Human impacts lead to both global- and local-scale environmental changes and cause the ongoing decline of Coral Reefs documented through the Anthropocene. Stressors including increasing sea surface temperatures and physical habitat destruction may completely extirpate most Corals before the end of the century. Ultimately, worldwide action to decrease the carbon dioxide emissions that drive global change is essential to preserve the ecologic and economic services of reefs. Under less extreme conditions, variation in Coral’s stress response (colony to colony and species to species) and mortality is observed. However, understanding the mechanisms of resistance and resilience of Corals to environmental change is paramount to ongoing management and restoration efforts at local scales. The impacts of climate change together with pollution, over-exploitation, and habitat destruction severely impact coastal (Coral) ecosystems. In heavily urbanised regions, physical destruction and eutrophication reduce coral Cover and overall diversity with differential effects on species and individuals. From these, we can identify ‘winners and losers’ associated with any type of environmental change. Those colonies which appear more robust could then be effectively employed for conservation/restoration efforts and utilised to identify mechanisms that contribute to resistance and resilience and employed for assisted evolution approaches.
In a paper published in the journal Coral Reefs on 2 May 2020, Till Röthig of the Swire Institute of Marine Science and School of Biological Sciences at the University of Hong Kong, and the Aquatic Research Facility at the University of Derby, Henrique Bravo, also of the Swire Institute of Marine Science and School of Biological Sciences at the University of Hong Kong, and of the Groningen Institute for Evolutionary Life Sciences at the University of Groningen, Alison Corley, Tracey-Leigh Prigge, Arthur Chung, Vriko Yu, and Shelby McIlroy, again of the Swire Institute of Marine Science and School of Biological Sciences at the University of Hong Kong, Mark Bulling and Michael Sweet, also of the Aquatic Research Facility at the University of Derby, and David Baker, once again of the wire Institute of Marine Science and School of Biological Sciences at the University of Hong Kong, present the results of a study of the microniome of the Coral Oulastrea crispata, and how this varies across the highly urbanised coastline of Hong Kong, with a view to undestanding this species ability to survive in a tough and highly variable environment.
Corals are holobionts, relying on an inter-kingdom symbiosis that includes the Animal host, its endosymbiotic Dinoflagellates from the family Symbiodiniaceae, and a complex suite of other microbial partners including Bacteria, Archaea, Fungi, and Viruses. The members of this holobiont are tightly interlinked and appear to contribute together to its overall health, for instance, through nutrient acquisition and recycling. Further, they seem to play an important role in the resilience of their host to environmental change. For example, variation in their associated Symbiodiniaceae can confer thermal resilience with shifts in community composition leading to a more advantageous state, at least temporarily. The Coral’s Prokaryote community has also been identified as an important contributor to the holobiont’s environmental resilience. Changes in the Bacterial communities are repeatedly correlated with varying environmental parameters such as temperature or salinity.
Water quality can also significantly affect the microbiome of Corals and increase the probability of disease outbreaks. Low water quality and nutrient input, especially in urbanised habitats, result in increases in nitrogen, phosphorus, and turbidity/sedimentation, which affect the local spatial distribution of Corals and Coral diversity. Mostly laboratory-based studies show that increases in nutrients impair function, facilitate disease, and cause bleaching, the latter two often leading to increased chances of mortality. However, there is a lack of studies linking specific compounds from multi-stressor eutrophic environments (e.g. ammonium, nitrate, phosphate, or turbidity) directly to shifts in specific Coral microbiomes in situ. In most cases, the microbiome shifts or responds to any given stress event prior to visible signs of stress in the Coral itself. Therefore, it has been suggested that dysbiosis in the Coral microbiome could be used as a bioindicator for Coral health and contributes to their response to environmental perturbations.
Corals harbour a relatively high diversity of Prokaryotes within their microbiome. It is therefore difficult to untangle key members (belonging to the core microbiome and/or providing important functions) from merely transient associates, for instance, caught in the dynamic mucus layer or simply part of the holobiont’s usual food source. Recently, it has been argued that Corals, as hosts, may be divided into two main categories as has been previously described for Sponges. The first category includes those which regulate a conserved, stable microbiome (‘microbiome regulator’). The second category includes those which have greater ‘microbiome flexibility’ in their associates (‘microbiome conformer’) and therefore the potential for dynamic microbiome adjustment under environmental change. It has been hypothesised that climate change ‘winners’ will be the microbiome conformers which can adjust more readily to environmental change. This extends the Coral probiotic hypothesis whereby a Coral can counteract a possible negative dysbiosis by adjusting its composition.
This offers a glimmer of hope for the ability of Corals to adapt to global change in evolutionarily relatively short timescales. The same mechanisms of microbial turnover may confer intraspecific resilience to local environmental stressors. Hong Kong offers a punctuated water quality gradient that shapes Coral distributions on a relatively small spatial scale. Mirs Bay in the north-east is flushed by comparatively good quality oceanic waters and harbours at least 94 Coral species. In contrast, to the north-west, heavily eutrophied waters discharging from the Pearl River result in poor water quality, which has driven the regional extirpation of all but one hard Coral species, Oulastrea crispata. Röthig et al. capitalised on this species’ ubiquitous distribution to test whether microbial conformity (or regulation) underpins this species’ broad tolerance of environmental conditions. To do this, they examined several water parameters (temperature, salinity, dissolved oxygen, turbidity, pH, ammonia, nitrite, nitrate, total nitrogen, phosphorus, and chlorophyll a) as potential drivers of spatial differences in Oulastrea crispata’s microbiome.
A colony of the Coral, Oulastrea crispata, sometimes called the Zebra Coral for its distinctive black & white colouration. Charlie Veron/Corals of the World.
The marine habitats around Hong Kong are characterised by pronounced environmental fluctuations and intense anthropogenic pressures stemming from the world’s fifth busiest shipping port, coastal development, fishing, and public recreation associated with a city of over seven million inhabitants The marine habitats are diverse, with open-ocean conditions to the south-east and estuarine waters in the west and south-west. Seasonality leads to stark environmental changes in Hong Kong waters. In the dry winter season, the north-east monsoon (and the Coriolis force) results in a western current; thus, the effect of Pearl River freshwater inflow on the marine environment of Hong Kong is limited. In contrast, during the wet summer season, south-western monsoon winds drive the freshwater plume into Hong Kong waters. The Pearl River flows through the most densely populated region in the world and discharges about 350 billion cubic metres of freshwater, and 85 million tons of sediment per year, as well as large amounts of sewage and nutrients. The influence of estuarine waters decreases towards the north-east (Mirs Bay, Crescent Island), and this is reflected by a pronounced change in water quality and salinity. Overall, the water quality is particularly poor in the Pearl River Delta, Deep Bay, and Victoria and Tolo Harbour.
Water quality data from the Hong Kong Environmental Protection Department’s fixed sampling stations (1 m depth) were utilised to characterise the water quality gradient and ultimately identify variables that align with Coral microbiome differences. The parameters include temperature, salinity, disolved oxygen, turbidity, pH; and ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, phosphorus, and chlorophyll a concentrations. Nine stations were selected which covered five areas with relatively high Coral cover with consideration of local distribution patterns. These included: NM1 for Lantau Island, SM6 and SM18 for Lamma Island, PM9 and PM11 for Bluff Island, MM7 and MM3 for Crescent Island, and TM4 for Centre Island. Where two environmental sampling stations approximately equidistant to a field site were available, an average of the two was used. Measurements for this study were taken across three months (June, July, and August) in 2018. The stations outline a pronounced water quality and Coral diversity/distribution gradient.
Sampling sites for Coral and environmental parameters. Blue stations depict Hong Kong Environmental Protection Department fixed stations for water parameter measurements. Red stars indicate Coral sampling sites. The inlaid map shows the geographic location of Hong Kong. Röthig et al. (2020).
Owing to its high latitude location and close proximity to the Pearl River, the environment of Hong Kong is characterised by extremely variable conditions, which contribute to the high local biodiversity. The region is considered marginal for Coral growth. Coral coverage and diversity generally increase with distance from the outflow of freshwater and sediment from the Pearl River Estuary. Only one hard coral, Oulastrea crispata (family Faviidae), can cope with the adverse conditions at the sampling site at Lantau. At the opposite side of the water quality gradient, Crescent Island, with about 60 reported hard Coral species, provides a site of comparably high Coral diversity around Hong Kong. Oulastrea crispata is the most widely distributed Scleractinian Coral in Hong Kong and remarkably robust. Given that Oulastrea crispata does not occur closer to the Pearl River Delta, the habitat around Lantau may provide environmental conditions that are at the edge of the Coral’s environmental niche. Acclimatisation and/or adaptation to these challenging conditions may therefore include the contribution of all holobiont members. Compared to less challenged individuals, Oulastrea crispata specimens from Lantau provide an opportunity to test for a contribution of the microbiome to environmental flexibility.
Five replicate fragments (about 5 cm²) from the coral Oulastrea crispata were collected at a depth between 3 and 5 m from each of five locations, where possible. At Lantau, Bluff, and Crescent, one replicate each was sampled between 1 and 7 m. To exclude differences in these ‘outliers’, all samples from the same location were tested for consistency in colour, visual health state, and microbiome composition. Coral sampling locations were selected as close to the locations where water parameters were measured as possible, including Lantau, Lamma, Bluff, Crescent, and Centre. All corals were sampled within two days (18 and 19 August 2018) to reduce temporal variation. All colonies sampled were visually healthy and the same size to reduce any effect of age. A sterile hammer and chisel were utilised by the divers who wore nitrile gloves. Upon surfacing, the samples were rinsed with filtered seawater, wrapped in sterile aluminium foil, and flash-frozen in liquid nitrogen. All coral fragments were then stored at -20° C.
Environmental parameters were measured between June and August 2018 at all eight stations including Lantau, Lamma (two stations), Bluff (two stations), Crescent (two stations), and Centre Islands. Measured parameters were variable across the five sites. Differences are driven on temporal and spatial scales and reflect the variable nature of the environmental conditions at the survey sites of this study, especially considering that the summer months are characteristically not affected by the regionally strong seasonal variations. On a spatial scale, averages of the monthly measurements highlight pronounced water quality gradients. Water quality increases from Lantau to Lamma, Bluff, and Crescent, reflected by an average increase in salinity and a decrease in nutrients (ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, and total phosphorus), and chlorophyll a. A second water quality gradient in Tolo Harbour is also present from Centre to Crescent (i.e. an increase in salinity and decrease in ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, and chlorophyll a).
The temporal variability is reflected by pronounced fluctuations at any one site (such as salinity at Lantau, turbidity at Crescent, and total nitrogen at Bluff). The measurements further include ‘anomalies’ that likely stem from extreme weather events such as strong, seasonal rainfall, e.g. a salinity of 18.2 at Centre and a drop in chlorophyll a from 31.0 to 0.6 μg per litre within one month at Lantau. These variations will however contribute to the uniqueness of each habitat from which the corals were sampled.
Overall, Lantau was the most variable habitat, as this location exhibits the highest variations for disolved oxygen, ammonia nitrogen, nitrite nitrogen, total phosphorus, and chlorophyll a, but interestingly the lowest variability for temperature. In contrast, Bluff (most variable for total nitrogen; least variable for disolved oxygen, turbidity, pH, total phosphorus) together with Crescent (most variable for total nitrogen; least variable for salinity, nitrite nitrogen, nitrate nitrogen, total phosphorus, and chlorophyll a) may be considered more stable during the months assessed in Röthig et al.'s study.
Each correlation analysis (based on the three environmental data sets: August, average July and August, and avererage June, July, and August) yielded different correlations. After removal of correlating parameters for August 2018, only five out of the initial 11 environmental parameters remained for further analyses (temperature, salinity, disolved oxygen, nitrite, and total nitrogen). For the averaged months (July and August 2018), four parameters revealed no interdependencies (temperature, salinity, pH, and nitrate). Another four variables (disolved oxgyen, total nitrogen, total phosphorus, and chlorophyll a) were kept for analyses of the averages of June to August 2018.
A total of 501 319 DNA sequences were produced across 27 samples (i.e. five Oulastrea crispata samples for each of the five collection sites and two negative controls). After quality trimming and removal of singletons (i.e. sequences that occurred only once across the full data set) and unwanted sequences, 285 955 sequences with an average length of 292 base pairs were available for subsequent analyses. To evaluate bacterial community composition across samples, Röthig et al. classified all sequences to the family level and visualised the data in a stacked column plot. Interestingly, no Coral microbiome was dominated (in terms of relative abundance) by any one single taxon. The most abundant taxa included an unclassified family of each Gammaproteobacteria, Alphaproteobacteria, Rhodobacteraceae, Flavobacteriaceae, and Vibrionaceae, each contributing less than 10% on average. Variation occurred between Coral samples, including those from within the same site. However, the stacked column plots reveal no apparent trend between samples from different sites. The Bacterial community compositions of the coral samples are markedly different from the controls, which were used to identify contaminants and then disregarded from further analyses.
To examine differences in the corals’ bacterial community composition (between sites), sequences were clustered into Operational Taxonomic Units (clusters of organisms, grouped by DNA sequence similarity of a specific taxonomic marker gene; a pragmatic proxy for species in the absence of a full taxonomic study). A total of 11 669 distinct Operational Taxonomic Units were identified across all five sites and subsampled to 3051 reads per sample. The average number of unique observed Operational Taxonomic Units was lowest for the corals collected from Centre at 1173 Operational Taxonomic Units and highest for Lamma with 1426 Operational Taxonomic Units. The average estimated species richness ranged from 1218 for Corals at Centre to 1462 for Lamma. Despite the pronounced variations in water quality parameters, there were no significant differences in the number of unique Operational Taxonomic Units for any of the corals across the sites surveyed, indicating apparent structural stability in Oulastrea crispata’s Bacterial associates.
Bacterial community composition in Oulastrea crispata. Bacterial taxonomy stacked column plot on the phylogenetic level of family. Each colour represents one of the 14 most abundant families with over 6000 sequences each across all samples. All other taxa are grouped under the category ‘others’. Numbers in parentheses depict the number of distinct species within the family. Control 1, extraction control; Control 2, sequencing control. Röthig et al. (2020).
Oulastrea crispata is associated with a high diversity of Bacterial taxa, which is reflected by 11 669 distinct Operational Taxonomic Units across all 25 Coral samples, with an average of Operational Taxonomic Units per sample. To visualise this high diversity and to identify differences in the Corals’ bacterial community profiles, Operational Taxonomic Unit data were plotted in an ordination plot. The plot shows a separate clustering of samples from Lantau and Bluff and a cluster of four out of five samples from Centre, albeit both axes explaining only about 20% of the variation. When statistically assessing differences between the locations, an analysis of molecular variance revealed a significant difference between only two of the sites, Bluff and Lantau. However, non-significant differences between Bluff and Centre, Bluff and Lamma, Centre and Crescent, Centre and Lamma, Centre and Lantau, Crescent and Lantau, and Lamma and Lantau, indicate a generally high spatial heterogeneity. This heterogeneity is further corroborated by the linear discriminant analysis effect size analysis and the core microbiome (determined as Operational Taxonomic Units that occur at 100% of all samples). In total, 286 Operational Taxonomic Units were identified as indicator species representing the Coral-associated Bacterial communities at any given site, including 109 for Lantau, 66 for Lamma, 47 for Bluff, 29 for Crescent, and 35 for Centre. The core microbiome consisted of seven Operational Taxonomic Units (three unclassified Gammaproteobacteria, a Nitrospira sp., an unclassified Sandaracinaceae, an unclassified Cyclobacteriaceae, and a Ruegeria sp.). All core microbiome members are among the 35 most abundant across all samples (over 300 occurrences). The core microbiome shows a heterogeneous pattern without apparent abundance trends along the environmental gradient or between the sites. That said, one of the identified core members of the microbiome (Cyclobacteriaceae) was also identified as an indicator species for one of our sites, Lantau. However, its relative abundance at Lantau was only slightly higher than that at both Lamma and Centre.
Nitrospira sp., a chemolithoautotrophic nitrite-oxidising Bacterium, in the phylum Nitrospirae, which formed part of the core microbiome of the Oulastrea crispata Corals tested in Röthig et al.'s study. Boris Nowka & Eva Spieck/University of Hamburg/Phys.org.
To assess the explanatory power of environmental parameters with respect to the response of the Coral-associated Bacterial communities (i.e. spatial heterogeneity), redundancy analyses were performed on each of the three cleaned environmental data sets. The redundancy analysis employed one month of environmental data (closest to the sampling date) and considered temperature, salinity, disolved oxygen, nitrite, and total nitrogen based on the correlation analysis. It revealed that 22.9% of the observed spatial differences in the microbial communities were explained by the environmental variables included in the model. The results were confirmed by a significant permutation analysis. However, stepwise selection did not show any significance, with total nitrogen explaining most of the microbiome variations. When considering environmental data from two months (July and August), in this instance including temperature, salinity, pH, and nitrate, similar results were obtained, i.e. 22.9% of the observed variation was explained, further confirmed by a significant permutation analysis. Again, there was no significance in the stepwise selection contributing parameters, with nitrate explaining the most variation. When three months were taken into account, exploring the impacts of disolved oxygen, total nitrogen, total phosphorus, and chlorophyll a, 22.9% of the variation was again explained. The stepwise regression was also non-significant with total phosphorus explaining most of the variation, albeit without statistical significance. Conserved results in the redundancy analyses irrespective of the considered time frame indicate analytical robustness. However, relatively low explanatory values and the lack of significantly contributing environmental parameters suggest other factors play a role.
As with many other organisms, Coral health depends on the complex yet essential microbiome. However, poor water quality can disrupt the microbiome (dysbiosis) and lead to disease and mortality. This suggests that microbiome stability itself may contribute to the environmental robustness of the Coral holobiont. Indeed, some Corals appear to be able to conserve (regulate) their microbiome, while the alternative strategy appears to be microbial conforming. In the latter, Corals can adjust their microbial members to gain a temporarily more advantageous composition while maintaining structure and stability. Defining microbiome regulators and conformers has only recently been attempted in Corals, with a recent study having shown that common reef-building Corals like Pocillopora verrucosa and Acropora hemprichii can utilise these contrasting strategies. Pocillopora verrucosa maintained a structurally conserved microbiome along an environmental water quality gradient based on differences in evenness, richness, and diversity. To build on this concept of microbiome flexibility and how the ‘choice’ of strategy might be driven by the environment, Röthig et al. sampled a widely distributed coral, Oulastrea crispata, and assessed its microbiome composition (spatially, not temporally) across an environmental gradient consisting of dramatic changes in water quality. Interestingly, Oulastrea crispata's microbiome appears structurally stable even at the very poor water quality sites.
Coral microbial communities perform essential functions within the holobiont. While the mechanisms are not fully understood, microbes have been linked to holobiont carbon, sulphur, and nitrogen cycling, antimicrobial properties, and secondary metabolism. In response to environmental stress, Coral microbiome diversity has been shown to increase, possibly caused by disruption of the native microbiome in terms of stability and function, thereby allowing for invasion of atypical residents. The spatial and temporal variability of the microbiome in response to environmental conditions suggests that perhaps only a small number of taxa are obligate symbionts which make up a ‘core microbiome’. Poor water quality, in particular when originating from sewage, is known to affect Coral physiology and the associated microbiome. Studies along eutrophication gradients have demonstrated significant differences in the coral microbiomes according to the water quality/nutrient loads in different species, and such perturbation characteristically results in dysbiosis and disease. Importantly, upon removal of the stressor, affected Corals can quickly reorganise their microbiomes back to their original ‘healthy’ state.
Based on this body of evidence Röthig et al. anticipated, microbiome differences driven by the changes in water parameters would be visible for Oulastrea crispata along the west to east, low to high water quality gradient across which this species is distributed in Hong Kong. Röthig et al. show that the microbiome in Oulastrea crispata is highly diverse and lacks dominant taxa across a range of environmental conditions. The Coral microbiome at the lowest water quality sites (Lantau and to a lesser extent Centre) differed from sites with relatively better water quality (i.e. Bluff and Crescent), which is characterised by certain indicator species among the rich Coral-associated bacterial assemblages. Compared to other studies, however, the microbiome differences are moderate. For example, Corals exposed to pronounced environmental changes, arguably pushing them to their ‘limit’ in terms of environmental robustness, have been shown to strongly shift or restructure their microbial consortia, either as an acclimation process or as a consequence of dysbiosis. Strongly restructured microbiomes have been observed after exposing Corals to 20% increased salinity over 28 days, after relocating a deep-sea Coral from natural to aquaria settings, as a consequence of Coral disease, and as a result of anthropogenic disturbance. In comparison, Oulastrea crispata’s Bacterial microbiome exhibited the highest biodiversity in the highest stress site (Lantau), but was structurally stable at all sites based on similar alpha diversity indices.
Structural stability in Oulastrea crispata exposed to environmental perturbation is further corroborated by distribution patterns of putative pathogens. Environmentally challenged Corals often harbour increased numbers of the order Vibrionales that are considered as possible indicators or biomarkers highlighting environmental stress before visual signs of disease are present. However, many other studies have shown such bacteria are present in healthy Coral. In Röthig et al.'s study, Vibrionales are among the most common taxa of the Oulastrea crispata microbiome, with Vibrionaceae among the five most abundant families including the most abundant Operational Taxonomic Units in the study. Interestingly, Vibrionales are more common at sites with better water quality compared to the impacted sites, and the most abundant strain appears an indicator species for a mediumimpacted site (Lamma). The high contribution of Vibrio spp. found in this study, for all Corals, may be the result of all Oulastrea crispata showing signs of environmental stress. However, the fact that there was a lower relative abundance of Vibrio spp. in Corals from more polluted sites does not support this explanation. Despite all the Coral microbiomes being dominated by a high relative number of Vibrio spp., all Coral colonies were macroscopically healthy at the time of sampling indicating this Corals’ ability to resist a high level of environmental pressure.
Vibrio Bacteria (Vibrionales, Gammaproteobacteria), are the causative agent of Cholera in Humans and are generally associated with poor health and environmental stress in Corals, but appear to be part of the normal microbiome of Oulastrea crispata. Safe Drinking Water Foundation.
Coral reefs typically occur in shallow, oligotrophic, light-flooded tropical regions where environmental parameters are stable, with largely detrimental effects resulting from variation in temperature and salinity. Gradients in water quality correlate well with Coral diversity and species distribution as shown previously at the same location of this study. Examination of the environmental data indicated both high spatial and temporal variations in the measured parameters that create a gradient in water quality along the Hong Kong coastline. Data collected over the three months nearest our sampling times (June to August 2018) included temperatures ranging from 27.3 to 30.4° C, salinity ranging from 18.2 to 31.8, disolved oxygen from 4.1 to 8.5 mg per litre, turbidity from 0.6 to 5.3 Nephelometric Turbidity Units, pH from 7.7 to 8.5, total nitrogen from 8.21 to 55.69 μmol per litre, and chlorochlorophyll a from 0.6 to 31 μg per litre. These also aligned well with previous notable variations; temperature 13.9°C (winter)–31.9° C (summer), salinity 9.8–35.2, pH 7.4–8.5, turbidity 0.3–52 Nephelometric Turbidity Units, disolved oxygen 1.5–11.8 mg per litre, 10.7–678 μmol per litre total nitrogen, and 0.2–100 μg per litre chlorophyll a.
Despite such variation in the environmental parameters, the Corals’ microbiome only partly aligned with the differences. To link microbiome composition and specific water quality parameters, Röthig et al. employed redundancy and regression analyses. Interestingly, the three different correlation analyses performed for this study all indicated slightly different interactions between the parameters, a result which may be driven by the high variability of these parameters across the sites surveyed. Based on the redundancy analyses, the overall contribution of environmental parameters to microbiome differences accounts for only about 23% of the observed variability, and this appears to be largely independent of the considered timescale or parameters. This may be, at least in part, caused by the different distances and time spans for which the environmental parameters and the Coral samples were taken. Surprisingly, Röthig et al. were unable to identify any specific environmental parameter which contributed to the observed microbiome differences. This is in contrast to sediment-associated microbes sampled in the region which showed increased microbial diversity with decreasing water quality, highlighting the ability of the Coral holobiont to structure and maintain a distinct microbial community to that of the surrounding environment.
Oulastrea crispata’s overall limited microbiome flexibility, based on the spatial structural stability under an extreme environmental gradient, suggests that the Coral maintains a conserved microbiome and therefore fits into the ‘microbiome regulator’ category. Given that Oulastrea crispata is the most environmentally versatile Coral located around Hong Kong, this does not fit with the current working hypothesis that ‘microbiome conformers’ may be more environmentally resilient. However, the previously hypothesised phylogenetic origin of microbiome stability does still correspond with the hypothesis. Like Pocillopora verrucosa, the ‘microbiome regulator’ Oulastrea crispata is (1) from the robust clade, albeit with unresolved evolutionary relationships, and (2) seems to form generally stable associations with its Algal endosymbionts. The recurring stability of both host–Bacterial and host–Dinoflagellate associations is an interesting trend that warrants further investigation. Repeated sampling of Oulastrea crispata’s microbiome considering Hong Kong’s strong seasonality could gain further insight into its temporal stability and contribution to environmental resilience. Based on the current data, it may be that the wide environmental tolerance of Oulastrea crispata is largely a function of the host itself, which is highly flexible in its ecology and physiology. The Coral is widely distributed in the Pacific, invasive in the Mediterranean and Korea, and resistant to both high and low temperatures. Further, under laboratory conditions, Oulastrea crispata reveals a high trophic flexibility and can survive for more than one year in the dark or under daylight conditions in a simple set-up without feeding. Finally, Oulastrea crispata is a generalist, with a flexible reproduction strategy (asexual and sexual, brooder, and broadcast spawner). Taken together, these traits point to this Coral having a ‘winning’ strategy to survive climate change and other environmental perturbations.
Röthig et al. characterised the Bacterial microbiome of Oulastrea crispata along a pronounced water quality gradient in a highly urbanised environment. Their results describe a spatially and temporally highly variable environment along the entire gradient. Reflecting their environment, the Coral microbiomes were complex. However, the microbiome variations do not appear pronounced (at least on a spatial scale) and only partially align with environmental differences (explaining about 23%). Oulastrea crispata appears to be extremely flexible with regard to the environment in which it can thrive. Röthig et al.'s data suggest that its Bacterial microbiome either contributes little to this ability and/or that microbiome flexibility (i.e. ability to conform) does not necessarily confer environmental robustness to the holobiont. That said, it remains difficult to assign Corals into microbiome ‘conformers’ or ‘regulators’ without a direct comparative framework; therefore, more data are required to understand the nature of microbiome flexibility and its potential role in the holobiont’s mechanisms of adaptation to environmental change.
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