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Across the world, coral species are threatened by rising rates of disease, a major factor leading to the decline of coral populations. With global temperatures on the rise due to anthropogenic climate change, the environments in which they thrive are being altered . Unable to escape habitats that become unfavourable, they are forced to adapt to changing environmental conditions. Interactions between corals and their surface microbes likely play an important role in adaptation, and understanding this relationship could allow scientists to detect and characterize the diseases that attack them.
In a paper recently published in Nature, Ochsenkun and colleagues found that unique metabolites and microbial communities harboured by coral surfaces act as indicators of biotic and abiotic stress, and thus investigated their influence on coral health .
Using current knowledge of corals and their environment, the researchers aim to determine the mechanism of infection, specifically for white-syndrome type diseases. Along with their symbiotic microbial communities and associated metabolites, corals possess a layer of mucus that acts as their first line of defence .
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However, environmental stress alters the microbial composition and structure of the mucus, which ultimately leaves the corals vulnerable to disease. Ochsesnkun and colleagues wanted to determine whether corals do in fact secrete molecules that are retained in the mucus layer, and help to either fend off harmful bacteria or attract beneficial ones. The study confirmed that six particular molecules were expressed in very difference abundances, found exclusively in either healthy samples or in diseased coral samples. These findings suggest that they may be vital to disease prevention or microbiome regulation.
Figure 1 | Sample ranges.
Ochsenkun et al. used a minimally invasive technique to collect samples from Acropora (left) and Platygyra (right) coral species. Using a syringe with an attached soft silicon nozzle, samples were taking at the 3 distances shown: 0cm representing the coral surface, 5cm above the coral surface, and 50cm as the seawater control (with the assumption that microbial communities found in 0cm and 5cm samples would be very different from control samples). The structure of each coral species led to different levels of correlation between 0cm and 5cm samples.
The team collected samples from Acropora or Platygyra coral species at random spots around the Saadiyat Reef in Abu Dhabi. Using a minimally invasive technique, these samples were drawn at three distances relative to the coral surface: 0 cm (coral surface), 5 cm, and 50 cm (seawater control) (Fig. 1). A number of replicate samples were taken at these distances from visibly healthy tissues, as well as tagged infected tissues for proper microbiome comparison. Both Acropora and Platygyra species contained 50% noticeably healthy tissue and 50% white-syndrome diseased tissue. From these samples, they were able to sequence microbial DNA, analyze the metabolites, and identify the contained compounds.
After examining microbial compositions, unsurprisingly, results for all of the samples were dominated by Pelagibacteriales, one of the most common and abundant groups of bacteria found in marine environments . However, the researchers excluded this finding, as they believed the dominance of this particular bacterium could hinder the analysis of abundance patterns of other prokaryotes present in the sample. The remainder of the bacterial composition was found to contain several other types of bacteria, including Endozoicomaceae, which is thought to be a symbiont of corals . Bacterial and other biological components were much more clustered in 0 cm samples than at other distances, suggesting that coral surfaces may harbor unique biological communities that maintain a symbiotic relationship and possibly contribute to protection against disease.
Analysis and identification of molecular features from Acropora and Platygyra proved that a large percentage are shared between both coral species. Although up to 93% of features specific to a particular species were detected in samples at all three distances, the remainder were found exclusively in 0 cm, or in both 0 cm and 5 cm samples, not in the seawater control. Higher levels of organic nitrogen and sulfur were observed at coral surface and 5 cm samples as well. From the 18 molecules that were found to be common amongst both species, 6 of these were either more abundant only in diseased coral or only in healthy coral samples.
These findings prove significant in determining the true nature of surface-associated molecules. Those most abundant in healthy samples likely play a role in protection against disease or symbiotic microbe colonization, while those most abundant in diseased samples could function as part of the coral's defense mechanism. Understanding the effect of these molecules and bacteria on coral health is vital to the survival and conservation of coral populations which serve as an important habitat for so many aquatic species. Since there are several different pathogens affecting corals, future research should likely encompass a wider range of coral-specific diseases, rather than limiting it to white syndrome-type diseases only. Although more research is required to fully comprehend the relationship between corals and their microbiomes, Ochsenkun at al. provide some valuable insight on the topic.
NICOLE CORBETTAcross the world coral species are threatened by. (2019, Dec 08). Retrieved from https://studymoose.com/nicole-corbettacross-the-world-coral-species-are-threatened-by-example-essay
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