The structural framework provided by corals is crucial for reef
ecosystem function and services, but high seawater temperatures can be
detrimental to the calcification capacity of reef-building ...organisms. The Red
Sea is very warm, but total alkalinity (TA) is naturally high and
beneficial for reef accretion. To date, we know little about how such
detrimental and beneficial abiotic factors affect each other and the balance
between calcification and erosion on Red Sea coral reefs, i.e., overall
reef growth, in this unique ocean basin. To provide estimates of present-day
reef growth dynamics in the central Red Sea, we measured two metrics of reef
growth, i.e., in situ net-accretion/-erosion rates (Gnet)
determined by deployment of limestone blocks and ecosystem-scale carbonate
budgets (Gbudget), along a cross-shelf gradient (25 km,
encompassing nearshore, midshore, and offshore reefs). Along this gradient, we assessed
multiple abiotic (i.e., temperature, salinity, diurnal pH fluctuation,
inorganic nutrients, and TA) and biotic (i.e., calcifier and epilithic
bioeroder communities) variables. Both reef growth metrics revealed similar
patterns from nearshore to offshore: net-erosive, neutral, and net-accretion
states. The average cross-shelf Gbudget was 0.66 kg
CaCO3 m−2 yr−1, with the highest budget of 2.44 kg
CaCO3 m−2 yr−1 measured in the offshore reef. These
data are comparable to the contemporary Gbudgets from the western
Atlantic and Indian oceans, but lie well below “optimal reef production”
(5–10 kg CaCO3 m−2 yr−1) and below maxima recently
recorded in remote high coral cover reef sites. However, the erosive forces
observed in the Red Sea nearshore reef contributed less than observed
elsewhere. A higher TA accompanied reef growth across the shelf gradient,
whereas stronger diurnal pH fluctuations were associated with negative
carbonate budgets. Noteworthy for this oligotrophic region was the positive effect of
phosphate, which is a central micronutrient for reef building corals. While
parrotfish contributed substantially to bioerosion, our dataset also
highlights coralline algae as important local reef builders. Altogether, our
study establishes a baseline for reef growth in the central Red Sea that
should be useful in assessing trajectories of reef growth capacity under
current and future ocean scenarios.
Despite the importance of deep-sea corals, our current understanding of their ecology and evolution is limited due to difficulties in sampling and studying deep-sea environments. Moreover, a recent ...re-evaluation of habitat limitations has been suggested after characterization of deep-sea corals in the Red Sea, where they live at temperatures of above 20 °C at low oxygen concentrations. To gain further insight into the biology of deep-sea corals, we produced reference transcriptomes and studied gene expression of three deep-sea coral species from the Red Sea, i.e. Dendrophyllia sp., Eguchipsammia fistula, and Rhizotrochus typus. Our analyses suggest that deep-sea coral employ mitochondrial hypometabolism and anaerobic glycolysis to manage low oxygen conditions present in the Red Sea. Notably, we found expression of genes related to surface cilia motion that presumably enhance small particle transport rates in the oligotrophic deep-sea environment. This is the first study to characterize transcriptomes and in situ gene expression for deep-sea corals. Our work offers several mechanisms by which deep-sea corals might cope with the distinct environmental conditions present in the Red Sea As such, our data provide direction for future research and further insight to organismal response of deep-sea coral to environmental change and ocean warming.
Increasing temperatures on a global scale and locally deteriorating water quality affect coral distribution and health. Mechanisms that convey environmental robustness are poorly understood and have ...been attributed to the coral host, algal symbionts, and prokaryotic associates. Flexibility of the host’s (bacterial) microbiome has been suggested to contribute to environmental robustness, but the underlying mechanisms are unclear. We therefore utilised the vastly contrasting water quality gradient present along Hong Kong’s highly urbanised coastline to explore whether flexibility in the microbiome of
Oulastrea crispata
relates to spatial variations in temperature, salinity, dissolved oxygen, pH, nitrate, nitrite, ammonia, total nitrogen, phosphorus, turbidity, and chlorophyll a. We identified differences in the coral microbiomes between sites, but the measured environmental variables only explained ~ 23% of the variation suggesting other factors are contributing substantially. The observed structural complexity of the microbiome (based on alpha diversity indices) appears to be relatively conserved across the environmental gradient even at sites where no other hard coral can survive. Therefore, we conclude that, at least in
O. crispata
, flexibility in the microbiome does not appear to underpin the robustness of this broadly distributed coral.
Climate change is fundamentally altering marine and coastal ecosystems on a global scale. While the effects of ocean warming and acidification on ecology and ecosystem functions and services are ...being comprehensively researched, less attention is directed toward understanding the impacts of human‐driven ocean salinity changes. The global water cycle operates through water fluxes expressed as precipitation, evaporation, and freshwater runoff from land. Changes to these in turn modulate ocean salinity and shape the marine and coastal environment by affecting ocean currents, stratification, oxygen saturation, and sea level rise. Besides the direct impact on ocean physical processes, salinity changes impact ocean biological functions with the ecophysiological consequences are being poorly understood. This is surprising as salinity changes may impact diversity, ecosystem and habitat structure loss, and community shifts including trophic cascades. Climate model future projections (of end of the century salinity changes) indicate magnitudes that lead to modification of open ocean plankton community structure and habitat suitability of coral reef communities. Such salinity changes are also capable of affecting the diversity and metabolic capacity of coastal microorganisms and impairing the photosynthetic capacity of (coastal and open ocean) phytoplankton, macroalgae, and seagrass, with downstream ramifications on global biogeochemical cycling. The scarcity of comprehensive salinity data in dynamic coastal regions warrants additional attention. Such datasets are crucial to quantify salinity‐based ecosystem function relationships and project such changes that ultimately link into carbon sequestration and freshwater as well as food availability to human populations around the globe. It is critical to integrate vigorous high‐quality salinity data with interacting key environmental parameters (e.g., temperature, nutrients, oxygen) for a comprehensive understanding of anthropogenically induced marine changes and its impact on human health and the global economy.
Salinity changes and their influence on ecosystem structure and function. Increasing temperatures enhance hydrological cycling, resulting in increased meltwater and shifts in evaporation and precipitation patterns that affect global ocean salinity patterns. Local impacts, such as land use practice further modulate terrestrial runoff patterns, affecting coastal ecosystems. Enhanced variation or shifts in salinity impact diversity, growth, and survival of key species. Sea level rise connected with salinization as well as ecotone shifts and trophic cascades may contribute to substantially altered ecosystem structure and functionality.
Scleractinian corals are assumed to be stenohaline osmoconformers, although they are frequently subjected to variations in seawater salinity due to precipitation, freshwater run‐off and other ...processes. Observed responses to altered salinity levels include differences in photosynthetic performance, respiration and increased bleaching and mortality of the coral host and its algal symbiont, but a study looking at bacterial community changes is lacking. Here, we exposed the coral Fungia granulosa to strongly increased salinity levels in short‐ and long‐term experiments to disentangle temporal and compartment effects of the coral holobiont (i.e. coral host, symbiotic algae and associated bacteria). Our results show a significant reduction in calcification and photosynthesis, but a stable microbiome after short‐term exposure to high‐salinity levels. By comparison, long‐term exposure yielded unchanged photosynthesis levels and visually healthy coral colonies indicating long‐term acclimation to high‐salinity levels that were accompanied by a major coral microbiome restructuring. Importantly, a bacterium in the family Rhodobacteraceae was succeeded by Pseudomonas veronii as the numerically most abundant taxon. Further, taxonomy‐based functional profiling indicates a shift in the bacterial community towards increased osmolyte production, sulphur oxidation and nitrogen fixation. Our study highlights that bacterial community composition in corals can change within days to weeks under altered environmental conditions, where shifts in the microbiome may enable adjustment of the coral to a more advantageous holobiont composition.
Coral reefs are subject to coral bleaching manifested by the loss of endosymbiotic algae from coral host tissue. Besides algae, corals associate with bacteria. In particular, bacteria residing in the ...surface mucus layer are thought to mediate coral health, but their role in coral bleaching is unknown. We collected mucus from bleached and healthy Porites lobata colonies in the Persian/Arabian Gulf (PAG) and the Red Sea (RS) to investigate bacterial microbiome composition using 16S rRNA gene amplicon sequencing. We found that bacterial community structure was notably similar in bleached and healthy corals, and the most abundant bacterial taxa were identical. However, fine-scale differences in bacterial community composition between the PAG and RS were present and aligned with predicted differences in sulfur- and nitrogen-cycling processes. Based on our data, we argue that bleached corals benefit from the stable composition of mucus bacteria that resemble their healthy coral counterparts and presumably provide a conserved suite of protective functions, but monitoring of post-bleaching survival is needed to further confirm this assumption. Conversely, fine-scale site-specific differences highlight flexibility of the bacterial microbiome that may underlie adjustment to local environmental conditions and contribute to the widespread success of Porites lobata.
The existence of coral reef ecosystems critically relies on the reef carbonate framework produced by scleractinian corals and calcareous crusts (i.e., crustose coralline algae). While the Red Sea ...harbors one of the longest connected reef systems in the world, detailed calcification data are only available from the northernmost part. To fill this knowledge gap, we measured in situ calcification rates of primary and secondary reef builders in the central Red Sea. We collected data on the major habitat-forming coral genera
Porites
,
Acropora
, and
Pocillopora
and also on calcareous crusts (CC) in a spatio-seasonal framework. The scope of the study comprised sheltered and exposed sites of three reefs along a cross-shelf gradient and over four seasons of the year. Calcification of all coral genera was consistent across the shelf and highest in spring. In addition,
Pocillopora
showed increased calcification at exposed reef sites. In contrast, CC calcification increased from nearshore, sheltered to offshore, exposed reef sites, but also varied over seasons. Comparing our data to other reef locations, calcification in the Red Sea was in the range of data collected from reefs in the Caribbean and Indo-Pacific; however,
Acropora
calcification estimates were at the lower end of worldwide rates. Our study shows that the increasing coral cover from nearshore to offshore environments aligned with CC calcification but not coral calcification, highlighting the potentially important role of CC in structuring reef cover and habitats. While coral calcification maxima have been typically observed during summer in many reef locations worldwide, calcification maxima during spring in the central Red Sea indicate that summer temperatures exceed the optima of reef calcifiers in this region. This study provides a foundation for comparative efforts and sets a baseline to quantify impact of future environmental change in the central Red Sea.
Microbes associated with deep-sea corals remain poorly studied. The lack of symbiotic algae suggests that associated microbes may play a fundamental role in maintaining a viable coral host via ...acquisition and recycling of nutrients. Here we employed 16 S rRNA gene sequencing to study bacterial communities of three deep-sea scleractinian corals from the Red Sea, Dendrophyllia sp., Eguchipsammia fistula, and Rhizotrochus typus. We found diverse, species-specific microbiomes, distinct from the surrounding seawater. Microbiomes were comprised of few abundant bacteria, which constituted the majority of sequences (up to 58% depending on the coral species). In addition, we found a high diversity of rare bacteria (taxa at <1% abundance comprised >90% of all bacteria). Interestingly, we identified anaerobic bacteria, potentially providing metabolic functions at low oxygen conditions, as well as bacteria harboring the potential to degrade crude oil components. Considering the presence of oil and gas fields in the Red Sea, these bacteria may unlock this carbon source for the coral host. In conclusion, the prevailing environmental conditions of the deep Red Sea (>20 °C, <2 mg oxygen L
) may require distinct functional adaptations, and our data suggest that bacterial communities may contribute to coral functioning in this challenging environment.
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