Posts Tagged 'protists'

Coral symbiosis carbon flow: a numerical model study spanning cellular to ecosystem levels

Corals rely on a symbiotic relationship with algae (zooxanthellae), which reside in the host tissue and play a critical role for host metabolism through photosynthesis, respiration, carbon translocation, and calcification. These processes affect coral reefs on different scales from cellular to organismal and ecosystem levels. A process-based dynamic model was developed and coupled with a one-dimensional (1-D) biogeochemical model to describe coral photosynthesis, respiration, and carbon translocation at the cellular level, calcification and ion transport in different coral polyp components (i.e., coelenteron, calcifying fluid) at the organismal level; and the exchange of material between corals and the ambient seawater at the ecosystem level. Major processes controlling the carbon budget in internal symbiosis were identified. For the symbiont, photosynthesis is the primary carbon source and translocation to the host is the major sink. For the host, most of the carbon translocated from the symbiont is lost through mucus leakage. In the host dissolved inorganic carbon (DIC) pool, most of the carbon is obtained from the surrounding seawater through uptake; photosynthesis and calcification are the major sinks of DIC. Based on a series of scenario studies, the model produced increase of photosynthesis rate with decline of calcification rate under higher air pCO2 and associated carbonate chemistry variabilities in different polyp components. The model results support the hypothesis that elevated pCO2 stimulates photosynthesis, resulting in a reduced supply of DIC to calcification. Such coupled models allow the exploration of process-based mechanisms, complementing laboratory and field studies.

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Effect of temperature and pH on the Millepora alcicornis and Mussismilia harttii corals in light of a spectral reflectance response

The increase in carbon dioxide (CO2) atmospheric levels contributes to the rise in temperature and ocean acidification; consequently, it directly impacts coral reefs. The increase in seawater temperature is the primary factor that causes the collapse of coral-algal symbiosis, which can be followed by coral death and, generally, ocean acidification impairs biogenic calcification and promotes dissolution of carbonate substrata. These harmful effects on corals associated with the continuous increase in CO2 atmospheric levels raise widespread concerns about the coral reef decline, intensifying the efforts to understand/monitor their effects on these organisms. The objective of this study was to evaluate the physiological effect of temperature increase, water acidification (i.e. decrease in pH), and their effects combined (temperature increase with water acidification), through the reflectance analyses and maximum photosynthetic capacity of zooxanthellae (Fv/Fm) in two coral species: Millepora alcicornis and Mussismilia harttii. Fragments of four large colonies of each specie were collected, fragmented, and submitted to four different treatments for 15 days: (i) control treatment (under identical temperature and pH conditions observed in the sampling seawater site), (ii) temperature treatment (with an increase temperature of around ≅2ºC); (iii) water acidification treatment (with a decrease of nearly 0.3 in pH); and (iv) a treatment of combined effects from water temperature rising and acidification. Spectral reflectance and Fv/Fm were measured from samples of these species in a marine mesocosm. Data of reflectance, first and second-order derivative, area under the curve, full width at half maximum (FWHM), depth values and the Fv/Fm were used to classify the coral species and treatments through the linear discriminant analysis (LDA). Coral samples were exposed to the increased temperature bleached, whilst decreased pH caused a slight reduction in reflectance albedo with minimal effects on Fv/Fm. The combined factors (treatment iv) triggered a bleaching response, presenting spectral reflectance and colouring patterns similar to those observed in bleached corals, especially for M. alcicornis. The two-way ANOVA indicated statistically meaningful spectral differences between treatments for the second-order derivatives at 634 nm and for Fv/Fm values. However, there was no statistically meaningful interaction effect due to the treatment type and coral species response for the second-order derivative at 670 nm and to the Fv/Fm values. LDA classified the corals’ species and the corals in different treatment, using their spectral responses and Fv/Fm results, with high accuracy (96.7% and 73.3%, respectively), reinforcing its application for coral physiology evaluation and species classification. The control and combined groups achieved the best classification scores, with only one misclassification.

Continue reading ‘Effect of temperature and pH on the Millepora alcicornis and Mussismilia harttii corals in light of a spectral reflectance response’

Potential resilience to ocean acidification of benthic foraminifers living in Posidonia oceanica meadows: the case of the shallow venting site of Panarea

This research shows the results regarding the response to acidic condition of the sediment and Posidonia foraminiferal assemblages collected around the Panarea Island. The Aeolian Archipelago represents a natural laboratory and a much-promising study site for multidisciplinary marine research (carbon capture and storage, geochemistry of hydrothermal fluids and ocean acidification vs. benthic and pelagic organisms). The variability and the complexity of the interaction of the ecological factors characterizing extreme environments such as shallow hydrothermal vents did not allow us to carry out a real pattern of biota responses in situ, differently from those observed under controlled laboratory conditions. However, the study provides new insights into foraminiferal response to increasing ocean acidification (OA) in terms of biodiversity, faunal density, specific composition of the assemblages and morphological variations of the shells. The study highlights how the foraminiferal response to different pH conditions can change depending on different environmental conditions and microhabitats (sediments, Posidonia leaves and rhizomes). Indeed, mineral sediments were more impacted by acidification, whereas Posidonia microhabitats, thanks to their buffer effect, can offer “refugia” and more mitigated acidic environment. At species level, rosalinids and agglutinated group represent the most abundant taxa showing the most specific resilience and capability to face acidic conditions.

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The coral reef-dwelling Peneroplis spp. shows calcification recovery to ocean acidification conditions

Large Benthic Foraminifera are a crucial component of coral-reef ecosystems, which are currently threatened by ocean acidification. We conducted culture experiments to evaluate the impact of low pH on survival and test dissolution of the symbiont-bearing species Peneroplis spp., and to observe potential calcification recovery when specimens are placed back under reference pH value (7.9). We found that Peneroplis spp. displayed living activity up to 3 days at pH 6.9 (Ωcal < 1) or up to 1 month at pH 7.4 (Ωcal > 1), despite the dark and unfed conditions. Dissolution features were observed under low Ωcal values, such as changes in test density, peeled extrados layers, and decalcified tests with exposed organic linings. A new calcification phase started when specimens were placed back at reference pH. This calcification’s resumption was an addition of new chambers without reparation of the dissolved parts, which is consistent with the porcelaneous calcification pathway of Peneroplis spp. The most decalcified specimens displayed a strong survival response by adding up to 8 new chambers, and the contribution of food supply in this process was highlighted. These results suggest that porcelaneous LBF species have some recovery abilities to short exposure (e.g., 3 days to 1 month) to acidified conditions. However, the geochemical signature of trace elements in the new calcite was impacted, and the majority of the new chambers were distorted and resulted in abnormal tests, which might hinder the specimens’ reproduction and thus their survival on the long term.

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Acidification impacts and acclimation potential of foraminifera

Ocean acidification is expected to negatively affect many ecologically important organisms. Here we explored the response of Caribbean benthic foraminiferal communities to naturally discharging low-pH waters similar to expected future projections for the end of the 21st century. At low-pH (~ 7.7 pH units), low calcite saturation, agglutinated and symbiont-bearing species were relatively more abundant, indicating higher resistance to potential carbonate chemistry changes. Diversity and other taxonomical metrics declined steeply with decreasing pH despite exposure of this ecosystem for millennia to low pH conditions, suggesting that tropical foraminifera communities will be negatively impacted under acidification scenarios SSP3-7.0 and SSP5-8.5. The species Archaias angulatus, a major contributor to sediment production in the Caribbean was able to calcify at conditions more extreme than those projected for the late 21st century (7.1 pH units), but the calcified tests were of lower density than those exposed to high-pH ambient conditions (7.96 pH units), indicating that reef foraminiferal carbonate budget might decrease. Smaller foraminifera were highly sensitive to decreasing pH and our results demonstrate their potential as indicators to monitor increasing OA conditions.

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Nocturnal acidification: a coordinating cue in the Euprymna scolopes–Vibrio fischeri Symbiosis

The Vibrio fischeriEuprymna scolopes symbiosis has become a powerful model for the study of specificity, initiation, and maintenance between beneficial bacteria and their eukaryotic partner. In this invertebrate model system, the bacterial symbionts are acquired every generation from the surrounding seawater by newly hatched squid. These symbionts colonize a specialized internal structure called the light organ, which they inhabit for the remainder of the host’s lifetime. The V. fischeri population grows and ebbs following a diel cycle, with high cell densities at night producing bioluminescence that helps the host avoid predation during its nocturnal activities. Rhythmic timing of the growth of the symbionts and their production of bioluminescence only at night is critical for maintaining the symbiosis. V. fischeri symbionts detect their population densities through a behavior termed quorum-sensing, where they secrete and detect concentrations of autoinducer molecules at high cell density when nocturnal production of bioluminescence begins. In this review, we discuss events that lead up to the nocturnal acidification of the light organ and the cues used for pre-adaptive behaviors that both host and symbiont have evolved. This host–bacterium cross talk is used to coordinate networks of regulatory signals (such as quorum-sensing and bioluminescence) that eventually provide a unique yet stable environment for V. fischeri to thrive and be maintained throughout its life history as a successful partner in this dynamic symbiosis.

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Synergistic effects of ocean acidification and warming on coral host, Symbiodinium and nutrients exchange-based symbioses indicated by metatranscriptome

Global climate changes e.g. ocean acidification and warming caused by anthropogenic emission of CO2 are the greatest global threat to coral reef ecosystems. However, compared with the knowledge of Symbiodinium, little is known about the synergistic effects of combined ocean acidification and warming on the coral host and coral-Symbiodinium symbioses. In this study, metatranscriptomic analysis was performed to reveal the response of coral host and its symbiotic Symbiodinium to acidification (A), warming (H) and combined acidification and acidification (AH), using branching A. valida and massive G. fascicularis as models in a laboratory simulation system. RNA-Seq-based differently expressed genes (DEGs), together with coral’s morphological change, suggested the synergistic effects of AH on the coral host and coral-Symbiodinium symbioses, e.g. photosynthesis inhibition and negative effect on nutrients exchange between the host and its Symbiodinium. Particularly, AH showed a far greater impact on coral host than on Symbiodinium. These findings provide novel insights into the molecular mechanism of coral holobionts’ response to future extreme ocean acidification and warming, meanwhile highlight the molecular evidence for the different tolerance of branching and massive corals to environmental changes.

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Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) is recognized by a major negative carbon isotope (δ13C) excursion (CIE) signifying an injection of isotopically light carbon into exogenic reservoirs, the mass, source, and tempo of which continue to be debated. Evidence of a transient precursor carbon release(s) has been identified in a few localities, although it remains equivocal whether there is a global signal. Here, we present foraminiferal δ13C records from a marine continental margin section, which reveal a 1.0 to 1.5‰ negative pre-onset excursion (POE), and concomitant rise in sea surface temperature of at least 2°C and a decline in ocean pH. The recovery of both δ13C and pH before the CIE onset and apparent absence of a POE in deep-sea records suggests a rapid (< ocean mixing time scales) carbon release, followed by recovery driven by deep-sea mixing. Carbon released during the POE is therefore likely more similar to ongoing anthropogenic emissions in mass and rate than the main CIE.

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Interactive effects of acidification and copper exposure on the reproduction and metabolism of coral endosymbiont Cladocopium goreaui

Highlights

  • Acidification raised growth by Fv/Fm, nutrient uptake and biomolecular biosynthesis.
  • Copper pollution alone decreased algal reproduction through toxic effects.
  • Combined stressor repressed reproduction through downregulated aromatic amino acid.
  • Abstract

Ocean acidification resulting from increased CO2 and pollution from land-sourced toxicants such as copper have been linked to coral cover declines in coastal reef ecosystems. The impacts of ocean acidification and copper pollution on corals have been intensively investigated, whereas research on their effects on coral endosymbiont Symbiodiniaceae is limited. In this study, reproduction, photosynthetic parameters, nutrient accumulation and metabolome of Symbiodiniaceae Cladocopium goreaui were investigated after a weeklong treatment with acute CO2-induced acidification and copper ion. Acidification promoted algal reproduction through increased nutrients assimilation, upregulated citrate cycle and biomolecular biosynthesis pathway, while copper exposure repressed algal reproduction through toxic effects. The combined acidification and copper exposure caused the same decline in algal reproduction as copper exposure alone, but the upregulation of pentose phosphate pathway and the downregulation of aromatic amino acid biosynthesis. These results suggest that copper pollution could override the positive effects of acidification on the symbiodiniacean reproduction.

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Physiological acclimatization in Hawaiian corals following a 22-month shift in baseline seawater temperature and pH

Climate change poses a major threat to coral reefs. We conducted an outdoor 22-month experiment to investigate if coral could not just survive, but also physiologically cope, with chronic ocean warming and acidification conditions expected later this century under the Paris Climate Agreement. We recorded survivorship and measured eleven phenotypic traits to evaluate the holobiont responses of Hawaiian coral: color, Symbiodiniaceae density, calcification, photosynthesis, respiration, total organic carbon flux, carbon budget, biomass, lipids, protein, and maximum Artemia capture rate. Survivorship was lowest in Montipora capitata and only some survivors were able to meet metabolic demand and physiologically cope with future ocean conditions. Most M. capitata survivors bleached through loss of chlorophyll pigments and simultaneously experienced increased respiration rates and negative carbon budgets due to a 236% increase in total organic carbon losses under combined future ocean conditions. Porites compressa and Porites lobata had the highest survivorship and coped well under future ocean conditions with positive calcification and increased biomass, maintenance of lipids, and the capacity to exceed their metabolic demand through photosynthesis and heterotrophy. Thus, our findings show that significant biological diversity within resilient corals like Porites, and some genotypes of sensitive species, will persist this century provided atmospheric carbon dioxide levels are controlled. Since Porites corals are ubiquitous throughout the world’s oceans and often major reef builders, the persistence of this resilient genus provides hope for future reef ecosystem function globally.

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Modelling antifouling compounds of macroalgal holobionts in current and future pH conditions

Marine macroalgae are important ecosystem engineers in marine coastal habitats. Macroalgae can be negatively impacted through excessive colonization by harmful bacteria, fungi, microalgae, and macro-colonisers and thus employ a range of chemical compounds to minimize such colonization. Recent research suggests that environmental pH conditions potentially impact the functionality of such chemical compounds. Here we predict if and how naturally fluctuating pH conditions and future conditions caused by ocean acidification will affect macroalgal (antifouling) compounds and thereby potentially alter the chemical defence mediated by these compounds. We defined the relevant ecological pH range, analysed and scored the pH-sensitivity of compounds with antifouling functions based on their modelled chemical properties before assessing their distribution across the phylogenetic macroalgal groups, and the proportion of sensitive compounds for each investigated function. For some key compounds, we also predicted in detail how the associated ecological function may develop across the pH range. The majority of compounds were unaffected by pH, but compounds containing phenolic and amine groups were found to be particularly sensitive to pH. Future pH changes due to predicted average open ocean acidification pH were found to have little effect. Compounds from Rhodophyta were mainly pH-stable. However, key algal species amongst Phaeophyceae and Chlorophyta were found to rely on highly pH-sensitive compounds for their chemical defence against harmful bacteria, microalgae, fungi, and biofouling by macro-organisms. All quorum sensing disruptive compounds were found the be unaffected by pH, but the other ecological functions were all conveyed in part by pH-sensitive compounds. For some ecological keystone species, all of their compounds mediating defence functions were found to be pH-sensitive based on our calculations, which may not only affect the health and fitness of the host alga resulting in host breakdown but also alter the associated ecological interactions of the macroalgal holobiont with micro and macrocolonisers, eventually causing ecosystem restructuring and the functions (e.g. habitat provision) provided by macroalgal hosts. Our study investigates a question of fundamental importance because environments with fluctuating or changing pH are common and apply not only to coastal marine habitats and estuaries but also to freshwater environments or terrestrial systems that are subject to acid rain. Hence, whilst warranting experimental validation, this investigation with macroalgae as model organisms can serve as a basis for future investigations in other aquatic or even terrestrial systems.

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Physiological responses of the symbiotic shrimp Ancylocaris brevicarpalis and its host sea anemone Stichodactyla haddoni to ocean acidification

Highlights

  • Low pH condition have triggered the lipid peroxidation in anemone and shrimp.
  • AP showed less values in shrimp and anemone could be because of low pH stress.
  • Antioxidant enzymes showed upward tendency as an indicator for oxidative stress.
  • Short term exposure had adversely affected the physiology of anemone and shrimp.

Abstract

In this study, the physiology of symbiotic ‘peacock-tail’ shrimp Ancylocaris brevicarpalis and its host ‘Haddon’s carpet’ sea anemone Stichodactyla haddoni were tested under lowered pH (7.7) and control (8.1) conditions. The biochemical responses such as digestive enzyme (AP), organic acids (lactate and succinate), oxidative damages (MDA), antioxidants metabolites/enzymes (ASC, GSH, SODCATAPXGPX, GR, POX, and PHOX), and detoxification enzyme (GST) were measured. The AP showed insignificantly reduced values in both the organisms in lowered pH conditions compared to control indicating the effect of abiotic stress. The hierarchical clustering analysis indicated low MDA in sea anemone can be explained by higher POX, APX, GR, ASC, and GSH levels compared to shrimps. However, the detoxification enzyme GST showed less activity in sea anemones compared to shrimps. The results suggest that A. brevicarpalis and sea anemone S. haddoni may have deleterious effects when exposed to short-term acidification stress.

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Can areas of high alkalinity freshwater discharge provide potential refugia for marine calcifying organisms?

The Springs Coast of Florida, USA (northeast Gulf of Mexico), includes an extensive system of springs and spring-fed streams that discharge billions of liters of water daily. The spring waters have high alkalinity and high calcium concentrations due to the Paleogene limestone lithology of this region. Benthic foraminifers are abundant on the shallow shelf, including Archaias angulatus which hosts chlorophyte symbionts. This study was motivated by the hypothesis that areas of discharge from limestone lithofacies may provide refugia for calcifying organisms during ocean acidification. Environmental data and sediment samples were available from 41 sites at depths <8 m. Benthic foraminiferal species identified (142) included 65 porcelaneous, 65 hyaline, and 12 agglutinated taxa, with 13 species sufficiently common to make up ≥2% total relative abundance. Overall, 58% of the specimens were porcelaneous and most of the remainder were hyaline. Smaller miliolids dominated in samples from most of the inshore polyhaline sites (22–30), while hyaline taxa co-dominated the more offshore sites (salinities >30), representing a distribution reversal compared to previous reports from Gulf of Mexico coastal habitats. The elevated alkalinity and calcium ion concentrations associated with freshwater discharge from limestone lithofacies allows Ar. angulatus and small miliolids to thrive in polyhaline waters.

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Regulation of the coral-associated bacteria and symbiodiniaceae in Acropora valida under ocean acidification

Ocean acidification is one of many stressors that coral reef ecosystems are currently contending with. Thus, understanding the response of key symbiotic microbes to ocean acidification is of great significance for understanding the adaptation mechanism and development trend of coral holobionts. Here, high-throughput sequencing technology was employed to investigate the coral-associated bacteria and Symbiodiniaceae of the ecologically important coral Acropora valida exposed to different pH gradients. After 30 days of acclimatization, we set four acidification gradients (pH 8.2, 7.8, 7.4, and 7.2, respectively), and each pH condition was applied for 10 days, with the whole experiment lasting for 70 days. Although the Symbiodiniaceae density decreased significantly, the coral did not appear to be bleached, and the real-time photosynthetic rate did not change significantly, indicating that A. valida has strong tolerance to acidification. Moreover, the Symbiodiniaceae community composition was hardly affected by ocean acidification, with the C1 subclade (Cladocopium goreaui) being dominant among the Symbiodiniaceae dominant types. The relative abundance of the Symbiodiniaceae background types was significantly higher at pH 7.2, indicating that ocean acidification might increase the stability of the community composition by regulating the Symbiodiniaceae rare biosphere. Furthermore, the stable symbiosis between the C1 subclade and coral host may contribute to the stability of the real-time photosynthetic efficiency. Finally, concerning the coral-associated bacteria, the stable symbiosis between Endozoicomonas and coral host is likely to help them adapt to ocean acidification. The significant increase in the relative abundance of Cyanobacteria at pH 7.2 may also compensate for the photosynthesis efficiency of a coral holobiont. In summary, this study suggests that the combined response of key symbiotic microbes helps the whole coral host resist the threats of ocean acidification.

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Proteome and microbiota analyses characterizing dynamic coral-algae-microbe tripartite interactions under simulated rapid ocean acidification

Highlights

  • pH changes had a significant effect on the coral proteome- and 16S-profiling.
  • Maintenance of coral-algae-microbe interactions is a mechanism in coping with OA.
  • Proteome analysis identified some core biological pathways in OA early responses.
  • OA influences the microbial community, potentially compromising holobiont fitness.

Abstract

Ocean acidification (OA) is a pressing issue currently and in the future for coral reefs. The importance of maintenance interactions among partners of the holobiont association in the stress response is well appreciated; however, the candidate molecular and microbial mechanisms that underlie holobiont stress resilience or susceptibility remain unclear. Here, to assess the effects of rapid pH change on coral holobionts at both the protein and microbe levels, combined proteomics and microbiota analyses of the scleractinian coral Galaxea fascicularis exposed to three relevant OA scenarios, including current (pHT = 8.15), preindustrial (pHT = 8.45) and future IPCC-2100 scenarios (pHT = 7.85), were conducted. The results demonstrated that pH changes had no significant effect on the physiological calcification rate of G. fascicularis in a 10-day experiment; however, significant differences were recorded in the proteome and 16S profiling. Proteome variance analysis identified some of the core biological pathways in coral holobionts, including coral host infection and immune defence, and maintaining metabolic compatibility involved in energy homeostasis, nutrient cycling, antibiotic activity and carbon budgets of coral-Symbiodiniaceae interactions were key mechanisms in the early OA stress response. Furthermore, microbiota changes indicate substantial microbial community and functional disturbances in response to OA stress, potentially compromising holobiont health and fitness. Our results may help to elucidate many complex mechanisms to describe scleractinian coral holobiont responses to OA and raise interesting questions for future studies.

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Effects of variable daily light integrals and elevated CO2 on the adult and juvenile performance of two Acropora corals

Reef-building corals are subject to multi-day periods of reduced light and progressive ocean acidification. We experimentally assessed how adult and early post-settlement Acropora tenuis and A. hyacinthus corals responded to contrasting daily light integrals (DLI) and to multi-day variability in DLI, and whether contrasting DLIs altered the effects of ocean acidification. Four light treatments—three with stable DLIs (12.6, 7.6, 2.5 mol photons m−2 d−1) and one with variable DLI that averaged 7.6 mol photons m−2 d−1 were fully crossed with two levels of pCO2 (400 and 900 ppm) in a 63-day aquarium experiment. Adult coral growth and protein content declined as average DLI declined, regardless of whether DLI was stable or variable. In both species, photoacclimation was insufficient to compensate for low DLI, although both effective (φPSII) and maximum (Fv/Fm) quantum yields of photosystem two varied by < 5% between all stable DLI treatments. Under variable DLI, both species adjusted their φPSII on the day of change in DLI, whereas Fv/Fm remained relatively constant despite five-fold difference in DLI between days. Elevated CO2 increased protein content in adult A. tenuis at all DLIs, but otherwise had little effect on measured parameters. For juveniles, both species had reduced survival at low DLI due to overgrowth by Peyssonnelia algae, and A. tenuis growth was fastest at low DLI. Our study shows that the effects of multi-day periods of DLI reductions accumulate over time for corals, negatively affecting Acropora adult growth rates and juvenile survival, and hence slowing reef recovery after disturbance.

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The history, biological relevance, and potential applications for polyp bailout in corals

Corals have evolved a variety of stress responses to changing conditions, many of which have been the subject of scientific research. However, polyp bailout has not received widespread scientific attention, despite being described more than 80 years ago. Polyp bailout is a drastic response to acute stress in which coral colonies break down, with individual and patches of polyps detaching from the colony and the calcareous skeleton Polyps retain their symbiotic partners, have dispersal ability, and may undergo secondary settlement and calcification. Polyp bailout has been described worldwide in a variety of anthozoan species, especially in Scleractinia. It can be induced by multiple natural stressors, but also artificially. Little is known about the evolutionary and ecological potential and consequences of breaking down modularity, the dispersal ability, and reattachment of polyps resulting from polyp bailout. It has been shown that polyp bailout can be used as a model system, with promise for implementation in various research topics. To date, there has been no compilation of knowledge on polyp bailout, which prompted us to review this interesting stress response and provide a basis to discuss research topics and priorities for the future.

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The promotion of stress tolerant Symbiodiniaceae dominance in juveniles of two coral species due to simulated future conditions of ocean warming and acidification

The symbiotic relationship between coral and its endosymbiotic algae, Symbiodiniaceae, greatly influences the hosts’ potential to withstand environmental stress. To date, the effects of climate change on this relationship has primarily focused on adult corals. Uncovering the effects of environmental stress on the establishment and development of this symbiosis in early life stages is critical for predicting how corals may respond to climate change. To determine the impacts of future climate projections on the establishment of symbionts in juvenile corals, ITS2 amplicon sequencing of single coral juveniles was applied to Goniastrea retiformis and Acropora millepora before and after exposure to three climate conditions of varying temperature and pCO2 levels (current and RCP8.5 in 2050 and 2100). Compared to ambient conditions, juvenile corals experienced shuffling in the relative abundance of Cladocopium (C1m, reduction) to Durusdinium (D1 and D1a, increase) over time. We calculated a novel risk metric incorporating functional redundancy and likelihood of impact on host physiology to identify the loss of D1a as a ‘low risk’ to the coral compared to the loss of “higher risk” taxa like D1 and C1m. Although the increase in stress tolerant Durusdinium under future warming was encouraging for A. millepora, by 2100, G. retiformis communities displayed signs of symbiosis de-regulation, suggesting this acclimatory mechanism may have species-specific thresholds. These results emphasize the need for understanding of long-term effects of climate change induced stress on coral juveniles and their potential for increased acclimation to heat tolerance through changes in symbiosis.

Originality Statement Here we assessed changes in the uptake and establishment of Symbiodiniaceae in the early lifehistory stages of two coral species under future climate scenarios. Our study represents the first such assessment of future climate change projections (increased temperature and pCO2) influencing Symbiodiniaceae acquisition and specifically shows a community structure dominated by the stress tolerant genus Durusdinium. We also develop a novel risk metric that includes taxonomic function and redundancy to estimate the impact of symbiont taxa changes on coral physiology. Through the risk metric, we relate the stress-induced changes in symbiont community structure to the likelihood of functional loss to better understand the extent to which these changes may lead to a decrease in coral health.

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Changes in physiological performance and protein expression in the larvae of the coral Pocillopora damicornis and their symbionts in response to elevated temperature and acidification

Highlights

  • Thermal stress reduced and acidification increased larval photosynthesis in P. damicornis.
  • Thermal stress decreased light reaction and carbon fixation in endosymbionts.
  • Thermal stress decreased dissolved inorganic carbon transport in coral host and symbionts.
  • Acidification upregulated photosystem I iron‑sulfur center protein levels in endosymbionts.

Abstract

Climate change causes ocean warming and acidification, which threaten coral reef ecosystems. Ocean warming and acidification cause bleaching and mortality, and decrease calcification in adult corals, leading to changes in the composition of coral communities; however, their interactive effects on coral larvae are not comprehensively understood. To examine the underlying molecular mechanisms of larval responses to elevated temperature and pCO2, we examined the physiological performance and protein expression profiles of Pocillopora damicornis at two temperatures (29 and 33 °C) and pCO2 levels (500 and 1000 μatm) for 5 d. Extensive physiological and proteomic changes were observed in coral larvae. The results indicated a significant decrease in net photosynthesis (PNET) and autotrophic capability (PNET/RD) of larvae exposed to elevated temperature but a marked increase in PNET and PNET/RD of larvae exposed to high pCO2 levels. Elevated temperature significantly reduced endosymbiont densities (to approximately 70%) and photochemical efficiency, indicating that warming impaired host-symbiont symbiosis. Expression of photosynthesis-related proteins, the photosystem (PS) I reaction center subunits IV and XI as well as oxygen-evolving enhancer 1, was downregulated at higher temperatures in symbionts, whereas expression of the PS I iron‑sulfur center protein was increased under high pCO2 conditions. Furthermore, expression of phosphoribulokinase (involved in the Calvin cycle) and phosphoenolpyruvate carboxylase (related to the C4 pathway) was downregulated in symbionts under thermal stress; this finding suggests reduced carbon fixation at high temperatures. The abundance of carbonic anhydrase-associated proteins, which are predicted to exert biochemical roles in dissolved inorganic carbon transport in larvae, was reduced in coral host and symbionts at high temperatures. These results elucidate potential mechanisms underlying the responses of coral larvae exposed to elevated temperature and acidification and suggest an important role of symbionts in the response to warming and acidification.

Continue reading ‘Changes in physiological performance and protein expression in the larvae of the coral Pocillopora damicornis and their symbionts in response to elevated temperature and acidification’

Pliocene decoupling of equatorial Pacific temperature and pH gradients

Ocean dynamics in the equatorial Pacific drive tropical climate patterns that affect marine and terrestrial ecosystems worldwide. How this region will respond to global warming has profound implications for global climate, economic stability and ecosystem health. As a result, numerous studies have investigated equatorial Pacific dynamics during the Pliocene (5.3–2.6 million years ago) and late Miocene (around 6 million years ago) as an analogue for the future behaviour of the region under global warming1,2,3,4,5,6,7,8,9,10,11,12. Palaeoceanographic records from this time present an apparent paradox with proxy evidence of a reduced east–west sea surface temperature gradient along the equatorial Pacific1,3,7,8indicative of reduced wind-driven upwelling—conflicting with evidence of enhanced biological productivity in the east Pacific13,14,15 that typically results from stronger upwelling. Here we reconcile these observations by providing new evidence for a radically different-from-modern circulation regime in the early Pliocene/late Miocene16 that results in older, more acidic and more nutrient-rich water reaching the equatorial Pacific. These results provide a mechanism for enhanced productivity in the early Pliocene/late Miocene east Pacific even in the presence of weaker wind-driven upwelling. Our findings shed new light on equatorial Pacific dynamics and help to constrain the potential changes they will undergo in the near future, given that the Earth is expected to reach Pliocene-like levels of warming in the next century.

Continue reading ‘Pliocene decoupling of equatorial Pacific temperature and pH gradients’

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