Tuesday, 24 December 2013

Levels of immunity parameters underpin bleaching and disease susceptibility of reef corals.

Since our lecture on coral diseases, I was curious about the coral immune system. This paper focused on variables commonly associated with invertebrate immunity and investigated their relationship with regards to coral susceptibility (bleaching and disease) with both hard (Scleractinia) and soft (Alcyonacea) corals spanning 10 families from the Great Barrier Reef. Whilst this paper mainly focuses on coral bleaching, it mentioned several times how the mechanisms of defence against coral bleaching are similar to the defence strategies employed during infection.

This paper regarded four main variables, which were used to make up a Constituent immunity number. These variables were presence/frequency of melanin, size of melanin containing granular cells, phenoloxidase (PO) activity, and concentrations of fluorescent proteins (FPs.)

  •         Presence/frequency of melanin
  •         Size of melanin containing granular cells

The melanin-synthesis pathway is a key invertebrate defence mechanism, triggered by physical injury or invasion by foreign organisms. When activated, it enables the deposition of encapsulating melanins and produces highly cytotoxic intermediates such as reactive oxygen species (ROS).

The 15 species investigated all contained melanin-containing granular cells in the free body wall tissue, however the distribution and density of melanin containing granular cells varies within species. Melanin frequency was negatively correlated with disease susceptibility (deducted from field studies of disease prevalence) – ergo corals with the lowest melanin frequency showed the highest disease prevalence. The spatial distribution of melanin is also important in the effectiveness of coral defence e.g. having UV- and visible-light-absorbing melanain in epidermal cell layers may protect zooxanthellae from excess UV/light in the environment.

It has been proposed melanin-containing granular cells are potential amebocytes, a mobile possible phagocytic defence mechanism which characteristically aggregate in area of injury or invasion. A mobile defence mechanism is further beneficial as entry points of attack are unpredictable.

  •        Phenoloxidase (PO) activity – which indicates melanin pathway activity.

This is another indication of melanin pathway activity, and once again was present in all coral families, while showing a significant variation of activity dependent on species.

  •         Concentrations of fluorescent proteins (FPs)

Known to remit light at different wavelengths, the biological role of FPs is still not fully understood, however they do show reactive oxygen scavenging properties which would provide protection during high oxidise stress conditions such as coral bleaching and pathogen invasion. FPs are present in unbleached but still compromised tissues suggests that they are involved in immunity against not just coral bleaching, but also infection. Like other variables investigated, concentrations varied significantly among coral families.


Immunity parameters were strongly interdependent with size of melanin-containing granular cells, PO activity and FP concentrations. There was a significant positive correlation between Constituent immunity and both bleaching and disease susceptibility.

Consistent differences in disease susceptibility across families suggest taxa-specific levels of disease resistance and investment in immunity mechanisms. Immunity was found to explain over 78% of interfamily variation in bleaching susceptibility, and 83% of disease susceptibility, providing evidence that the coral host itself plays a major role in pathogen and bleaching resistance.

Another interesting point raised by the paper, is the idea that Immunity varies among individuals due to species specific differences in allocation of energy to certain traits. This paper uses life history theory to predict the species which have higher immune functions, will have allocated less energy into growth and/or reproduction. This idea was backed up by the fact that the fast growing/branching species of Acropora (which requires a vast amount of energy to promote rapid colonial growth) was seen to demonstrate the lowest immune rank, which in turn contributed to a high susceptibility to bleaching and disease.

The paper also mentioned the necessity of a holist approach in understanding bleaching and disease with regards to corals – due to the combined effects of several base defence mechanisms in these species: which I though was a point that needed reiterating, as often we get so focused in science we find ourselves drawing conclusions between A and B – without taking into consideration the whole network of mechanisms which are going on in the microscopic world!

Palmer, C. V., Bythell, J. C., & Willis, B. L. (2010). Levels of immunity parameters underpin bleaching and disease susceptibility of reef corals. The FASEB Journal, 24(6), 1935-1946.


  1. I also found the mechanisms behind the immunity of corals really fascinating after that lecture...I am just wondering whether the paper suggested that the synthesis and frequency, of melanin, is a dominant form of immunity against pathogens (and hence is the primary way of fighting infection), or whether this mechanism acts synergistically with other factors on a more equal level to maintain coral health (e.g. with the fluorescent proteins)?
    I think you are right to assume about the complex systems that influence the health of coral reefs, especially as they don't seem to really mention an important factor, the zooxanthellae, as my dissertation is based on the thermotolerance of Scleractinian corals, and many other papers have made it clear to me that the mechanisms involved in their resilience are heavily influenced by the species specificity of both the coral host and the algae in the symbiotic relationship (I thought it was worth mentioning as you have already touched on the high variability of immunity and susceptibility amongst coral taxa alone). I am also just wondering whether the susceptibility/ tolerance of the algae to environmental stresses that can lead to bleaching and diseases, can also affect the level of immunity to pathogens. For instance, do the different clades of zooxanthellae (and hence their coral hosts) that are more tolerant to bleaching also contribute to a higher immunity, as you mentioned that the mechanisms against bleaching and diseases are similar?

    You mentioned that the higher concentration of melanin in corals can lead to greater photoprotection from UV radiation, but I think the authors could have also used a common technique to measure the effects of UV (by determining the photochemical efficiency and/ or photoinhibition of the symbiotic algae) to support this assumption.

    1. Hi Caroline, this study is indeed fascinating and shows correlation between those immune parameters and bleaching and disease susceptibility.

      Please don't get me wrong but, I think the statement that you have made "the mechanisms of defence against coral bleaching are similar to the defence strategies employed during infection" is very confusing and I am sceptical about it.

      As per the research I've done, I believe that "coral bleaching" and "coral disease" are totally different things. There is association between them. Disease prevalence is observed usually after a bleaching event because bleached corals are stressed and hence very weak and compromised. Similarly, some pathogens like Vibrio shiloi may induce bleaching.

      However, coral diseases are caused by infection/invasion of one and/or more than one pathogen/s whereas bleaching is a stress response which is brought about by complex set of biochemical reactions that occur in coral-dinoflagellate symbiosis, finally leading to loss of dinoflagellate symbiont (zooxanthellae) and in many cases mortality of both the coral host and its symbiont zooxanthellae.
      In general, cellular immune mechanisms (innate immunity) are used against invading pathogens and the cell may get rid of them through cellular processes like autophagy and apoptosis.
      Establishment of symbiosis between coral host and zooxanthellae is also kind of infection of coral host cell by the zooxanthellae. But in this case, this infection is favorable or necessary. To avoid apoptosis by innate immunity of coral host, the immune mechanisms of coral host are suppressed. In this context, "sphingosine rheostat" - a molecular immune mechanism made of sphingolipid sphingosine (Sph) and its phosporylated form sphingosine-1-phosphate (S1P) plays a role in symbiosis stability and bleaching. S1P is linked with symbiosis stability and Sph is linked with dissociation of symbiosis(bleaching) through apoptosis. Stress somehow disturbs balance between Sph and S1P towards Sph, leading to apoptosis and bleaching. See Detournay & Weis 2011 and Rodriguez-Lanetty et al. 2006 for more details on this.
      In this perspective bleaching is defined as modified host innate immune response of coral host where under stressful conditions coral host innate immunity treats its symbiont like a pathogen and removes it. See review by Weis (2008) on mechanisms of bleaching.
      Thus, bleaching itself is thought as an immune response against zooxanthellae under stressful conditions.

    2. Furthermore, I guess that the mechanisms that are involved in defence against thermal and light stress induced-bleaching would include mechanisms that ameliorate photoinhibition and photo-damage to Photosystem II of zooxanthellae chloroplast, which includes ability of non photochemical quenching, switching to cyclic electron flow from linear electron flow, xanthophyll cycling, ability of carrying out light-induced dissociation of antennae protein complexes, lipid composition of thylakoid membrane of zooxanthellae chloroplast etc. The zooxanthellae subcladal types that are competent in having these and many other abilities and qualities, would influence how would they defend thermal stress-induced bleaching. Similarly, abilities of host and its zooxanthellae to induce numerous heat shock proteins and antioxidant enzymes and having secondary metabolites like Mycosporine-like amino acids (e.g. Mycosporine-like-glucine) would also influence how corals resist bleaching because macromolecular damage and heavy oxidative stress is faced by coral-dinoflagellate symbiosis during thermal stress that lead to bleaching. There are possibly many more factors that would influence this, I have given just few examples that I know. There is special importance of thermal history in acclimating zooxanthellate cnidarians against bleaching. I am looking in this area for my dissertation. There is also potential role of epigenetics in thermal acclimation in corals.
      In contrast to these, the strategies employed during infection would be innate immune pathways and other holobiont aspects that we have discussed in the lectures, which include associated microbial communities producing antibiotics and other molecules that interfere with the quorum sensing of pathogens and in turn protect their host from the pathogenic and opportunistic microbes.

  2. With this level of understanding in coral disease and bleaching, is it fair to say that we may be able to help ecologically important coral habitats to survive through anthropologically-induced coral diseases/bleaching by introducing a genetically modified zooxanthellae, more able to readily defend themselves against photo and bacterial/viral stressors? Do you think this would provide some relief for coral reef systems?

  3. Disregarding the whole moral issue that comes with genetic modification ;)

    1. Hi Rachel,

      I don't think that using genetically modified zooxanthellae is going to help corals in any way and I even don't think that people would invest money in doing that because -
      1. As far coral bleaching is regarded, it is a natural phenomenon in which cnidarian host loose their symbiont zooxanthellae cells and in some cases they can also acquire them back. Acquiring tolerant zooxanthellae back immediately after loosing sensitive ones under bleaching conditions is the adaptive bleaching hypothesis. Alternatively, corals re-establish their zooxanthellae population after return of normal environmental (temperature) conditions. This process of zooxanthellae going out from the coral host and coming back again in the coral host is a dynamic process and which also occurs under changing seasons (Fitt et al. 2000).
      Hence, say if you develop a genetically modified zooxanthellae and insert it in a coral host; there are very little or no chances of it staying in the same coral host for longer time. It will be expelled. Apart from that there is trade-off between thermal tolerance of zooxanthellae and it's other physiological characteristics which is why coral host would not maintain thermally tolerant zooxanthellae population under normal environmental conditions because they cannot contribute enough photosynthetic products necessary for growth of the reef corals.
      2. Just considering zooxanthellae for finding solutions on problems of corals would just not work. Because bleaching is failure of symbiosis between zooxanthellae and its cnidarian host. I mean in these processes the role of the host is as important as of zooxanthellae. Thus, just using GM zooxanthellae would not work at all.
      3. Coral-zooxanthellae symbiosis (+ holobiont side) is relatively strong to environmental odds. It is showing acclimatization and adaptation to changing environmental conditions (see Thompson & Woesik 2009; Maynard et al. 2008).

      I am optimistic that corals with their partners in some way would cope with the human-activity related global climate changes. To help them coping with the changing environment, what all we need to do is to minimise impacts of local stressors as Rohwer & Youle 2010 have described in their book. Local stressors include overfishing, pollution, adding nutrients to reef water through addition of sewage and also through cutting mangrove forests etc.