could be underestimated if only total cells were counted or only the chlorophyll a concentration was measured. The results indicate that these three methods measure different properties of an algal culture and are complementary to each other in assessing the quality and quantity of an algal population. Direct algal counting offers a reliable numerical assessment for cell population abundance. Chlorophyll a concentration was closely correlated to the total cell count. In the presence of mussels, in vivo fluorescence did not correlate with either algal cell counts or chlorophyll a concentration, indicating that the measurement of in vivo fluorescence may be misleading for estimating algal abundance under different culture conditions. This revised version was published online in August 2006 with corrections to the Cover Date.