Physiological resilience of a temperate soft coral to ocean warming and acidification

Short-term ocean acidification decreases pulsation and growth of the widespread soft coral Xenia umbellata

Coral reefs may experience lower pH values as a result of ocean acidification (OA), which has negative consequences, particularly for calcifying organisms. Thus far, the effects of this global factor have been mainly investigated on hard corals, while the effects on soft corals remain relatively understudied. We therefore carried out a manipulative aquarium experiment for 21 days to study the response of the widespread pulsating soft coral Xenia umbellata to simulated OA conditions. We gradually decreased the pH from ambient (~8.3) to three consecutive 7-day long pH treatments of 8.0, 7.8, and 7.6, using a CO2 dosing system. Monitored response variables included pulsation rate, specific growth rate, visual coloration, survival, Symbiodiniaceae cell densities and chlorophyll a content, photosynthesis and respiration, and finally stable isotopes of carbon (C) and nitrogen (N) as well as CN content. Pulsation decreased compared to controls with each consecutive lowering of the pH, i.e., 17% at pH 8.0, 26% at pH 7.8 and 32% at pH 7.6, accompanied by an initial decrease in growth rates of ~60% at pH 8.0, not decreasing further at lower pH. An 8.3 ‰ decrease of δ13C confirmed that OA exposed colonies had a higher uptake and availability of atmospheric CO2. Coral productivity, i.e., photosynthesis, was not affected by higher dissolved inorganic C availability and none of the remaining response variables showed any significant differences. Our findings suggest that pulsation is a phenotypically plastic mechanism for X. umbellata to adjust to different pH values, resulting in reduced growth rates only, while maintaining high productivity. Consequently, pulsation may allow X. umbellata to inhabit a broad pH range with minimal effects on its overall health. This resilience may contribute to the competitive advantage that soft corals, particularly X. umbellata, have over hard corals.

Metabolic profiling of Mytilus coruscus mantle in response of shell repairing under acute acidification

Mytilus coruscus is an economically important marine bivalve mollusk found in the Yangtze River estuary, which experiences dramatic pH fluctuations due to seasonal freshwater input and suffer from shell fracture or injury in the natural environment. In this study, we used intact-shell and damaged-shell M. coruscus and performed metabolomic analysis, free amino acids analysis, calcium-positive staining, and intracellular calcium level tests in the mantle to investigate whether the mantle-specific metabolites can be induced by acute sea-water acidification and understand how the mantle responds to acute acidification during the shell repair process. We observed that both shell damage and acute acidification induced alterations in phospholipids, amino acids, nucleotides, organic acids, benzenoids, and their analogs and derivatives. Glycylproline, spicamycin, and 2-aminoheptanoic acid (2-AHA) are explicitly induced by shell damage. Betaine, aspartate, and oxidized glutathione are specifically induced by acute acidification. Our results show different metabolic patterns in the mussel mantle in response to different stressors, which can help elucidate the shell repair process under ocean acidification. furthermore, metabolic processes related to energy supply, cell function, signal transduction, and amino acid synthesis are disturbed by shell damage and/or acute acidification, indicating that both shell damage and acute acidification increased energy consumption, and disturb phospholipid synthesis, osmotic regulation, and redox balance. Free amino acid analysis and enzymatic activity assays partially confirmed our findings, highlighting the adaptation of M. coruscus to dramatic pH fluctuations in the Yangtze River estuary.

The Open acidification Tank Controller: An open-source device for the control of pH and temperature in ocean acidification experiments

Ocean acidification is the process by which the increase in atmospheric CO₂ causes a corresponding increase in seawater CO₂ and results in lowering the seawater pH. While this process is likely to have substantial impacts on marine ecosystems, research into the effect of ocean acidification has been limited by the high costs of quality tools to perform ocean acidification treatments in the lab. The Open Acidification Tank Controller is designed to reduce the cost of ocean acidification research by providing a device that can monitor and control pH and temperature of aquaria as well as or better than commercially available research-grade devices, but for less than $250 USD per aquarium. The device is centered around an Arduino Mega 2560 and is assembled into a 3D printed housing. It monitors pH using a BNC glass pH probe and temperature using a three-wire waterproof PT100 temperature sensor. The Open Acidification Tank Controller also features web-based parameter reporting, and data storage to a micro-SD card. This device can hold aquarium pH and temperature at given setpoints, ramp between two values over a user-defined time period, or produce a sine-wave fluctuation in values.

Excess labile carbon promotes diazotroph abundance in heat-stressed octocorals

Nitrogen limitation is the foundation of stable coral-algal symbioses. Diazotrophs, prokaryotes capable of fixing N₂ into ammonia, support the productivity of corals in oligotrophic waters, but could contribute to the destabilization of holobiont functioning when overstimulated. Recent studies on reef-building corals have shown that labile dissolved organic carbon (DOC) enrichment or heat stress increases diazotroph abundance and activity, thereby increasing nitrogen availability and destabilizing the coral-algal symbiosis. However, the (a)biotic drivers of diazotrophs in octocorals are still poorly understood. We investigated diazotroph abundance (via relative quantification of nifH gene copy numbers) in two symbiotic octocorals, the more mixotrophic soft coral Xenia umbellata and the more autotrophic gorgonian Pinnigorgia flava, under (i) labile DOC enrichment for 21 days, followed by (ii) combined labile DOC enrichment and heat stress for 24 days. Without heat stress, relative diazotroph abundances in X. umbellata and P. flava were unaffected by DOC enrichment. During heat stress, DOC enrichment (20 and 40 mg glucose l^-1) increased the relative abundances of diazotrophs by sixfold in X. umbellata and fourfold in P. flava, compared with their counterparts without excess DOC. Our data suggest that labile DOC enrichment and concomitant heat stress could disrupt the nitrogen limitation in octocorals by stimulating diazotroph proliferation. Ultimately, the disruption of nitrogen cycling may further compromise octocoral fitness by destabilizing symbiotic nutrient cycling. Therefore, improving local wastewater facilities to reduce labile DOC input into vulnerable coastal ecosystems may help octocorals cope with ocean warming.

Heat wave induces oxidative damage in the Chinese pond turtle (Mauremys reevesii) from low latitudes

Introduction

Global warming has led to frequent heat waves, causing global organisms to face severe survival challenges. However, the way in which heat waves threaten the fitness and survival of animals remains largely unclear. Oxidative damage and immunity are widely considered the link between heat waves and threats to animals.

Methods

To evaluate the oxidative damage caused by heat waves and to reveal the physiological resistance to heat waves by the antioxidant defense of animals from different latitudes, we exposed both high-latitude (Zhejiang) and low-latitude (Hainan) populations of Chinese pond turtle (Mauremys reevesii) to simulate heat waves and a moderate thermal environment for 1 week, respectively. Next, we compared the oxidative damage by malondialdehyde (MDA) and antioxidant capacity by superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and total antioxidant capacity (T-AOC) in the liver tissues and evaluated the innate immunity by serum complement protein levels (C3, C4) and lysozyme activity in plasma of turtles.

Results and discussion

We found that heat waves significantly increased the content of MDA and the activity of CAT, whereas it decreased the activity of SOD, T-AOC, and GSH/GSSG in turtles from low latitudes. Furthermore, heat waves increased CAT activity but decreased GSH/GSSG in turtles from high latitudes. Although the turtles from high latitudes had higher levels of innate immunity, the heat waves did not affect the innate immunity of C3, C4, or lysozyme in either population. These results indicate that the low-latitude population suffered higher oxidative damage with lower antioxidant capacities. Therefore, we predict that Chinese pond turtles from low latitudes may be more vulnerable to heat waves caused by climate warming. This study reveals the physiological and biochemical resistance to heat waves in Chinese pond turtles from different latitudes and highlights the importance of integrative determination of fitness-related responses in evaluating the vulnerability of ectotherms from different latitudes to climate warming.

Transcriptional response of the azooxanthellate octocoral Scleronephthya gracillimum to seawater acidification and thermal stress

The stress responses to increased seawater temperature and marine acidification were investigated using a microarray to reveal transcriptional changes in S. gracillimum. For the study, corals were exposed to different stress experiments; high temperature only (26 °C, 28 °C and 30 °C), low-pH only (pH 7.5, pH 7.0 and pH 6.5) and dual stress experiments (28 °C + pH 7.8, 28 °C + pH 7.5 and 28 °C + pH 7.0), mortality and morphological changes in 24 h exposure experiments were investigated. The survival rates of each experimental group were observed. The gene expression changes in single and dual stress exposed coals were measured and the differentially expressed genes were classified with gene ontology analysis. The top three enriched gene ontology terms of DEGs in response to dual stress were metal ion binding (23.4%), extracellular region (17.2%), and calcium ion binding (12.8%). The gene showing the greatest increase in expression as a response to the dual stress was hemagglutinin/amebocyte aggregation factor, followed by interferon-inducible GTPase 5 and the gene showing the greatest decrease as a response to the dual stress was Fas-associating death domain-containing protein, followed by oxidase 2. These results represented the transcriptomic study focused on the stress responses of the temperate asymbiotic soft coral exposed to single and dual stresses. The combined effect of thermal and acidification stress on corals triggered the negative regulation of ion binding and extracellular matrix coding genes and these genes might serve as a basis for research into coral-specific adaptations to stress responses and global climate change.

Transcriptional response of the calcification and stress response toolkits in an octocoral under heat and pH stress

Up to one-third of all described marine species inhabit coral reefs, but the future of these hyperdiverse ecosystems is insecure due to local and global threats, such as overfishing, eutrophication, ocean warming and acidification. Although these impacts are expected to have a net detrimental effect on reefs, it has been shown that some organisms such as octocorals may remain unaffected, or benefit from, anthropogenically induced environmental change, and may replace stony corals in future reefs. Despite their potential importance in future shallow-water coastal environments, the molecular mechanisms leading to the resilience to anthropogenically induced stress observed in octocorals remain unknown. Here, we use manipulative experiments, proteomics and transcriptomics to show that the molecular toolkit used by Pinnigorgia flava, a common Indo-Pacific gorgonian octocoral, to deposit its calcium carbonate skeleton is resilient to heat and seawater acidification stress. Sublethal heat stress triggered a stress response in P. flava but did not affect the expression of 27 transcripts encoding skeletal organic matrix (SOM) proteins. Exposure to seawater acidification did not cause a stress response but triggered the downregulation of many transcripts, including an osteonidogen homologue present in the SOM. The observed transcriptional decoupling of the skeletogenic and stress-response toolkits provides insights into the mechanisms of resilience to anthropogenically driven environmental change observed in octocorals.

The Draft Genome of an Octocoral, Dendronephthya gigantea

Coral reefs composed of stony corals are threatened by global marine environmental changes. However, soft coral communities of octocorallian species, appear more resilient. The genomes of several cnidarians species have been published, including from stony corals, sea anemones, and hydra. To fill the phylogenetic gap for octocoral species of cnidarians, we sequenced the octocoral, Dendronephthya gigantea, a nonsymbiotic soft coral, commonly known as the carnation coral. The D. gigantea genome size is ∼276 Mb. A high-quality genome assembly was constructed from PacBio long reads (29.85 Gb with 108× coverage) and Illumina short paired-end reads (35.54 Gb with 128× coverage) resulting in the highest N50 value (1.4 Mb) reported thus far among cnidarian genomes. About 12% of the genome is repetitive elements and contained 28,879 predicted protein-coding genes. This gene set is composed of 94% complete BUSCO ortholog benchmark genes, which is the second highest value among the cnidarians, indicating high quality. Based on molecular phylogenetic analysis, octocoral and hexacoral divergence times were estimated at 544 MYA. There is a clear difference in Hox gene composition between these species: unlike hexacorals, the Antp superclass Evx gene was absent in D. gigantea. Here, we present the first genome assembly of a nonsymbiotic octocoral, D. gigantea to aid in the comparative genomic analysis of cnidarians, including stony and soft corals, both symbiotic and nonsymbiotic. The D. gigantea genome may also provide clues to mechanisms of differential coping between the soft and stony corals in response to scenarios of global warming.