R. van Woesik

Bleaching increases likelihood of disease on Acropora palmata (Lamarck) in Hawksnest Bay, St John, US Virgin Islands

Anomalously high water temperatures may enhance the likelihood of coral disease outbreaks by increasing the abundance or virulence of pathogens, or by increasing host susceptibility. This study tested the compromised-host hypothesis, and documented the relationship between disease and temperature, through monthly monitoring of Acropora palmata colonies from May 2004 to December 2006, in Hawksnest Bay, St John, US Virgin Islands (USVI). Disease prevalence and the rate of change in prevalence showed a positive linear relationship with water temperature and rate of change in water temperature, respectively, but only in 2005 during prolonged periods of elevated temperature. Both bleached and unbleached colonies showed a positive relationship between disease prevalence and temperature in 2005, but the average area of disease-associated mortality increased only for bleached corals, indicating host susceptibility, rather than temperature per se, influenced disease severity on A. palmata.

Coral communities and reef growth in the southern Great Barrier Reef

Abstract. Fringing reef development is limited around 22° S along the inner Great Barrier Reef, although there is substantial development north and south of this latitude. This study examined the relationships among coral communities and the extent of reef development. Reefs were examined to determine coral composition, colony abundance, colony size and growth form between the latitudes 20°S and 23°S. Major reef framework builders (scler- actinian genus Acropora and families Faviidae and Poritidae) dominated reefs north and south of 22°S, but declined significantly at 22°S where foliose and encrusting corals (Turbinaria and Montipora spp.) were most common. Porites spp. were present at 22° S but had encrusting morphologies. Consistently high turbidity at this latitude, caused by a 10 m tidal range and strong tidal flows, resuspends silts from the shallow shelf, and appears to have precluded reef development throughout the Holocene, by limiting the abundance, stunting the growth, and shortening the life expectancies of reef framework corals. The distinctions between ‘natural’ and ‘human-induced’ degradation may be interpreted on the basis of the relationship between Holocene development and current benthic community longevity. A mismatch between substantial past reef building capacity (a broad and/or thick reef) and non-existent or limited present reef-building capacity could signify anything from a long-period, natural cycle to an unprecedented deterioration in ecosystem function caused by human influence.

Symbiotic zooxanthellae provide the host-coral Montipora digitata with polyunsaturated fatty acids

We compared the fatty acid composition of the host-coral Montipora digitata with the fatty acid composition in the corals endosymbiotic dinoflagellates (zooxanthellae). Fatty acids as methyl esters were determined using gas chromatography (GC) and verified by GC-mass spectrometry. We found the main difference between the fatty acids in the host and their symbionts were that zooxanthellae supported higher proportions of polyunsaturated fatty acids. The presence of fatty acids specific to dinoflagellates (i.e. 18:4omega3, 22:5omega3 and 22:6omega3) in the host tissue suggests that zooxanthellae provide the coral host not only with saturated fatty acids, but also with diverse polyunsaturated fatty acids.

Photo-acclimation dynamics of the coral Stylophora pistillata to low and extremely low light

Abstract Photo-acclimation dynamics of the symbiotic coral Stylophora pistillata to lowering light intensities in the range of 95% to 0.8% of incident surface photosynthetic active radiation (PAR0) was studied. Coral colonies were sampled from 1- to 2-m depths in open and shaded habitats from the fringing reef of Sesoko Island (near the Tropical Biosphere Research Center, University of the Ryukyus) Okinawa, Japan. Photo-acclimation of exterior branches of coral colonies were examined in outdoor aquarium, where light regime and feeding were similar to field conditions. Two photo-acclimation reactions were studied: (1) changes in chlorophyll concentrations in zooxanthellae; (2) changes in zooxanthellar population density in coral branches. In parallel, we measured an average volume of zooxanthellae, proliferating zooxanthellae frequency (PZF) and degrading zooxanthellae frequency (DZF). It was shown that the coral S. pistillata can survive and acclimate to a wide range of light intensities from 95% to 0.8% PAR0. Acclimation to low light (30% and 8% PAR0) involves maximizing the light harvesting capacity by increasing photosynthetic pigment concentration in zooxanthellae and zooxanthellar population density in coral branches. Under extremely low light (0.8% PAR0), the coral lost zooxanthellae by degradation (perhaps digestion) and retained zooxanthellae-accumulated high concentrations of chlorophyll. The photo-acclimation process is dynamic and immediate. Changes in pigment concentrations in zooxanthellae occurred within 2–4 days and changes in zooxanthellar population densities occurred within 40 days. Zooxanthellae population densities were regulated by changes in rates of division and degradation of symbiotic cells.

Photo-acclimation of the hermatypic coral Stylophora pistillata while subjected to either starvation or food provisioning.

This study investigated the photo-acclimation capacity of the coral Stylophora pistillata (Esper). Outer branches of coral colonies, taken from 2 m, were subjected to 90, 20, or 3% of incident surface photosynthetic active radiation (PAR(0)), or kept in total darkness. The corals were maintained either in filtered seawater (i.e., under starvation), or in seawater that had daily additions of zooplankton (rotifers). The experiments were maintained for 31 days. Zooxanthellae population densities and chlorophyll concentrations increased in S. pistillata fragments subjected to 20 and 3% PAR(0). The zooxanthellae densities decreased after 6 days in corals kept in total darkness, although chlorophyll concentrations remained higher. Corals that were fed and subjected to 90% PAR(0) showed lower degrading zooxanthellae frequencies, higher photosynthetic and respiration rates, and higher chlorophyll concentrations than corals in the same light regime under starvation. Complete acclimation to dim (20% PAR(0)) and low (3% PAR(0)) light was only apparent for corals fed with zooplankton. Changes in zooxanthellae population densities occurred through differential rates of zooxanthellae division and degradation.

Strict paternal transmission of mitochondrial DNA of Chlamydomonas species is explained by selection against maternal nucleoids

Summary. The non-Mendelian inheritance of organelle DNA is common in most plants and animals. Here we examined inheritance mechanisms involved in the transfer of mitochondrial DNA. We successively backcrossed (to F5) two interfertile strains of the unicellular isogamous haploid algae Chlamydomonas reinhardtii and Chlamydomonas smithii to match nuclear backgrounds and examine transmission patterns of mitochondrial DNA by PCR analysis of cob gene sequences. Mitochondrial DNA was strictly transmitted paternally. To investigate the behavior of parental mitochondrial DNA, we used F5 progeny to form zygotes and isolated single zygotes. The results showed selective disappearance of maternal mitochondrial nucleoids occurred between 3 and 6 h after zygote formation.

Effects of coral colony morphologies on mass transfer and susceptibility to thermal stress

This study tested the hypothesis that corals of the same species, but of varying size and shape, may respond differently to thermal stress because of different mass transfer capacities. High mass transfer rates are an advantage under thermal stress, and mass transfer rates are assumed to scale with size. Yet large, corymbose Acropora colonies are more vulnerable to thermal stress than small corymbose Acropora colonies. We took a two-tiered approach to examine the differences in the susceptibility of different coral morphologies to thermal stress. Firstly, the response of several coral species of different sizes and shapes were measured in the field through a thermal stress event. Secondly, diffusion experiments were conducted using gypsum-coral models of different morphologies to estimate mass transfer rates, to test whether dissolution rates differed in accordance with colony morphology and colony size. Coral colonies with a high height to diameter ratio were subjected to more partial mortality than flat colonies. These results agree with mass transfer theory. The diffusion experiments showed that in a low-flow environment, small encrusting colonies had higher rates of dissolution than large flat or small branched colonies. These results, however, disagree with mass transfer theory. We show that the volume of space between colony branches predicts the response to thermal stress in the field. Small encrusting colonies were most likely to maintain mass transfer and were therefore more likely to survive thermal stress than large branched colonies. We predict that an increase in the frequency and intensity of thermal stresses may see a shift from large branched coral colonies to both small colonies, and flat-massive colonies with low aspect ratios.

Shading reduces coral-disease progression

The growing incidence of tropical-marine diseases is attributed to increases in pathogen prevalence and virulence associated with global warming. Additionally, the compromised-host hypothesis suggests that rising ocean temperatures may increase disease activity by making the corals more susceptible to ubiquitous pathogens. We tested the effects of reducing irradiance stress on coral-disease progression rates by shading corals showing signs consistent with white-plague disease. Our results showed that white-plague disease on shaded corals progressed significantly more slowly than on controls. Although the mechanisms are unknown, this study suggests that light intensity influences the rate of coral-disease progression.

Relationships between the history of thermal stress and the relative risk of diseases of Caribbean corals

The putative increase in coral diseases in the Caribbean has led to extensive declines in coral populations. Coral diseases are a consequence of the complex interactions among the coral hosts, the pathogens, and the environment. Yet, the relative influence that each of these components has on the prevalence of coral diseases is unclear. Also unknown is the extent to which historical thermal-stress events have influenced the prevalence of contemporary coral diseases and the potential adjustment of coral populations to thermal stress. We used a Bayesian approach to test the hypothesis that in 2012 the relative risk of four signs of coral disease (white signs, dark spots, black bands, and yellow signs) differed at reef locations with different thermal histories. We undertook an extensive spatial study of coral diseases at four locations in the Caribbean region (10(3) km), two with and two without a history of frequent thermal anomalies (approximately 4-6 years) over the last 143 years (1870-2012). Locations that historically experienced frequent thermal anomalies had a significantly higher risk of corals displaying white signs, and had a lower risk of corals displaying dark spots, than locations that did not historically experience frequent thermal anomalies. By contrast, there was no relationship between the history of thermal stress and the relative risk of corals displaying black bands and yellow signs, at least at the spatial scale of our observations.

Spatial differences and seasonal changes of net carbonate accumulation on some coral reefs of the Ryukyu Islands, Japan

This study sought to understand short-term spatial changes in accretion and erosion on (experimental) carbonate blocks on three coral reefs of the Ryukyu Islands, Japan. The principal objectives were to differentiate net accretion/erosion according to season, location, depth and substrate-type. At all locations the summer season showed more positive net weight changes and higher coralline algal coverage than the winter season. Windward reefs revealed higher net accretion and higher coralline algal coverage than leeward reefs. Massive (Holocene) Porites blocks showed highest net loss, followed by Pleistocene carbonate and (Holocene) Acropora blocks. High population densities of Echinometra mathaei (de Blainville) were recorded on reefs adjacent to large human populations and overall net carbonate loss significantly correlated to densities of E. mathaei type A.