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 Physiological and adaptive basis of thermal tolerance of scleractinian corals

The current and predicted scenarios of global warming bring to light an urgent need to explore the capacity of scleractinian corals and their microbial symbionts to acclimatize and/or adapt to the increasing frequency of elevated seawater temperatures on regional and global scales. In the IMAGES Lab we are currently focused on discerning the genetic response of coral species to global environmental changes by looking at the molecular mechanisms behind physiological thermal tolerance and by identifying the genetic traits under global-change-induced selection. This area of research is crucial for a complete understanding of the potential of tropical corals to survive and adapt to rapid global climate changes.

In this project we conduct gene expression studies (DNA microarrays and RNA-seq) which allow us to identify the genes that are regulating thermal tolerance whether under simulated or controlled experimental settings. With this knowledge we will be able to monitor these genes by comparing the plastic response of populations exposed to different thermal histories in the field (natural habitat).

Moreover, we use genetic markers to monitor whether environmental changes, such as seawater temperature increases, influence corals and their symbionts at the genomic level to assess the selective pressure imposed by environmental changes and to measure the potential of coral populations to respond to global climate change by evolutionary adaptation. Specific questions that we are currently addressing are:

1) What are the molecular level effects that account for thermal tolerance ranges? and How do these effects differ from those observed in thermal injury?

2) What type of changes in gene expression is needed to achieve thermal tolerance/acclimation? and How is the transient nature of the phenomenon of acquired thermal tolerance?

3) What is the pattern of expression of thermal tolerant genes in natural populations? and How do these transcription profiles correlate to the natural thermal history of these coral populations?

 

Impact of ocean warming on the immunological response of corals to pathogenic microbial agents

As the effects of climate change have become increasingly visible over the past three decades, coral reefs have suffered detrimental effects from warming oceans, resulting in a critical decline in coral populations. On one hand, the effect of increasing seawater temperature causes a physiological disruption of the symbiotic association between corals and their symbiotic dinoflagellates, compromising the important source of nutrients provided by the algal symbionts. On the other hand, a correlated effect has been documented with increases of water temperature and the frequency of coral diseases. Recent work supports the hypothesis that opportunistic infectious pathogens, whose virulence and growth is enhanced by increased temperature, thrive and become aggressive during hot seawater thermal anomalies, which then cause coral mortality. It is not clear, however, if increases of seawater temperatures also compromise the immune defense system of corals and therefore affect their ability to fight infection. This effect has barely been studied, as we still know very little about the immune defense repertoire of scleractinian corals used to fight microbial pathogens. This is extremely relevant if we are to understand how corals will respond to the increasing frequency of diseases associated with ocean warming linked to global climate changes. In my lab we are examining how the immune response of corals to microbial agents is affected by thermal stress challenges, and whether corals have the capacity to acclimatize to elevated temperature and therefore resist the detrimental thermal effects on their immune systems. Specific questions that we are currently addressing are:

1) Is the resistance response (immune competence) of corals to microbial pathogenic challenges affected by thermal stress?

 2) Does the microbial community associated with corals change during the thermal stress? If so, does this change correlate with change of immune competence?

 3) Can we identify molecular-level effects that account for the reduced immune-competence in corals?

 4) Can corals which have been thermally-acclimatized by exposure to short periods of high temperatures respond physiologically better to microbial pathogenic agents than corals that are not thermally-acclimatized? If so, does this response correlate to altered microbial community associated with the acclimatized coral?

 5) Does the bacterial community associated with thermally acclimatized corals change after exposure to high temperature?

 

Uncovering the role of the symbiotic microbial consortia in coral thermal tolerance

Over the last decade, studies on the microbial communities associated to reef corals have made us realize the large diversity and highly complex biological interactions that compose the coral holobiont. While it has been widely documented that there is a dynamic microbial biota living on the surface (mucus) and in the tissue of many coral species, the role of these microorganisms in the reef ecosystem and their contribution to coral well-being remain, for the large part, unclear. Therfore, my lab is particular interested to develope a better understanding of the microbial community associated with scleractinian corals in the acclimatization process (thermal tolerance) to elevated seawater temperatures linked to global warming. For this we study the chnges of the composition and the community-level gene expression profiles of the microbial consortia associated with corals that undergo thermal acclimatization and determine whether gene expression changes of the microbial community play a role in coral thermal tolerance.Parallel to the comprehension of the coral host physiology, a clear understanding of the responses of the coral-associated microbial symbionts to thermal stress is of paramount importance in the face of intensifying threats on the world’s coral reefs. Specific questions that we are currently addressing are:

1) How do changes in microbial community structure from acclimatized corals compare with corals that are thermally injured and undergo coral bleaching?

 2) Is there a functional metabolic shift of the microbial community during the process of thermal tolerance of the coral holobiont?

 3) How do these functional changes correlate with shifts in the composition/structure of the microbial community during the process of thermal tolerance of the coral holobiont?

 4) How do the functional changes in the microbial community from acclimatized corals compare with corals that are thermally injured and undergo coral bleaching?

 5) How transient are these structural/functional shifts of the microbial community in thermally acclimatized corals?  Will the microbial community return to the pre-stress profiles in terms of composition, structure and function?

 

Reservoirs and ecology of free-living Symbiodinium

Ecological studies have started showing that coral reef ecosystems worldwide are undergoing major shifts in community structure from coral-dominated communities to seaweed-dominated assemblages. Whether these changes are just temporary or irreversible shift phases in coral reef communities is not clear and still under discussion based on the current findings. More importantly, the capacity of coral recovering is currently unknown.

The thrive of communities dominated by scleractinian corals would depend both on healthy growth rates of already established coral colonies but also on the successful establishment of new coral recruits derived from sexual reproduction. The success of these new coral recruits would not only rely on finding available and adequate substratum to settle on the reef, but very importantly and crucially on finding and acquiring their symbiotic dinoflagellates (Symbiodinium) which would help the coral host to live and grow. As most coral species acquire their symbiotic dinoflagellates from the environment (i.e. horizontal or open-system transmission) in each generation, it is crucial to understand if these environmental reservoirs of symbionts are not also compromised by the local and global community changes occurring within coral reef ecosystems due to natural and anthropogenic stressors. The problem is that we still do not know what the actual habitat reservoirs used by Symbiodinium during the free-living stage are, as our understanding of the ecology of free-living Symbiodinium is precarious.

In IMAGES, we are aimed at answering a core set of open questions that are pivotal for forming a better understanding of the distribution and population fluctuations of free-living Symbiodinium in coral reef habitats. some these questions are:

1) Is there any preference of free-living Symbiodinium to live in sediment, water column, coral rubble and/or on macroalgae found in the reef environment?

2) What is the spatial-temporal fluctuation of free-living Symbiodinium abundance associated with these reef habitats?

3) Is there any particular pattern of association between strains of Symbiodinium and functional-form (morphological architecture) and/or species of macroalgae?