Posts Tagged 'protists'

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

Published 16 February 2022 Science Closed
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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, SOD, CAT, APX, GPX, 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?

Published 3 February 2022 Science Closed
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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

Published 20 January 2022 Science Closed
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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

Published 11 January 2022 Science Closed
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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

Published 23 December 2021 Science Closed
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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

Published 17 November 2021 Science Closed
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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

Published 12 November 2021 Science Closed
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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

Published 2 November 2021 Science Closed
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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.

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Pliocene decoupling of equatorial Pacific temperature and pH gradients

Published 25 October 2021 Science Closed
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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.

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Distribution and abundances of planktic foraminifera and shelled pteropods during the polar night in the sea-ice covered Northern Barents Sea

Published 25 October 2021 Science Closed
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Planktic foraminfera and shelled pteropods are important calcifying groups of zooplankton in all oceans. Their calcium carbonate shells are sensitive to changes in ocean carbonate chemistry predisposing them as an important indicator of ocean acidification. Moreover, planktic foraminfera and shelled pteropods contribute significantly to food webs and vertical flux of calcium carbonate in polar pelagic ecosystems. Here we provide, for the first time, information on the under-ice planktic foraminifera and shelled pteropod abundance, species composition and vertical distribution along a transect (82°–76°N) covering the Nansen Basin and the northern Barents Sea during the polar night in December 2019. The two groups of calcifiers were examined in different environments in the context of water masses, sea ice cover, and ocean chemistry (nutrients and carbonate system). The average abundance of planktic foraminifera under the sea-ice was low with the highest average abundance (2 ind. m–3) close to the sea-ice margin. The maximum abundances of planktic foraminifera were concentrated at 20–50 m depth (4 and 7 ind. m–3) in the Nansen Basin and at 80–100 m depth (13 ind. m–3) close to the sea-ice margin. The highest average abundance (13 ind. m–3) and the maximum abundance of pteropods (40 ind. m–3) were found in the surface Polar Water at 0–20 m depth with very low temperatures (–1.9 to –1°C), low salinity (<34.4) and relatively low aragonite saturation of 1.43–1.68. The lowest aragonite saturation (<1.3) was observed in the bottom water in the northern Barents Sea. The species distribution of these calcifiers reflected the water mass distribution with subpolar species at locations and depths influenced by warm and saline Atlantic Water, and polar species in very cold and less saline Polar Water. The population of planktic foraminifera was represented by adults and juveniles of the polar species Neogloboquadrina pachyderma and the subpolar species Turborotalita quinqueloba. The dominating polar pteropod species Limacina helicina was represented by the juvenile and veliger stages. This winter study offers a unique contribution to our understanding of the inter-seasonal variability of planktic foraminfera and shelled pteropods abundance, distribution and population size structure in the Arctic Ocean.

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Heritable variation and lack of tradeoffs suggest adaptive capacity in Acropora cervicornis despite negative synergism under climate change scenarios

Published 15 October 2021 Science Closed
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Knowledge of multi-stressor interactions and the potential for tradeoffs among tolerance traits is essential for developing intervention strategies for the conservation and restoration of reef ecosystems in a changing climate. Thermal extremes and acidification are two major co-occurring stresses predicted to limit the recovery of vital Caribbean reef-building corals. Here, we conducted an aquarium-based experiment to quantify the effects of increased water temperatures and pCO2 individually and in concert on 12 genotypes of the endangered branching coral Acropora cervicornis, currently being reared and outplanted for large-scale coral restoration. Quantification of 12 host, symbiont and holobiont traits throughout the two-month-long experiment showed several synergistic negative effects, where the combined stress treatment often caused a greater reduction in physiological function than the individual stressors alone. However, we found significant genetic variation for most traits and positive trait correlations among treatments indicating an apparent lack of tradeoffs, suggesting that adaptive evolution will not be constrained. Our results suggest that it may be possible to incorporate climate-resistant coral genotypes into restoration and selective breeding programmes, potentially accelerating adaptation.

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Decrease in volume and density of foraminiferal shells with progressing ocean acidification

Published 8 October 2021 Science Closed
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Rapid increases in anthropogenic atmospheric CO2 partial pressure have led to a decrease in the pH of seawater. Calcifying organisms generally respond negatively to ocean acidification. Foraminifera are one of the major carbonate producers in the ocean; however, whether calcification reduction by ocean acidification affects either foraminiferal shell volume or density, or both, has yet to be investigated. In this study, we cultured asexually reproducing specimens of Amphisorus kudakajimensis, a dinoflagellate endosymbiont-bearing large benthic foraminifera (LBF), under different pH conditions (pH 7.7–8.3, NBS scale). The results suggest that changes in seawater pH would affect not only the quantity (i.e., shell volume) but also the quality (i.e., shell density) of foraminiferal calcification. We proposed that pH and temperature affect these growth parameters differently because (1) they have differences in the contribution to the calcification process (e.g., Ca2+-ATPase and Ω) and (2) pH mainly affects calcification and temperature mainly affects photosynthesis. Our findings also suggest that, under the IPCC RCP8.5 scenario, both ocean acidification and warming will have a significant impact on reef foraminiferal carbonate production by the end of this century, even in the tropics.

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The Bouraké semi-enclosed lagoon (New Caledonia) – a natural laboratory to study the lifelong adaptation of a coral reef ecosystem to extreme environmental conditions (update)

Published 17 September 2021 Science Closed
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According to current experimental evidence, coral reefs could disappear within the century if CO2 emissions remain unabated. However, recent discoveries of diverse and high cover reefs that already live under extreme conditions suggest that some corals might thrive well under hot, high-pCO2, and deoxygenated seawater. Volcanic CO2 vents, semi-enclosed lagoons, and mangrove estuaries are unique study sites where one or more ecologically relevant parameters for life in the oceans are close to or even worse than currently projected for the year 2100. Although they do not perfectly mimic future conditions, these natural laboratories offer unique opportunities to explore the mechanisms that reef species could use to keep pace with climate change. To achieve this, it is essential to characterize their environment as a whole and accurately consider all possible environmental factors that may differ from what is expected in the future, possibly altering the ecosystem response.

This study focuses on the semi-enclosed lagoon of Bouraké (New Caledonia, southwest Pacific Ocean) where a healthy reef ecosystem thrives in warm, acidified, and deoxygenated water. We used a multi-scale approach to characterize the main physical-chemical parameters and mapped the benthic community composition (i.e., corals, sponges, and macroalgae). The data revealed that most physical and chemical parameters are regulated by the tide, strongly fluctuate three to four times a day, and are entirely predictable. The seawater pH and dissolved oxygen decrease during falling tide and reach extreme low values at low tide (7.2 pHT and 1.9 mg O2 L−1 at Bouraké vs. 7.9 pHT and 5.5 mg O2 L−1 at reference reefs). Dissolved oxygen, temperature, and pH fluctuate according to the tide by up to 4.91 mg O2 L−1, 6.50 ∘C, and 0.69 pHT units on a single day. Furthermore, the concentration of most of the chemical parameters was 1 to 5 times higher at the Bouraké lagoon, particularly for organic and inorganic carbon and nitrogen but also for some nutrients, notably silicates. Surprisingly, despite extreme environmental conditions and altered seawater chemical composition measured at Bouraké, our results reveal a diverse and high cover community of macroalgae, sponges, and corals accounting for 28, 11, and 66 species, respectively. Both environmental variability and nutrient imbalance might contribute to their survival under such extreme environmental conditions. We describe the natural dynamics of the Bouraké ecosystem and its relevance as a natural laboratory to investigate the benthic organism’s adaptive responses to multiple extreme environmental conditions.

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Microbes support enhanced nitrogen requirements of coral holobionts in a high CO2 environment

Published 6 September 2021 Science Closed
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Ocean acidification is posing a threat to calcifying organisms due to the increased energy requirements of calcification under high CO2 conditions. The ability of scleractinian corals to cope with future ocean conditions will thus depend on their ability to fulfill their carbon requirement. However, the primary productivity of coral holobionts is limited by low nitrogen (N) availability in coral reef waters. Here, we employed CO2 seeps of Tutum Bay (Papua New Guinea) as a natural laboratory to understand how coral holobionts offset their increased energy requirements under high CO2 conditions. Our results demonstrate for the first time that under high pCO2 conditions, N assimilation pathways of Pocillopora damicornis are jointly modified. We found that diazotroph-derived N assimilation rates in the Symbiodiniaceae were significantly higher in comparison to an ambient CO2 control site, concomitant with a restructured diazotroph community and the specific prevalence of an alpha-proteobacterium. Further, corals at the high CO2 site also had increased feeding rates on picoplankton and in particular exhibited selective feeding on Synechococcus sp., known to be rich in N. Given the high abundance of picoplankton in oligotrophic waters at large, our results suggest that corals exhibiting flexible diazotrophic communities and capable of exploiting N-rich picoplankton sources to offset their increased N requirements may be able to cope better in a high pCO2 world.

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Experimental reef communities persist under future ocean acidification and warming

Published 31 August 2021 Science Closed
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Coral reefs are among the most sensitive ecosystems affected by ocean acidification and warming, and are predicted to shift from net accreting calcifier-dominated systems to net eroding algal-dominated systems over the coming decades. Here we present a long-term experimental study examining the responses of entire mesocosm coral reef communities to acidification (-0.2 pH units), warming (+ 2°C), and combined future ocean (-0.2 pH, + 2°C) treatments. We show that under future ocean conditions, net calcification rates declined yet remained positive, corals showed reduced abundance yet were not extirpated, and community composition shifted while species richness was maintained. Our results suggest that under Paris Climate Agreement targets, coral reefs could persist in an altered functional state rather than collapse.

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How do coral reefs respond to climate change? Investigating the role of Symbiodiniaceae community composition on coral performance under long-term exposure to warming and acidification

Published 25 August 2021 Science Closed
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Coral reefs are one of the most biodiverse and ecologically important ecosystems on the planet, but they are increasingly threatened by ocean acidification and warming. Changes in environmental factors can cause the coral to expel their endosymbiotic community of single-celled dinoflagellates (family: Symbiodiniaceae), leaving them more vulnerable to disease and mortality. One proposed method through which coral can acclimatize to the fluctuations in ocean temperature is by shuffling their Symbiodiniaceae community compositions to increase the relative proportions of temperature-tolerant symbionts. However, the effects of ocean acidification on Symbiodiniaceae community compositions are still unknown. Here, I present data from a two-year mesocosm experiment investigating the effects of long-term exposure to ocean acidification and warming on Symbiodiniaceae community composition in eight common species of Hawaiian coral. Coral were collected from six geographically distinct locations around O’ahu and exposed to predicted end-of-century temperature and pH conditions for two years. Next Generation Sequencing (NGS) of the ribosomal internal transcribed spacer 2 (ITS2) region of the Symbiodiniaceae provided detailed insight into the distinct symbiont types and the changes in community composition resulting from environmental stressors. Our findings indicate that temperature is a more significant driver of changes to the Symbiodiniaceae community compositions than pH in Hawaiian corals. We additionally demonstrate that the changes in symbiont communities in response to experimental temperature and pCO2 conditions arise from the shuffling of current symbionts and the incorporation of novel symbionts from the environment, which has implications for future coral resilience against climate change.

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Impact of ocean acidification on the intestinal microflora of the Pacific oyster Crassostrea gigas

Published 24 August 2021 Science Closed
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Highlights

  • Ocean acidification changed the community structure of the intestinal microflora in Crassostrea gigas.
  • The relative abundance of Firmicutes and the Firmicutes/Bacteroides ratio decreased under ocean acidification.
  • Mycoplasma was significantly enriched under ocean acidification.
  • The pathways related to proliferation were significantly enhanced in the intestinal microflora under ocean acidification.

Abstract

The intestinal microflora is critical for the health of hosts by affecting their nutrient absorption and immune response. Increasing evidences demonstrate that environmental stress can lead to the dysbiosis of intestinal microflora, which increases the susceptibility of host to pathogens. Ocean acidification (OA) is one of the greatest environmental threats for marine mollusks with the negative effects on growth and calcification, but its impact on intestinal microflora is poorly understood. In the present study, the intestinal microflora of the Pacific oyster Crassostrea gigas reared at seawater with pH values of 8.1 (control group), 7.8 (AC78 group) and 7.4 (AC74 group) were characterized and compared using 16S rRNA gene sequencing. The composition of oyster intestinal microflora changed significantly after acidification, while no significant difference of α-diversity was observed between the control and acidification groups. At the phylum level, the relative abundance of Firmicutes decreased in acidification groups, and the Firmicutes/Bacteroides ratio in AC78 and AC74 groups were 0.53 and 0.31-fold of that in the control group, respectively. At the genus level, the intestinal microflora was dominated by Mycoplasma, which was significantly enriched in the two acidification groups. LEfSe analysis showed that Mycoplasma was one of the most discriminative biomarkers in the AC78 group, while Alteromonadales, Amphritea, Salinirepens and Alteromonas were the biomarker taxa in the AC74 group. The functional prediction results indicated that the pathways related to protein and nucleic acid synthesis were enriched in the two acidification groups, while those related to carbohydrate catabolism were blocked in the AC78 group but enhanced in the AC74 group. These results suggested that the relative abundance of probiotic bacteria decreased upon ocean acidification, which favored the proliferation of pathogenic species in the intestine of oysters. The increased consumption of nutrients caused by microflora proliferation would aggravate the susceptibility of oysters to pathogens. Under greater OA stress, the intestinal microflora would enhance the competition for energy source with their hosts, consequently posing a great challenge to the host health. The information contributed to the better understanding of the oyster-microflora interactions under environmental stress.

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The role of symbiotic algae in the acclimatization of Oculina arbuscula to ocean acidification

Published 18 August 2021 Science Closed
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Ocean acidification (OA) caused by CO2 emissions is projected to decrease seawater pH to 7.6 by 2100. Scleractinian corals are at risk because excess H+ in seawater binds to carbonate (CO3 2-), reducing its availability for CaCO3 skeletons. The energy demand for skeletal growth increases as pH decreases because corals must actively purge excess H+ from their seawater sourced calcifying fluid to maintain high calcification rates. In scleractinian corals it is hypothesized that photosynthesis by symbiotic algae is critical to meet this increased energy demand. To test this hypothesis, I conducted laboratory and field studies with Oculina arbuscula, a facultatively symbiotic coral common in the southeastern U.S.A., which exhibits resilience to seasonal fluctuations in pCO2 that drive pH to as low as 7.8 in the summer. In the lab, aposymbiotic and symbiotic O. arbuscula colonies were exposed to a pH of 8.0 or 7.6 for 51 days to test the role of the algal symbiosis in maintaining energy reserves, calcifying fluid pH, skeletal organic matrix and calcification rates during OA. To supplement this controlled laboratory experiment, I transplanted 20 coral colonies to a seafloor CO2 monitoring platform, exploiting the natural variation in pCO2 that occurs in Georgia coastal waters. The relationship between calcifying fluid pH and seawater pH was tested for in these samples using boron stable isotopes in coral skeletons. Contrary to the hypothesis, both aposymbiotic and symbiotic O. arbuscula colonies maintained similar calcification rates when exposed to OA. Upregulation of calcifying fluid pH, likely fueled by metabolic energy derived from heterotrophy, was the primary acclimatory mechanism detected. Symbiotic algae were associated with higher coral lipid reserves and denser skeletons, but neither of these variables were affected by seawater pH. Corals growing offshore maintained a consistent calcifying fluid pH in the face of seasonal fluctuations in seawater pH and other environmental variables such as temperature and turbidity. The results of my study provide valuable insight into how O. arbuscula has evolved to survive harsh conditions of seasonally low pH levels characteristic of southeastern U.S.A. coastal waters and the mechanisms that may contribute to its future resilience to increasing OA.

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Ocean acidification alters the thermal performance curves of brooded larvae from the reef coral Pocillopora damicornis

Published 10 August 2021 Science Closed
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Establishing the thermal reaction norm of coral larvae under elevated pCO2 is crucial to anticipate how larval dispersal and population maintenance may be affected by future climate change. Here, we characterized the functional relationship between temperature (27−33 °C) and larval performance of the reef coral Pocillopora damicornis under two pCO2 levels. The results showed that the temperature threshold of larvae was between 32 and 33 °C, as evidenced by the abrupt declines in photochemical efficiency and symbiont density, whereas no oxidative damage was observed between 27 and 33 °C and elevated pCO2 did not influence any of these parameters. In addition, larval respiration and photosynthesis rates exhibited parabolic responses to temperature, and this relationship conformed to the Gaussian–Gompertz model with an optimal temperature around 31.5 °C, which was approximately 2.5 °C above the summer mean temperature, suggesting the potential for thermal acclimation. Most importantly, elevated pCO2 significantly enhanced the larval photosynthesis and the stimulatory effect of elevated pCO2 on the photosynthetic rates and capacity was more pronounced in cool and warm temperatures, indicative of shifted thermal sensitivity under high pCO2. These results suggest that ocean acidification could alter the thermal performance curves and tolerance window of brooded P. damicornis larvae, with profound and important implications for larval ecology in a future changing ocean.

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Acidification can directly affect olfaction in marine organisms

Published 30 July 2021 Science Closed
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In the past decade, many studies have investigated the effects of low pH/high CO2 as a proxy for ocean acidification on olfactory-mediated behaviours of marine organisms. The effects of ocean acidification on the behaviour of fish vary from very large to none at all, and most of the maladaptive behaviours observed have been attributed to changes in acid–base regulation, leading to changes in ion distribution over neural membranes, and consequently affecting the functioning of gamma-aminobutyric acid-mediated (GABAergic) neurotransmission. Here, we highlight a possible additional mechanism by which ocean acidification might directly affect olfaction in marine fish and invertebrates. We propose that a decrease in pH can directly affect the protonation, and thereby, 3D conformation and charge distribution of odorants and/or their receptors in the olfactory organs of aquatic animals. This can sometimes enhance signalling, but most of the time the affinity of odorants for their receptors is reduced in high CO2/low pH; therefore, the activity of olfactory receptor neurons decreases as measured using electrophysiology. The reduced signal reception would translate into reduced activation of the olfactory bulb neurons, which are responsible for processing olfactory information in the brain. Over longer exposures of days to weeks, changes in gene expression in the olfactory receptors and olfactory bulb neurons cause these neurons to become less active, exacerbating the problem. A change in olfactory system functioning leads to inappropriate behavioural responses to odorants. We discuss gaps in the literature and suggest some changes to experimental design in order to improve our understanding of the underlying mechanisms and their effects on the associated behaviours to resolve some current controversy in the field regarding the extent of the effects of ocean acidification on marine fish.

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