Latitudinal Patterns in the Distribution of Algal Symbionts (Symbiodinium spp.) in Reef Corals of Madagascar, and their Response to Thermal Disturbance

The island continent of Madagascar spans nearly 13.5o of latitude in the SW Indian Ocean. Its coastline includes a number of well developed coral reefs, ranging from tropical Nosy Bé (NW Madagascar, 12oS) and Vohemar (Volhmarina, NE Madagascar, 13oS) to subtropical Tuléar (Toliara , SW Madagascar, 23.5oS), as well as temperate coral communities at Fort Dauphin (Tolagnaro, SE Madagascar, 25oS). Given the range of environmental conditions experienced by reef corals at these different sites, Madagascar represents an ideal location to study the distribution of algal symbionts (Symbiodinium spp.) in these coral hosts. To investigate the effect of latitudinal gradients in temperature on Symbiodinium distributions, 220 samples from 27 coral genera in 12 families were collected from these 4 sites in September 2001. To test the stability of these distributions over time, a further 337 samples were collected from the Nosy Bé and Tuléar regions in March 2007 and November 2009. Symbiodinium communities were screened using Denaturing Gradient Gel Electrophoresis (DGGE) to analyze the internal transcribed spacer-2 (ITS-2) region of Symbiodinium ribosomal DNA, with individual symbiont taxa identified by sequencing individual DGGE bands. Significant differences were found in the Symbiodinium cladal composition of reef corals at different sites, with corals at northern sites containing a higher relative frequency of Symbiodinium in clade D (occurring as mixed clade C+D communities) than southern sampling sites. Nominal logistic analysis of the distribution of symbionts found a significant effect of coral taxa and site, but not of sea surface temperature metrics (environmental data obtained from NOAA’s Coral Reef Watch satellite-derived data) in determining the distribution of different symbionts. Rarefaction analysis indicated there were no differences in Symbiodinium richness (at either the clade or the subtype level) between different sites, or between different sampling intervals. Differences existed in the subcladal composition of dominant ITS-2 types found in congeners at different latitudes, with corals in the genus Acropora being dominated by Symbiodinium C3 (specifically subtype C3z) in northern sites, and C1 in southern sites. Symbiont communities changed between 2001 and 2007/2009, with increases in mixed Symbiodinium C+D assemblages occurring at southern sites that had experienced temperature stress during the intervening period. Decreases in mixed Symbiodinium communities occurred at northern sites, which were not as severely affected by thermal stress. It is suggested that the latitudinal gradients in Symbiodinium found in Madagascar, and the environmental controls on community structure described here, provide important insight into how coral species in this understudied area can adapt or acclimatize to changing environmental conditions through shifts in the composition of their symbiont communities. This will help improve our understanding of how projected climate change in the SW Indian Ocean will affect survival trajectories for coral reefs in the region

Fast Growth May Impair Regeneration Capacity in the Branching Coral Acropora muricata

International audienceRegeneration of artificially induced lesions was monitored in nubbins of the branching coral Acropora muricata at two reef-flat sites representing contrasting environments at Réunion Island (21u079S, 55u329E). Growth of these injured nubbins was examined in parallel, and compared to controls. Biochemical compositions of the holobiont and the zooxanthellae density were determined at the onset of the experiment, and the photosynthetic efficiency (F v /F m) of zooxanthellae was monitored during the experiment. Acropora muricata rapidly regenerated small lesions, but regeneration rates significantly differed between sites. At the sheltered site characterized by high temperatures, temperature variations, and irradiance levels, regeneration took 192 days on average. At the exposed site, characterized by steadier temperatures and lower irradiation, nubbins demonstrated fast lesion repair (81 days), slower growth, lower zooxanthellae density, chlorophyll a concentration and lipid content than at the former site. A trade-off between growth and regeneration rates was evident here. High growth rates seem to impair regeneration capacity. We show that environmental conditions conducive to high zooxanthellae densities in corals are related to fast skeletal growth but also to reduced lesion regeneration rates. We hypothesize that a lowered regenerative capacity may be related to limited availability of energetic and cellular resources, consequences of coral holobionts operating at high levels of photosynthesis and associated growth

Biochemical properties of nubbins of <i>Acropora muricata</i> at the onset of the lesion regeneration experiment.

*<p><i>p</i><0.05;</p>**<p><i>p</i><0.01;</p>***<p><i>p</i><0.001.</p

Environmental conditions.

<p>Daily average sea surface temperature (SST; ± SD, gray area) and cumulative daily solar radiation during the experimental period at (<b>a</b>) Planch’Alizé and (<b>b</b>) Kiosque.</p

Growth of <i>Acropora muricata</i>.

<p>The mean (± SD) relative increase in the projected surface area (‰ d⁻¹) of control and injured nubbins by site.</p

Calcification and lesion healing of <i>Acropora muricata</i>.

<p>Relationship between calcification rates and lesion healing for injured nubbins from Planch’Alizé and Kiosque.</p