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a Relationship between respiration, photosynthesis, and calcification. The products of coral respiration (water, carbon dioxide) are used as substrates in zooxanthellae photosynthesis, and the products of photosynthesis (oxygen, glucose) are used as substrates in respiration. The energy required for coral calcification is generated by respiration. b Effects of oxygen on coral respiration, photosynthesis, and calcification. (i) At low oxygen concentrations, respiration is limited by oxygen availability. At high oxygen concentrations, respiration is largely independent of oxygen availability. (ii) At low oxygen concentrations, limited respiration reduces photosynthesis. At high oxygen concentrations, reactive oxygen species (ROS) and photorespiration reduce photosynthesis. (iii) At low oxygen concentrations, limited photosynthesis and respiration reduce calcification. At high oxygen concentrations, ROS and reduced photosynthesis reduce calcification
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Coral reefs are suffering unprecedented declines worldwide. Most studies focus on stressors such as rising temperatures, nutrient pollution, overfishing, and ocean acidification as drivers of this degradation. However, recent mass mortality events associated with low oxygen on coral reefs indicate that oxygen is a critical factor that can be limiti...
Citations
... In particular, temperature is an important factor as it regulates an organism's metabolism and was highly correlated with depth, assessed as a key driver of MCE community changes (Pyle et al. 2016). Temperature variability (i.e., delta) is acting as a proxy for the thermocline, internal waves, and oxygen or nutrient availability (Hosegood et al. 2019;Nelson and Altieri 2019). In this study, PAR was found to contribute to the distribution of all the clusters, except the deepest one (150-160 m), where light was no longer available (Diaz et al. 2023c). ...
To support conservation efforts, accurate mapping of marine organism community’ distribution has become more critical than ever before. While previous mapping endeavours have primarily focused on easily accessible shallow‐water habitats, there remains limited knowledge about the ecosystems lying beyond SCUBA diving depths, such as mesophotic coral ecosystems (MCEs, ~30–150 m). MCEs are important habitats from an ecological and conservation perspective, yet little is known about the environmental factors that shape these ecosystems and their distribution, particularly in the Indian Ocean region. The goals of this study are to (1) predict the spatial distribution and extent of distinct benthic communities and MCEs in the Chagos Archipelago, central Indian Ocean, (2) test the effectiveness of a range of environmental and topography derived variables to predict the location of MCEs around Egmont Atoll and the Archipelago, and (3) independently validate the models produced. In addition, we compared the MCEs predicted extent in the Archipelago for the models derived from high‐resolution multibeam and low‐resolution GEBCO bathymetry data. Using maximum entropy modelling, all models resulted in excellent (> 0.9) performances, for AUC and threshold‐dependent metrics, predicting extensive and previously undocumented MCEs across the entire Archipelago with, however, differences in the predicted extent between the high‐ and low‐resolution models. Independent validation resulted in fair (> 0.7 AUC) and poor (> 0.6 AUC) performances for the high‐resolution and low‐resolution models, respectively. Photosynthetically active radiation (PAR), temperature, chlorophyll‐a, and topographically derived variables were identified as the most influential predictors. In conclusion, this study provides the first prediction of the distribution of MCEs and their distinct benthic communities in the Archipelago. It highlights their significance in terms of potential extent and response to various environmental factors, supporting decision making for prioritising future survey sites to study MCEs across the Archipelago and targeting ecologically important areas for conservation.
... Coral-associated fishes may play a key role in oxygenating and increasing water flow around the surface of branching corals which is vital to the metabolism of corals and their symbiotic algae (Nelson and Altieri 2019). This process may be particularly key at night, when the center of a branching coral can become anoxic due to a lack of water flow, in combination with respiration by the coral in the absence of photosynthesis by Symbiodiniacea (Shashar et al. 1993). ...
On coral reefs, hard corals (order Scleractinia) provide habitat for an extraordinary diversity of reef fishes, many of which have close associations with the coral. Coral can provide these fishes nesting sites, food and protection from predators. The fishes can also benefit the corals, through fish-derived services resulting from, for example, diurnal and nocturnal movements, foraging, and nutrient excretion. Here, we synthesize how coral-associated fishes (i.e., coral reef fishes that have prolonged or permanent dependency on live coral) positively contribute to the health and resilience of corals. We highlight how certain coral-associated fishes offer a variety of benefits to corals, including enhanced oxygenation, nutrient subsidies and recycling, sediment removal, and protection against predators and parasites. Our review highlights the critical role that some coral-associated fishes have in supporting reef function, and in some cases, conferring resilience.
... We deployed 13 temperature loggers, spread out across the tanks, which recorded tank water temperature every 10 min. To improve water flow and prevent anoxia within experimental units, we elevated units above the tank floor with test-tube racks and added water pumps and air bubblers near the bottom of the tanks, which helped keep oxygen levels high (oxygen: 8.18 mg l −1 , dissolved oxygen: 128.45% using a Hanna H198194 pH/EC/DO multiparameter; natural range: 50-200% dissolved oxygen [45]). Because C. ungulatus is highly responsive to movement and accustomed to hiding in coral interstices, we provided refugia for crabs in the form of an approximately 5 cm long section of polyvinyl chloride (PVC) pipe (approximately 2 cm diameter) placed in each experimental unit. ...
Mutualisms can increase the ability of foundation species to resist individual stressors, but it remains unclear whether mutualisms can also ameliorate co-occurring stressors for habitat-forming species. To examine whether a suspected mutualist could improve foundation species’ resistance to multiple stressors, we tested how a common coral-dwelling crab affected corals exposed to macroalgal contact and physical wounding during a widespread heat stress event using flow-through tanks supplied with seawater from a nearby reef flat. High temperatures on the reef flat, which raised the temperature in our tanks, appeared to trigger rapid tissue loss in experimental corals, but the amount of tissue lost by corals was strongly determined by treatment. Macroalgal contact increased, while the presence of a crab decreased, the amount of tissue lost. Although the effect of wounding was not strong in isolation, when wounding occurred in the presence of a crab, coral tissue loss unexpectedly decreased below that of all other treatments. We propose that wounding increased coral resistance to stress by attracting crabs—a result that appeared supported in a field experiment. These results highlight that mutualisms can interact with stressors in unexpected ways, buffering the effects of both local and global stressors on foundation species.
... However, there is sufficient evidence to say that dissolved oxygen is a critical resource on coral reefs and that oxygen limitation (i.e. hypoxia) results in non-linearities and feedbacks that contribute to ecological tipping points (Nelson and Altieri, 2019). The consequences of crossing these TPs are perhaps most dramatically evident in sudden mass mortality events, which has led to calls to accelerate research on the deoxygenation on coral reefs (Altieri et al., 2017). ...
Warm-water coral reefs are facing unprecedented human-driven threats to their continued existence as biodiverse functional ecosystems upon which hundreds of millions of people rely. These impacts may drive coral ecosystems past critical thresholds, beyond which the system reorganises, often abruptly and potentially irreversibly; this is what the Intergovernmental Panel on Climate Change (IPCC, 2022) define as a tipping point. Determining tipping point thresholds for coral reef ecosystems requires a robust assessment of multiple stressors and their interactive effects. In this perspective piece, we draw upon the recent global tipping point revision initiative (Lenton et al., 2023a) and a literature search to identify and summarise the diverse range of interacting stressors that need to be considered for determining tipping point thresholds for warm-water coral reef ecosystems. Considering observed and projected stressor impacts, we endorse the global tipping point revision's conclusion of a global mean surface temperature (relative to pre-industrial) tipping point threshold of 1.2 °C (range 1–1.5 °C) and the long-term impacts of atmospheric CO2 concentrations above 350 ppm, while acknowledging that comprehensive assessment of stressors, including ocean warming response dynamics, overshoot, and cascading impacts, have yet to be sufficiently realised. These tipping point thresholds have already been exceeded, and therefore these systems are in an overshoot state and are reliant on policy actions to bring stressor levels back within tipping point limits. A fuller assessment of interacting stressors is likely to further lower the tipping point thresholds in most cases. Uncertainties around tipping points for such crucially important ecosystems underline the imperative of robust assessment and, in the case of knowledge gaps, employing a precautionary principle favouring lower-range tipping point values.
... As sea surface temperatures rise and eutrophication becomes more prevalent, dissolved oxygen (DO) levels are decreasing owing to lower DO solubility (Keeling et al., 2010), while microbial blooms elevate respiratory quotas, resulting in a net depletion of oxygen (Rabalais et al., 2014). These shifts in oxygen availability pose a particular threat to tropical coastal ecosystems, where organisms and habitats already experience extreme fluctuations in oxygen over diel and seasonal cycles owing to high rates of benthic productivity (Nelson and Altieri, 2019;Pezner et al., 2023). The effect of deoxygenation on coastal fauna and flora has been most widely documented in temperate ecosystems, where a universal characterization of hypoxic conditions (i.e. the level at which a hypoxic stress response is detected) has been defined as any DO concentration at or below 2.8 mg l −1 (Diaz and Rosenberg, 2008). ...
... On tropical coral reefs, DO concentrations can fluctuate over seasonal and diurnal cycles, largely driven by benthic community metabolism (Cyronak et al., 2018;Pezner et al., 2023). Oxygen fluxes on coral reefs can cause seawater to vary from near-anoxic to hyperoxic conditions, sometimes over short periods of time (Nelson and Altieri, 2019). Therefore, some corals have adaptations that provide them the capacity to withstand natural cycles of oxygen depletion (Camp et al., 2017;Hughes et al., 2022). ...
Coastal deoxygenation poses a critical threat to tropical coral reefs. Dissolved oxygen (DO) depletion can cause hypoxia-induced stress and mortality for scleractinian corals. Coral hypoxic responses are species-specific and likely modulated by the duration and severity of low-DO conditions, although the physiological mechanisms driving hypoxia tolerance are not fully understood. In this study, the Caribbean corals Acropora cervicornis, Porites astreoides, and Siderastrea siderea were exposed to either severe (1.5 mg L−1 DO) or moderate (3.5 mg L−1 DO) deoxygenation or a control treatment (6 mg L−1 DO). All corals survived 2 weeks of deoxygenation but exhibited sublethal changes to coral metabolism after 1- and 2-week exposures, compared to controls. Maximum quantum yield (Fv/Fm) was suppressed after 1 week in both deoxygenation treatments in A. cervicornis, and after 2 weeks in S. siderea and P. astreoides exposed to severe or moderate treatments, respectively. Respiration rates were lower than controls in A. cervicornis and S. siderea after 1 and 2 weeks of severe deoxygenation. The reduced respiration of P. astreoides after 1 week of moderate deoxygenation returned to control levels in week 2. Overall coral metabolic budgets, assessed by ratios of gross photosynthesis to respiration (Pg:R), were more autotrophic, or photosynthesis-dominant, after 1 week of severe deoxygenation in S. siderea and P. astreoides, while Pg:R was not significantly different in A. cervicornis between treatments. These results reveal that some corals shift their metabolism to tolerate low-oxygen conditions and avoid bleaching or mortality, indicating that metabolic plasticity is an important aspect of coral resistance to deoxygenation.
... Dissolved oxygen (DO) has been deemed a "universal currency" in coastal marine ecosystems due to its fundamental role in aerobic respiration and dynamic cycling between organisms and the environment (Nelson and Altieri 2019). Seawater DO concentrations in coastal habitats can fluctuate between normoxia (≥ 5 mg DO L À1 ) and severe hypoxia (< 2 mg DO L À1 ; Rosenberg 1980;Pezner et al. 2023), with variability occurring over spatial and temporal scales (Altieri et al. 2017;Breitburg et al. 2018;Pezner et al. 2023). ...
... For example, the benthic sea anemone Nematostella vectensis (class Hexacorallia) is able to persist in shallow coastal estuaries that may experience frequent and severe hypoxic events (Diaz et al. 1992;Summers 2001;Kiddon et al. 2003;Baumann et al. 2015), though the mechanisms of resilience in this species remain unexplored. Despite being in the same class as N. vectensis, tropical reef-building corals are more sensitive to hypoxia (Altieri et al. 2017(Altieri et al. , 2021Nelson and Altieri 2019;Hughes et al. 2020). However, some coral species can maintain aerobic respiration and other key metabolic processes under hypoxic conditions (Johnson et al. 2021b;Dilernia et al. 2024), indicating some level of resilience against this stressor. ...
Seawater hypoxia is increasing globally and can drive declines in organismal performance across a wide range of marine taxa. However, the effects of hypoxia on early life stages (e.g., larvae and juveniles) are largely unknown, and it is unclear how evolutionary and life histories may influence these outcomes. Here, we addressed this question by comparing hypoxia responses across early life stages of three cnidarian species representing a range of life histories: the reef‐building coral Galaxea fascicularis, a broadcast spawner with horizontal transmission of endosymbiotic algae (family Symbiodiniaceae); the reef‐building coral Porites astreoides, a brooder with vertical endosymbiont transmission; and the estuarine sea anemone Nematostella vectensis, a non‐symbiotic broadcast spawner. Transient exposure of larvae to hypoxia (dissolved oxygen < 2 mg L⁻¹ for 6 h) led to decreased larval swimming and growth for all three species, which resulted in impaired settlement for the corals. Coral‐specific responses also included larval swelling, depressed respiration rates, and decreases in symbiont densities and function. These results indicate both immediate and latent negative effects of hypoxia on cnidarian physiology and coral–algal mutualisms specifically. In addition, G. fascicularis and P. astreoides were sensitized to heat stress following hypoxia exposure, suggesting that the combinatorial nature of climate stressors will lead to declining performance for corals. However, sensitization to heat stress was not observed in N. vectensis exposed to hypoxia, suggesting that this species may be more resilient to combined stressors. Overall, these results emphasize the importance of reducing anthropogenic carbon emissions to limit further ocean deoxygenation and warming.
... It ranges from 0 to 1, R = 1 signifying ocean-like conditions, while R = 0 indicates complete depletion of a limiting resource. Oxygen, rather than dissolved nutrients, is often the primary limiting resource in this context [27,28]. Corals are among the most efficient organisms adapted to thrive in oligotrophic waters, where excess nutrients can lead to eutrophication, subsequently reducing coral populations [13,29]. ...
In this work, we demonstrate that key aspects of the dynamical behavior of coral reefs at the macro scale, which evolve over time scales of centuries, can be accurately described using a model that integrates a few fundamental ecological and physical mechanisms. The model displays excitable behavior generating, among other dynamical regimes, traveling pulses and waves, which result in the formation of spatial structures resembling those observed in real reefs, without involving a classical pattern formation mechanism, like the Turing scenario. We conduct an in-depth exploration of the bifurcations exhibited by the model as a function of the two most ecologically significant parameters. This establishes the groundwork for using the presented model as a tool to explain coral reef formation.
... Decreased oxygen levels can affect essential physiological processes such as productivity, respiration, and calcification and often play a role in the outcome of interactions between corals and other organisms, as found from previous studies on coral reefs (Nelson and Altieri, 2019). Moreover, despite studies on cold-water corals being scarce, the predicted future reduction in the dissolved oxygen concentration (Sweetman et al., 2017) seems also to exert control on cold-water corals' biogeographic distribution, as also found in other reef-forming scleractinian species such as Desmophyllum pertusum in the North Atlantic (Tittensor et al., 2010). ...
Vulnerable Marine Ecosystems (VMEs) are recognised as having high ecological significance and susceptibility to disturbances, including climate change. One approach to providing information on the location and biological composition of these ecosystems, especially in difficult-to-reach environments such as the deep sea, is to generate spatial predictions for VME indicator taxa. In this study, the Random Forest algorithm was used to model the spatial distribution of density for 14 deep-water VME indicator taxa under current environmental conditions and future climate change scenarios (SSP2-4.5 and SSP3-7.0) within the New Zealand Territorial Sea and Exclusive Economic Zone (100-1500 m water depth) to evaluate potential changes in the location and distribution of density of these taxa over time. Overall, our species distribution models performed well for all taxa (mean AUC = 0.82; TSS = 0.56; r = 0.40) and predicted a considerable average reduction in density (54%) and habitat extent (61%), by the end of the 21st century under both climate change scenarios. Nevertheless, models identified regions that might serve as internal refugia (approximately 158,000 km 2), where some taxa are predicted to maintain the high densities predicted for current-day environmental conditions under future climatic conditions, and external refugia (approximately 121,000 km 2) where taxa were predicted to expand into new locations by the end of the 21st century. Our results represent a significant step forward as they provide predictions of the distribution of taxa densities, rather than just occurrence, under both present and future climatic conditions. Furthermore, these findings carry implications for ecosystem management and spatial planning, suggesting current marine spatial protection measures may not offer adequate protection to VME indicator taxa in the face of climate change. Additionally, activities like bottom trawling, present or future, may jeopardize climate refugia viability. Thus, a comprehensive assessment of cumulative effects on VME indicator taxa is recommended to establish effective protection measures for potential climate refugia, ensuring the continuity of essential ecosystem services.
... Photorespiration in zooxanthellae isolated from T. maxima clams increased at elevated oxygen levels [19]. Low dissolved oxygen concentrations are often observed in coral reef waters [20]. The photosynthesis of Tridacna sp. that evolved in such an environment may have been adapted to relatively low oxygen concentrations. ...
... The photosynthesis of Tridacna sp. that evolved in such an environment may have been adapted to relatively low oxygen concentrations. The occurrence of high oxygen concentrations observed in the coral-algal interface and water between coral branches [20] could negatively affect the photosynthesis and thus the survival of Tridacna sp. This finding also suggests the importance of controlling the oxygen concentration in the aquaculture of Tridacna sp. ...
The sea bivalve clam Tridacna crocea inhabiting the shallow sea of tropical and subtropical zones lives with the symbiotic alga zooxanthella in its mantle. Zooxanthellae algae perform photosynthesis and supply nutrients to T. crocea. Recently, the abundance of T. crocea has decreased rapidly due to overfishing in coastal areas in Okinawa, Japan. T. crocea culture systems for mass production will contribute to the conservation of T. crocea and thus marine ecosystems. Environmental control methods for T. crocea culture have not been established because of a lack of knowledge about the appropriate environmental conditions for T. crocea growth. The present study was initiated to obtain basic data for developing environmental control methods for T. crocea land-based aquaculture. The effects of water temperature, dissolved oxygen concentration, and photosynthetic photon flux density (PPFD) on the O2 exchange rates of the symbiotic system of T. crocea and zooxanthella, which are indicators of photosynthesis and respiration in the system, and the effect of daily integrated PPFD on T. crocea growth were investigated. Basic knowledge was obtained for the development of optimal environmental control technology for T. crocea clam culture. The optimum water temperature and dissolved oxygen concentration for photosynthesis in this symbiotic system were 28 °C, 5–6 mgO2 L−1 and 500 μmol m−2 d−1, respectively. The optimum daily integrated PPFD for clam growth was 20 mol m−2 d−1.
... Meanwhile, reduced light exposure at greater depths can constrain the photosynthetic activities of zooxanthellae (Kahng et al. 2019;López-Londoño et al. 2024). Additionally, very low water flow may result in the formation of boundary layers around the coral surface, hindering nutrient absorption and consequently suppressing coral respiration and growth (Nelson and Altieri 2019;Hughes et al. 2020). It is important to note that each location has unique conditions, and human impacts on coral reefs are highly dependent on the local context. ...
The Karimunjawa Marine National Park, situated in the Java Sea, Indonesia, is renowned for its rich biodiversity and lively coral reefs. However, amidst the backdrop of this natural beauty, concerns have been raised regarding the potential impacts of shipping activities on the health and diversity of these fragile ecosystems. The increase in maritime traffic, including commercial vessels, tourist boats, and fishing vessels, traversing through the Karimunjawa Marine National Park, raises significant environmental concerns. The movement of these vessels, especially along specific shipping routes, has the potential to disturb and damage coral reefs through various mechanisms. Hence, this study aimed to investigate the possible impacts of shipping routes on the coral abundance and diversity and the coral health in the Karimunjawa Islands, Java Sea, Indonesia.
This study categorized ship routes into West Route, East Route, and Non-Route and assessed coral health and diversity across 15 islands. Key metrics analyzed included coral disease prevalence, coral cover, diversity index, species richness, relative abundance, and evenness, using 15 × 2 m belt transects at 3 and 8 m depths with three repetitions each.
Statistical analysis revealed significant differences in coral abundance and species richness among ship-route groups, but no significant depth-related differences. These results suggest that while shipping routes affect certain aspects of coral health and diversity, other factors may be more influential in shaping coral disease prevalence and overall diversity in Karimunjawa reefs.
