Large-amplitude internal waves sustain coral health during thermal stress

Ocean warming is a major threat for coral reefs causing widespread coral bleaching and mortality. Potential refugia are thus crucial for coral survival. Exposure to large-amplitude internal waves (LAIW) mitigated heat stress and ensured coral survival and recovery during and after an extreme heat anomaly. The physiological status of two common corals, Porites lutea and Pocillopora meandrina, was monitored in host and symbiont traits, in response to LAIW-exposure throughout the unprecedented 2010 heat anomaly in the Andaman Sea. LAIW-exposed corals of both species survived and recovered, while LAIW-sheltered corals suffered partial and total mortality in P. lutea and P. meandrina, respectively. LAIW are ubiquitous in the tropics and potentially generate coral refuge areas. As thermal stress to corals is expected to increase in a warming ocean, the mechanisms linking coral bleaching to ocean dynamics will be crucial to predict coral survival on a warming plane

Growth rates and skeletal density of Desmophyllum dianthus - Effect of association with endolithic algae

It has been suggested that endolithic algae inside the skeleton of cold-water corals might have a symbiotic relationship with the coral host and would positively affect coral calcification. However, so far this hypothesis has not yet been further explored. This study investigated the effect of endolithic algae on the growth performance and skeletal density of the cold-water coral Desmophyllum dianthus at Fjord Comau, southern Chile. The fluorescent staining agent calcein was used to document coral growth by measuring the upward linear extension of septa for a period of one and a half years. Observations on skeletal density were recorded using x-ray computed tomography. The results of this study show a severe reduction of growth rates associated with the presence of endolithic algae. Infested individuals grew about half as fast as non-infested polyps with median value of 1.18μm/day compared to 2.76μm/day. Data on skeletal density revealed a similar – although not statistically significant – trend displaying mean values of 2.160g/cm³ compared to 2.294g/cm³, respectively. These results point towards a parasitic relationship between D. dianthus and its endolithic algae refuting the hypothesis of a mutually beneficial association. However, although this study appears to conclusively indicate a negative effect of the association of D. dianthus with endolithic algae, controversial evidence has been discovered regarding the mode of the relationship. Despite the decrease in growth performance, the coral host seems to benefit from a low transfer of metabolites from the endoliths to the coral tissue. Further research will be necessary to fully resolve the matter

Primär Produktion in Korallenriffen

Coral reef ecosystems sustain their high productivity in an oligotrophic environment by the efficient recycling of nutrients and tight benthic-pelagic coupling. The basis for productivity within coral reefs is the primary production, which is accomplished by diverse benthic photosynthetic organisms and, to a minor contribution, by phytoplankton in the waters aloft. Photosynthesis is determined by the availability of light, influenced by water depth and turbidity, and the availability of inorganic nutrients, essential for the organic synthesis of vital substances like amino acids. Benthic photosynthesis in coral reefs is characterised by the high abundance of manifold zooxanthellae bearing hosts, in particular scleractinian corals. Findings of the following chapters are presented from the organism to the ecosystem level (chapter 1-5), concerning organic matter recycling (nutrient availability) and energy conversion (light availability and photosynthesis) within a coral reef ecosystem. Nutrient and light availability vary likewise between ecosystems and are comparatively investigated at the Similan Islands in the Andaman Sea of the Indian Ocean (chapter 6-8), concerning environmental conditions, benthic community composition, primary production and heterotrophy

Primary production in coral reefs, Key players and adaptive strategies to natural environmental variations

Coral reef ecosystems sustain their high productivity in an oligotrophic environment by the efficient recycling of nutrients and tight benthic-pelagic coupling. The basis for productivity within coral reefs is the primary production, which is accomplished by diverse benthic photosynthetic organisms and, to a minor contribution, by phytoplankton in the waters aloft. Photosynthesis is determined by the availability of light, influenced by water depth and turbidity, and the availability of inorganic nutrients, essential for the organic synthesis of vital substances like amino acids. Benthic photosynthesis in coral reefs is characterised by the high abundance of manifold zooxanthellae bearing hosts, in particular scleractinian corals. Findings of the following chapters are presented from the organism to the ecosystem level (chapter 1-5), concerning organic matter recycling (nutrient availability) and energy conversion (light availability and photosynthesis) within a coral reef ecosystem. Nutrient and light availability vary likewise between ecosystems and are comparatively investigated at the Similan Islands in the Andaman Sea of the Indian Ocean (chapter 6-8), concerning environmental conditions, benthic community composition, primary production and heterotrophy

Gaining ground - cleaning with mesenterial filaments by the cold-water coral Desmophyllum dianthus?

The solitary cosmopolitan cold-water coral Desmophyllum dianthus (Esper, 1794) (A, scale bar approximately 1 cm) occurs as a deep-water emergent species in Patagonian fjords, Chile, and can form extensive banks (Försterra et al. 2005). This coral functions as a bioengineer by providing a 3D habitat for a diverse benthic community (Försterra et al. 2005). With skeletal growth, the living tissue is retracted by the organism to the upper part of the calyx, leaving the lower skeleton uncovered. Endolithic algae are then able to enter the tissue covered skeleton from below and start to encrust the tissue covered part of the skeleton (Försterra et al. 2005, 2008). They are visible as a greenish coat on the skeleton (A). The algae are able to bore into the skeleton (Försterra et al. 2005) and thus could interfere with the coral’s calcifying process. This process could result in a parasitic behavior (Försterra et al. 2012) rather than the previously hypothesized (Försterra and Häussermann 2008) symbiotic or even mutualistic relationship. This infection by algae may be the primary reason for the peculiar behavior we observed on several specimens both in situ (A) and in our aquaria system (B–D, scale bar approximately 1 cm). After an acclimatization time in our aquaria system, wild collected corals were well adapted and well fed, and secondarily expanded their tissue toward their lower skeleton (about 1 cm in 4 mo, CJ and GMS pers obs), recovering an area that had been without any tissue. Concomitantly, the corals everted their mesenterial filaments between this re-entered outer-skeleton and the covering tissue layer (A, B; C, D: both enlargements of B). During the eversion of the mesenterial filaments the new tissue generally appeared unusually pleated (C, D) in contrast to a less common smooth tissue surface (A). This was not a short-term or rare mechanism: corals maintained this behavior in the aquaria over multiple months. Several tropical coral species are known to eject their mesenterial filaments to digest prey too large to be swallowed (Yonge 1930). The tropical, branching coral Acropora pulchra (Brook, 1891) has even been observed to clean the substrate before it expands its tissue and skeleton (Roff et al. 2008). Our preliminary observations suggest a combination of both mechanisms in this cold-water coral. Mesenterial filaments could be digesting the growing algae below their tissue, but also may be cleaning the substrate (their own skeleton) prior to further tissue expansion

Turf algae-mediated coral damage in coastal reefs of Belize, Central America

Many coral reefs in the Caribbean experienced substantial changes in their benthic community composition during the last decades. This often resulted in phase shifts from scleractinian coral dominance to that by other benthic invertebrate or algae. However, knowledge about how the related role of coral-algae contacts may negatively affect corals is scarce. Therefore, benthic community composition, abundance of algae grazers, and the abundance and character of coral-algae contacts were assessed in situ at 13 Belizean reef sites distributed along a distance gradient to the Belizean mainland (12–70 km): Mesoamerican Barrier Reef (inshore), Turneffe Atoll (inner and outer midshore), and Lighthouse Reef (offshore). In situ surveys revealed significantly higher benthic cover by scleractinian corals at the remote Lighthouse Reef (26–29%) when compared to the other sites (4–19%). The abundance of herbivorous fish and the sea urchin Diadema antillarum significantly increased towards the offshore reef sites, while the occurrence of direct coral-algae contacts consequently increased significantly with decreasing distance to shore. About 60% of these algae contacts were harmful (exhibiting coral tissue damage, pigmentation change, or overgrowth) for corals (mainly genera Orbicella and Agaricia), particularly when filamentous turf algae were involved. These findings provide support to the hypothesis that (turf) algae-mediated coral damage occurs in Belizean coastal, near-shore coral reefs

Coral community composition and reef development at the Similan Islands, Andaman Sea, in response to strong environmental variations

The Similan Islands, a Thai archipelago in the Andaman Sea located near the shelf break, are subjected to frequent (up to several events per hour) and abrupt changes in physicochemical conditions, particularly during the dry season (NE monsoon, January through April) and to an intense monsoon season with strong surface wave action (May to October). The exposed west slopes of the islands feature more coral species, but lack a carbonate reef framework. By contrast, the sheltered east sides show a complex reef framework dominated by massive Porites. Our results suggest that the sudden changes in temperature, pH and nutrients (drops of up to 10°C and 0.6 U and increases of up to 9.4 μmol NOx l−1, respectively) due to pulsed upwelling events may rival the importance of surface waves and storms in shaping coral distribution and reef development