Atypical corals: lessons from these organisms living under extreme conditions
Coral reefs are one of the most threatened ecosystems by human activities. Given the changing environment, corals face a significant challenge to adapt to suboptimal or extreme conditions due to oxygen depletion, temperature increase, and ocean acidification amongst others.
In order to understand the adaptive capacity of corals, scientific research has been conducted in the wild, on reefs established under suboptimal (marginal) or extreme conditions. Why study them in the wild, instead of simulating conditions in a laboratory? By conducting these studies in the wild the exposure duration of reefs to specific conditions is maximized, allowing for observation of community interactions and biological processes under co-occurring stressors, which are difficult conditions to replicate in the laboratory. This can give some hints on how corals will acclimate or adapt facing changing conditions in the years to come. These ecosystems function as “natural laboratories”  to study the responses of organisms to conditions such as exposure to high temperatures, low pH due to proximity to volcanic vents, urban and high turbidity conditions, mesophotic* areas, among others.
In order to gather the scientific knowledge gathered so far on this subject, a review paper was published in 2020 in the scientific journal Coral Reefs, coordinated by the researcher John Burt from the University of Abu Dhabi, in collaboration with researchers from several universities and research centers in Australia, the United States, and Singapore.
Even if in the article the authors develop the results of several types of reefs (Fig 1), here only the reefs exposed to extreme temperatures and the reefs in turbid zones will be described.
Figure 1: Spatial (left) and temporal (right) distribution of scientific publications related to coral reefs under marginal and extreme environmental conditions. On the left, the size of the circles shows the number of publications in the different areas. On the right, the curve shows the number of scientific publications accumulated over time for each reef type (Source: ).
Corals’ resistance to high temperatures
The environmental conditions of shallow coral reefs are constantly fluctuating due to the tides’ variation, more recently intensified due to climate change. Corals are thus faced with numerous pressures from their living environment .
The heat waves of the last few years have put shallow water and lagoon reefs at the center of attention. Indeed, although the coral cover continues to decrease in most reef areas, some coral communities show a significant thermal tolerance. Through their study, scientists have identified the ways in which they stand and survive to high temperatures.
Several mechanisms have been identified at the basis of this resistance. One of them is the role of the microbiome. On one hand there is the plasticity of the composition of microorganisms associated with corals, which varies according to the habitat and allows the holobiont** to adapt to the changing environment .
On the other hand, studies in areas of high species endemicity and high water temperatures, such as the Persian Gulf, have shown that corals are more tolerant of high temperatures due to the permanent association with the thermo-resistant microalgal species Cladocopium thermophilum, amongst other mechanisms .
Nevertheless, the resistance to these extreme temperatures does not make reefs immune. The energetic cost of surviving to suboptimal heat conditions, such as in the Persian Gulf, results in smaller corals and minor calcification capacity and fecundity compared to similar species found in the Oman Sea . Also, studies have found that thermal stress can compromise the ability of corals to defend themselves from diseases, being more vulnerable to them .
Corals’ adaptation to turbid conditions
Turbid reefs contrast with clear water reefs in that they have a high suspended sediment content. Although sediments decrease light intensity and impact the photosynthesis made by the zooxanthellae, field studies have shown lower coral mortality on some turbid coastal reefs than on clear water reefs when exposed to similar temperatures . This resistance to thermal stress would be mainly explained by the low luminosity of the turbid environment, which limits the solar irradiation stress on corals, and their bleaching . Thus, turbidity can be perceived both as a threat and a means of survival during high temperatures’ events.
However, natural turbidity is often aggravated by sediment runoff accompanied by chemical pollutants (pesticides, insecticides, waste, sewage) . In this case, deleterious effects can be perceived on corals and fish. Indeed, when sediments are accompanied by pollutants, fish visibility can be impacted, impairing their ability to find shelter. In addition, the smallest particles accumulate on the fishes’ gills limiting the capacity of oxygen extraction . These data highlight the complexity of turbidity impacts on reefs, and why sedimentation and runoff are often recognized as a major threat to many coastal coral reefs.
Coral reefs have never been as impacted by anthropogenic pressures as they are today. It is therefore crucial to understand where marginal reefs are located and what are the different factors that allow them to survive in sub-optimal or extreme environmental conditions. The ability of these corals to cope with rising temperatures and high turbidity is a result of adaptations acquired through natural selection, allowing them to survive, notably through their symbiotic associations. Their study in the natural environment allows us to highlight the assets that these organisms possess that could be key elements to understand the fate of coral ecosystems facing the consequences of climate change.
* Mesophotic zone: mesophotic coral ecosystems exist in areas of poor solar illumination, often distributed from 30 to 40 meters depth.
** Holobiont: an assemblage composed of a host and species of microorganisms that live in association by mutualism or commensalism (microbiota). In corals, it is the assemblage of organisms which constitute the coral: the hosts (polyps), and the microorganisms in association (like zooxanthellae, bacteria and viruses).
For further information:
 Burt J, Camp E, Enochs I, Johansen J, MorganK, Riegl B, Hoey A (2020) Insights from extreme coral reefs in a changing world. Coral Reefs. 39, 495–507 https://doi.org/10.1007/s00338-020-01966-y
 Camp EF, Suggett DJ, Pogoreutz C, Nitschke MR, Houlbreque F, Hume BCC, Gardner SG, Zampighi M, Rodolfo-Metalpa R, Voolstra CR (2020) Corals exhibit distinct patterns of microbial reorganisation to thrive in an extreme inshore environment. Coral Reefs. https://doi.org/10.1007/s00338-019-01889-3.
 Aeby G, Howells E, Work T, Abrego D, Williams, Gareth & Wedding L& CaldwellJ & Moritsch Ma & Burt J (2020) Localized outbreaks of coral disease on Arabian reefs are linked to extreme temperatures and environmental stressors. Coral Reefs. 39. 10.1007/s00338-020-01928-4.
 Wagner DE, Kramer P, van Woesik R (2010) Species composition, habitat, and water quality influence coral bleaching in southern Florida. Mar Ecol Prog Ser 408:65–78.
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