Coral Reef Science:  Development Highlights

Habib Sekha

C. Ferrier-Pagès, F. Houlbrèque, E. Wyse, C. Richard, D. Allemand and F. Boisson. Bioaccumulation of zinc in the scleractinian coral Stylophora pistillata. Coral Reefs. December 2005, 636 - 645.

Abstract:

The uptake kinetics of zinc (Zn), an essential nutrient for both photosynthesis and calcification, in the tissue of S. pistillata showed that the transport of Zn is composed of a linear component (diffusion) at high concentrations and an active carrier-mediated component at low concentrations. The carrier affinity (K m=28 pmol l−1) was very low, indicating a good adaptation of the corals to low levels of Zn in seawater. Zn accumulation in the skeleton was linear; its level was dependent on the length of the incubation as well as on the external concentration of dissolved Zn. There was also a light-stimulation of Zn uptake, suggesting that zooxanthellae, through photosynthesis, are involved in this process. An enrichment of the incubation medium with 10 nM Zn significantly increased the photosynthetic efficiency of S. pistillata. This result suggests that corals living in oligotrophic waters might be limited in essential metals, such as zinc.

Comments:

Various discussions and publications have analyzed heavy metals in salt mixes, aquaria, food and supplements. From such discussions and publications we know that heavy metals such as zinc can become toxic if their concentration is high enough AND if they're in a bio-available form. Despite their toxicity at high concentration, they are essential for many important biological processes.

Zinc is an essential nutrient for photosynthesis, calcification and various other processes. The authors demonstrated that raising zinc's concentration by 0.65 ppb results in an increase in S. pistillata's (a coral some of the above authors often use in various studies) photosynthetic efficiency. I estimate that the final zinc concentration might have been around 1 ppb. The typical zinc concentration in average aquaria is usually higher than that; not all of the zinc, however, is in a bioavailable form.

The presence of an active carrier that's efficient at low zinc concentration, and an increase in zinc's uptake through light stimulation, suggest that various processes in the host and in symbionts affect zinc's uptake rates. Such effects and the increase in photosynthetic efficiency with zinc enrichment do suggest, according to the authors, that corals living in oligotrophic waters might be limited in zinc and other essential metals.

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Crabbe MJ, Smith DJ. Modeling variations in corallite morphology of Galaxea fascicularis coral colonies with depth and light on coastal fringing reefs in the Wakatobi Marine National Park (S.E. Sulawesi, Indonesia). Comput Biol Chem. 2006 Jan 4; [Epub ahead of print].

Abstract:

Coastal environments in the tropics can suffer from high sedimentation and low light levels. Galaxea fascicularis is a hermatypic coral that is relatively resilient to stress from bleaching and from sedimentation. Corallite dimensions - width, height, corallite densities and inter-corallite distances - of Galaxea fascicularis colonies varied significantly with depth, and so with incident light at the Sampela reef in the Wakatobi Marine National Park, S.E. Sulawesi, Indonesia. Modeling studies based on our data showed that corallite width decreased with the amount of incident light, while corallite height increased with the amount of incident light (r(2) values of 0.835 for width and 0.869 for height). In all colonies, inter-corallite distance decreased significantly with the amount of incident light (r(2)=0.89). Colony width/height ratio increased in a linear fashion with inter-corallite distance for all colonies studied (r(2)=0.65). Current flow as judged using tags did not appear to be a factor in variations of corallite dimensions. Our results suggest that under low light conditions, distribution and size of corallites may optimize heterotrophic nutrition, while in conditions where light is not limiting, distribution and size of corallites may be optimal for photosynthesis.

Comments:

Galaxea fascicularis taken from different depths from the Sampela reef, Sulawesi, Indonesia, were shown to have different corallite heights, widths and inter-corallite distances. Current flow was probably not a first order effect. On the other hand, strong correlations were found with the amount of incident light. With increasing light, corallite width decreased, height increased and inter-corallite distance decreased.

The authors suggest that the coral's variation (modulation) of its corallites' size and distribution is to either optimize food catching abilities or to optimize light absorption (photosynthesis). If they are correct, then Galaxea fascicularis that rely more on photosynthesis than heterotrophy would have relatively narrower but higher corallites and smaller inter-corallite distances than those that are more heterotrophic. Changes in aquarium parameters (light, skimming and feeding) might, therefore, also induce changes in corallite dimensions and abundance.

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Enriquez, S. Multiple scattering on coral skeletons enhances light absorption by symbiotic algae. Limnol. Oceanogr., 50(4), 2005, 1025-1032.

Abstract:

The success of symbiotic reef-building corals is largely determined by the efficiency with which they collect solar energy. Using thin coral laminae from the Caribbean scleractinian, Porites branneri, we characterize the absorption spectra of intact coral surfaces. Comparisons of absorption spectra from corals with a broad range of photosynthetic pigment densities, collected during a natural bleaching event, indicate that they are capable of collecting more than 85% of solar radiation with one order of magnitude less pigment density than terrestrial leaves. Measurements of the light-absorption efficiency as a function of pigment density reveal that symbiotic algae in intact P. branneri absorb between two and five times more light than freshly isolated symbionts. A theoretical model shows that multiple scattering by the skeleton can enhance the local light field, thus increasing absorption. As a result of this phenomenon, corals inhabiting high light environments can maximize their absorption capacity with low pigment investment while reducing self-shading in low-light environments. Local light field enhancements may have negative effects when corals are exposed to stressful conditions. During coral bleaching, increases in local irradiance associated with reductions in pigment density could exacerbate the negative effect of elevated temperatures. Symbiotic scleractinian corals are one of the most efficient solar energy collectors in nature, and the modulation of the internal light field by the coral skeleton may be an important driving force in the evolution of this group.

Comments:

The study above shows clearly that light harvesting is a cooperative effort by the symbionts and the host in the coral they investigated. The freshly isolated zooxanthellae from intact P. branneri absorbed only 20 - 50% of the light that shined on the coral. Because they corrected the data for pigment density, the difference might be real and not an artifact. The explanation they offer is based on a theoretical model that light scattering by the skeleton would enhance the light field locally, thereby increasing light absorption. I assume that they did eliminate this effect when measuring pigment densities. Nevertheless, they conclude the abstract by saying, "Symbiotic scleractinian corals are one of the most efficient solar energy collectors in nature…" This and the previous abstract both suggest that coral structure (at a relatively low structural level) helps to modulate light for zooxanthellae and the host as a function of ambient conditions.