Posts Tagged 'photosynthesis'

Response of Zostera japonica rhizosphere bacteria to ocean acidification

Published 24 July 2026 Science Leave a Comment
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Abstract

Amidst global climate change, the escalating atmospheric CO2 levels have intensified ocean acidification (OA), significantly impacting the structure and function of marine ecosystems. Seagrass beds, representative nearshore ecosystems, play a pivotal role in carbon sequestration, biodiversity preservation, and nearshore environmental equilibrium. Rhizosphere microorganisms within seagrass beds, essential components of the ecosystem, drive material cycling and energy flow. Their community structure and functions demonstrate heightened sensitivity to environmental variations. While previous studies have primarily focused on the effects of ocean acidification on seagrass hosts, limited attention has been given to the rhizosphere. Therefore, this study selected Zostera japonica as the focal species and systematically evaluated changes in the structure and function of the rhizosphere bacterial community across varying acidification levels (400 ppm, 1,000 ppm, 2,000 ppm CO2) within an ocean acidification context. The results revealed a significant decline in the richness and diversity of the rhizosphere bacterial community under acidification, accompanied by shifts in community composition characterized by an increase in the relative abundance of Bacteroidota and Tenacibaculum with escalating acidification levels. In high acidification conditions, bacterial network interactions exhibited a trend toward simplification; yet the number of key taxonomic units increases, and there was a shift in community assembly from stochastic to deterministic processes. Functional predictions indicated the enhancement of microbial carbon sequestration and nitrogen fixation under acidification, while denitrification and specific sulfur metabolism pathways were inhibited. This implies that in acidified environments, the rhizosphere bacterial community may enhance carbon and nitrogen fixation to uphold nutrient supply.

Importance

Against the background of escalating global climate change and ocean acidification, seagrass beds, as crucial blue carbon sink ecosystems, face formidable challenges to their ecological functions and stability. Rhizosphere microorganisms of seagrasses, serving as the “second genome” of the seagrass host, play a central role in material cycling, nutrient supply, and system stability within seagrass beds. They are a key biological component that supports seagrass adaptation to environmental changes. Therefore, investigating the response and adaptation mechanisms of seagrass rhizosphere bacterial communities under ocean acidification is essential for deepening our understanding of the stability and resilience of seagrass bed ecosystems.

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Water mass-driven variations in primary production and bacterial respiration during the spring bloom in the Fram Strait

Published 23 July 2026 Science Leave a Comment
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Highlights

  • Remote sensing indicates declining bloom in cold and developing bloom in warm water
  • Primary production peaks in polar and arctic surface water masses
  • Bacterial production is coupled to primary production despite thermal sensitivity
  • Oxygen-based community respiration is highest in warmer water masses
  • Oxygen-based and INT-based respiration estimates differ by an order of magnitude

Abstract

The Fram Strait is the primary oceanic gateway to the Arctic Ocean and has highly dynamic oceanographic conditions. Oceanographic conditions can shape community compositions, which is increasingly shown using molecular studies, but rate measurements remain scarce, especially for respiration in the Arctic Ocean. Here, we assessed primary production (PP), bacterial production (BP), bacterial abundances (BA), and community respiration (CR) using Winkler titrations and in vivo Iodo-Nitro-Tetrazolium (INT) reduction within the upper 50 m across water masses of the Fram Strait that had varying bloom conditions in spring 2021. We complemented in situ observations with remote sensing of sea surface temperature (SST) and chlorophyll-a (SSC) to infer bloom phenology in warm (θ>2°C), intermediate (2>θ>0°C), and cold (θ<0°C) waters using remote sensing machine-learning. In the cold, nutrient-rich surface waters, a subsiding spring bloom was associated with elevated PP, BP, and BA alongside a high temperature sensitivity (Q10) that indicates active microbial turnover. In contrast, the warm Atlantic-influenced waters exhibit a relatively lower PP but moderate cell-specific BP, suggesting bacterial maintenance metabolism under pre-bloom conditions. Notably, bacterial respiration (BR) estimates differed by 5- to 80-fold between the Winkler and in vivo INT methods. Although the true BR likely lies between these estimates, our results highlight the substantial microbial activity and underscore the need for more accurate BR measurements in Arctic studies.

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Changes in photosynthesis and grazing facilitate growth of a mixotrophic protist under ocean acidification and warming

Published 22 July 2026 Science Leave a Comment
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Summary

  • Mixotrophic protists capable of both photosynthesis and phagotrophy are key members of marine plankton communities. Yet, little is known about their responses to the combination of ocean acidification and warming.
  • A marine mixotrophic chrysophyte, Ochromonas CCMP2951, was subjected to two levels of pCO2 (300 and 800 ppm, resulting in pH of 8.2 and 7.8) and temperature (21°C and 26°C) in a factorial design.
  • Enhanced growth rates were observed in both the high CO2 and high temperature treatments, while cell size significantly decreased with temperature. Strongly decreased cellular phosphorus content led to increased N : P and C : P ratios of Ochromonas with temperature. Furthermore, warming increased grazing rates, while elevated CO2 reduced the Chl content but increased photosynthetic carbon acquisition, albeit only at low temperature. The combination of warming and elevated CO2 had antagonistic effects on the balance between autotrophic and heterotrophic carbon acquisition, keeping the net role of this mixotroph in the marine carbon cycle stable.
  • Altogether, both the direct stimulation of growth and the indirect effects of altered stoichiometry may favor mixotrophs under future ocean conditions. However, their contribution to future carbon cycling in complex natural communities will need further study.

Graphical Abstract

Grazing rates of Ochromonas CCMP2951 expressed as carbon obtained per mean cell volume (a) and Caut/het-photosynthetically fixed carbon divided by carbon obtained through grazing (b) in the four experimental treatments.

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Coastal phytoplankton response to acidification and warming under differing levels of nutrient availability

Published 12 June 2026 Science Closed
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Ocean acidification and warming will alter phytoplankton biomass and composition, yet despite numerous studies, there are few consistent responses on which to base predictions. To determine the responses of chlorophyll-a and phytoplankton size and composition to predicted lower pH (−0.33 to −0.5) alone, and also combined with elevated temperature (+2.5–3.5 °C), two mesocosm experiments were carried out in austral spring and autumn in temperate New Zealand coastal waters. Lower pH alone had no effect on chlorophyll-a in either experiment and, as the treatment pH was lower than the pH minimum recorded in a parallel four-year time series, this lack of response in chlorophyll-a was not attributable to prior in situ exposure. Conversely, chlorophyll-a increased under lower pH and warming in both experiments, with the large (>20 µm) phytoplankton size fraction showing opposing responses under nutrient deplete and replete conditions. Diatom biomass also increased in both treatments when nutrient availability was maintained, with a dominant pennate species Cylindrotheca clostridium emerging. The results highlight the value of contextual time series for experimental interpretation, and also the importance of assessing warming and acidification together using regionally representative nutrient concentrations, for prediction of coastal phytoplankton response to climate change.

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Seasonal upwelling shapes coral reef community structure and photophysiology on the Pacific Coast of Costa Rica

Published 3 June 2026 Science Closed
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Reef-building corals form the calcium-carbonate frameworks that underpin tropical coral reefs, yet global coral cover has declined by ~50% in recent decades, due to marine heatwaves and other stressors. Identifying refugia environments, such as upwelling systems, that buffer stress, promote recovery, and enhance resilience by promoting physiological plasticity that supports thermotolerance is therefore critical. Here, we compared benthic community composition, coral percent cover, and photo-physiology between an upwelling location in the Gulf of Papagayo and a non-upwelling location in Sámara on the Pacific coast of Costa Rica. Waters in Papagayo were cooler, more acidic, and richer in chlorophyll a. Reefs at this location exhibited higher crustose coralline algae, higher sea urchin cover, and lower macroalgae cover, compared to Sámara. Papagayo also showed higher stony coral cover, driven by Pocillopora spp., while Sámara was dominated by massive, heat-tolerant Porites spp.. When significant, photophysiological measurements showed 9.7 – 44.5% higher photosynthetic efficiency (Fv’/Fm’) in Papagayo corals and 19.94 – 42.75 % higher maximum photosynthetic rates (Pmax) in Sámara corals. These results highlight how contrasting environmental regimes within a relatively small geographic area can shape distinct coral community compositions and photophysiological strategies, with implications for identifying areas of reef persistence or refugia.

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Carbon allocation strategy of Thalassiosira weissflogii in response to elevated pCO2

Published 25 May 2026 Science Closed
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Diatoms are of significance in the marine ecosystem, playing a pivotal role in the sustenance of marine life and the transfer of carbon from the surface ocean to deeper waters. Although numerous studies have investigated the effects of elevated carbon dioxide (CO2) on marine diatoms across both short- and long-term adaptation scales, the molecular mechanisms governing chitin metabolism in response to ocean acidification remain poorly understood. In this study, we employed an integrated approach combining transcriptomic, metabolomic, and physiological analyses to examine the marine diatom Thalassiosira weissflogii following 40-day acclimation to high-CO2 conditions. Physiological studies have demonstrated that ocean acidification has the capacity to result in an augmentation of the C/N ratio, chitin content, maximum PSII quantum yield (Fv/Fm), and photosynthetic pigment content of T. weissflogii. Analysis of chlorophyll fluorescence dynamics further demonstrated enhanced primary photochemical efficiency of PSII in the acidified treatment group. Consistent with this, the transcriptome results also showed that the photosynthesis-related pathways were upregulated to meet the increased material and energy requirements after adaptation to elevated CO2 levels. More importantly, it was determined that acidification treatment resulted in the upregulation of chitin synthesis and the downregulation of chitin degradation in T. weissflogii, consequently leading to an augmentation in chitin content. These findings indicate that ocean acidification (high CO2, low pH) prompts T. weissflogii to prioritize the allocation of carbon resources to the synthesis of chitin. The synthesis of chitin may reinforce cell wall formation as an adaptive response to ocean acidification. Our research provides new insights into the marine acidification adaptation strategies of T. weissflogii.

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Seawater acidification accelerates growth but hastens decline in batch cultures of the marine diatom Thalassiosira pseudonana

Published 22 April 2026 Science Closed
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Diatoms are characterized by rapid cell division and a high capacity to adapt to environmental variability, and some species can form blooms when environmental conditions are favorable. Previous studies have largely focused on the bloom development phase, during which biomass accumulates rapidly, whereas the decline phase-despite its critical role in carbon export and microbial loop dynamics-has received far less attention. Here, we tracked changes in cell density and inorganic carbon utilization characteristics throughout the entire course of a simulated Thalassiosira pseudonana bloom under ambient (420 μatm) and elevated pCO2 (1000 μatm) conditions. Inhibitors of carbonic anhydrase and direct bicarbonate transporters were applied to investigate the characteristics of inorganic carbon utilization. The relationship between photosynthetic rate and inorganic carbon concentration was measured to assess inorganic carbon affinity. The simulated T. pseudonana bloom was characterized by rapid cell density accumulation, reaching a peak within 10 days, followed by a rapid decline without a distinct stationary phase. As the bloom progressed, photosynthetic rate and the maximum quantum yield of photosystem II declined, whereas the inorganic carbon affinity increased. Elevated CO2 enhanced growth and maximum quantum yield during the acceleration phase but resulted in an 86% higher fitted death rate during the decline phase. Regarding the relationship between photosynthetic rate and dissolved inorganic carbon concentration, elevated CO2 increased the maximum photosynthetic rate and half-saturation constant only during the acceleration phase. Collectively, these results indicate that seawater acidification can influence both biomass accumulation and decline intensity in diatom blooms, with potential consequences for carbon sequestration and its redistribution among biogeochemical pools.

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Unravelling marine benthic functioning shifts under ocean acidification

Published 10 April 2026 Science Closed
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Ocean acidification (OA) driven by increasing atmospheric CO2 is altering marine biodiversity. However, impacts of OA on ecosystem functioning at the community level, including calcification, primary production and nutrient uptake, remain largely unknown. Here, we conducted community transplant experiments at natural CO2 vents to assess how declining pH affects marine community species composition, biomass, and key ecosystem processes over time. Our results indicate that community shifts caused by declining pH lead to decreased biomass and calcification rates, while photosynthesis and nutrient uptake rates increased. By leveraging OA field model systems and in situ measurements of ecosystem functioning, this study provides critical insights into how OA-induced biodiversity loss reshapes the structure and functioning of temperate marine coastal ecosystems.

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Physiological responses of Swedish maerl to ocean acidification and warming

Published 17 March 2026 Science Closed
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Maerl, (Corallinales, Rhodophyta), are free-living calcareous algae found in coastal ecosystems. They form biogenic beds with complex structures in which other species can find refuge or on which other species can settle, which highlights their importance as an ecosystem. While many species have been investigated worldwide, maerl from the Swedish west coast are poorly studied. This report investigated both acidification and warming impacts on different physiological functions of Swedish maerl, including photosynthesis, respiration and calcification. The maerl were exposed to different pH levels and temperatures in both light and dark conditions to determine their physiological thresholds, where photosynthesis and respiration were measured via oxygen fluctuations, photosynthetic efficiency via PAM fluorometry and calcification via alkalinity titrations. It was found that neither photosynthetic nor respiratory oxygen exchange showed positive or negative trends when exposed to changes in pH. On the contrary, photosynthesis peaked at the natural ambient temperature of 16°C and respiration increased with increasing temperature. Photosynthetic efficiency also did not show any trends to pH changes. However, calcification showed a significant (p < 0.05) negative response to pH in both light and dark conditions, with the response more severe in dark conditions. This suggests that decreasing pH may induce skeletal dissolution, and that photosynthesis could help buffer internal responses to external conditions. Carbonate production at ambient conditions in the light was calculated to be 556 ± 54 g CaCO3 m-2 yr-1, showing that Swedish maerl are just as, if not more, productive than maerl found elsewhere. Overall, this report showed that photosynthetic and respiratory thresholds may not be reached with acidification and that temperature increases could instead have much more severe consequences. It also showed that calcification thresholds will be met sooner rather than later, depending on acidification rates, in darker conditions for maerl found in temperate and possibly polar regions.

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Multi-level holobiont dysregulation increases the ecological risk of combined ocean acidification and benzo[a]pyrene pollution to the reef-building coral Porites lutea

Published 17 March 2026 Science Closed
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Highlights

  • Combined ocean acidification and BaP induce holobiont dysregulation, evidencing by a decoupled Symbiodiniaceae proliferation and a collapse of the archaeal Nanoarchaeota-Halobacterota symbiosis.
  • The coral host shifts its defense strategy from antioxidant capacity to cellular homeostasis, while the bacterial community increases functional redundancy, revealing a costly acclimation mechanism.
  • The multi-level dysregulation demonstrates an underestimated ecological risk, highlighting that current single-stressor risk assessments are inadequate for protecting corals under complex pollution scenarios.

Abstract

Reef-building corals are increasingly threatened by the combined effects of global climate change and localized organic pollutants. However, the holistic impacts of co-exposure to ocean acidification (OA) and benzo[a]pyrene (BaP) on coral holobionts remain poorly understood. Here, we investigated the multi-level responses of the reef-building coral Porites lutea to short-term (7-day) exposure to OA (pH 7.80), BaP (10 µg/L), and their combination, by integrating physiological measurements with microbiome profiling (ITS2 and 16S rRNA). We found that combined stress was associated with a dysregulated response in Symbiodiniaceae, characterized by a significant increase in cell density without a parallel rise in chlorophyll content, suggesting a possible compensatory but inefficient proliferation response. Despite this, the dominant symbiont Cladocopium C15 remained stable. The bacterial diversity increased (e.g., enrichment of Ruegeria and Acanthopleuribacter, decline of Endozoicomonas), which may suggest enhanced functional redundancy, while the archaeal community was significantly restructured, most notably a marked decline of the putative obligate Nanoarchaeota–Halobacterota symbiosis. At the host level, combined stress was associated with suppressed antioxidant enzyme activities (SOD/POD) but upregulated genes related to protein folding (Hsp90) and calcium homeostasis (NCX1, VAMP4). These findings suggest a complex holobiont reconfiguration under combined stress, involving a stabilized core symbiont, altered microbiomes, and a shifted host defense strategy. Our study suggests that the ecological risk of combined OA and organic pollution may not be extrapolated from single-stressor responses, indicating the need to incorporate multi-stressor frameworks into coral reef risk assessments.

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Sex-specific physiological-biochemical and multi-omics responses of Sargassum thunbergii to ocean acidification

Published 10 March 2026 Science Closed
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Highlights

  • A multi-omics study on sexual dimorphism of macroalgae under OA.
  • Male S. thunbergii adopted a growth-oriented strategy under OA.
  • Female S. thunbergii showed a defense-oriented survival strategy under OA.
  • Fundamental trade-off between growth and defense underlay sex-specific responses.

Abstract

Ocean acidification (OA), driven by increasing atmospheric CO2 concentrations, poses significant threats to the ecologically important intertidal macroalgae. Multiple previous studies have indicated species-specific responses to OA, the sex-specific physiological-biochemical responses and underlying molecular mechanisms in dioecious macroalgae remain poorly understood. In this study, we investigated the responses of male and female Sargassum thunbergii to acidification treatment (2000 ppm CO2) by integrating physiological-biochemical, transcriptomic, and metabolomic analyses. Both sexes maintained photosynthetic performance, with increased maximum relative electron transport rates (rETRmax). Males exhibited a growth-oriented strategy, characterized by higher accumulation of storage compounds like triglycerides and up-regulation of genes related to the photosynthesis and biosynthesis pathways. In contrast, females displayed a survival-oriented strategy, with reduced carbon storage, increased soluble protein and phenolic substance contents, and up-regulation of genes related to defense- and stress-response pathways. These findings provided physiological-biochemical and molecular evidence for a growth and defense trade-off between male and female S. thunbergii under acidification treatment. Our study provided the mechanistic insights into the sex-specific responses of marine macroalgae to global climate change and highlighted the importance of accounting for sexual dimorphism in predicting the ecological resilience of intertidal macroalgae populations under future ocean conditions.

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Colony formation sustains the global competitiveness of nitrogen-fixing Trichodesmium under ocean acidification

Published 5 March 2026 Science Closed
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Anthropogenic carbon dioxide emissions drive ocean acidification. Trichodesmium, a key marine nitrogen-fixing cyanobacterium, displays contrasting growth responses to ocean acidification across morphotypes: negative in filamentous free trichomes but neutral or positive in colonies. However, lacking mechanistic understanding for these discrepancies has impaired our ability to predict Trichodesmium’s ecophysiological response. Here, we develop ecophysiological models to underpin mechanisms behind these divergent responses. For free trichomes, ocean acidification reduces nitrogen-fixing enzyme activity and photosynthetic energy production. In colonies, however, it alleviates copper and ammonia toxicity within the microenvironment—likely synergizing with enhanced iron acquisition—thereby outweighing minor benefit from relieved inorganic carbon limitation in the colony center. Projections suggest that globally, ocean acidification will reduce nitrogen fixation of trichomes by 16 ± 6% but increase that of colonies by 19 ± 24% within this century. By resolving morphotype-specific mechanisms, our study clarifies Trichodesmium’s adaptive strategies for sustaining its competitiveness and biogeochemical impacts in the changing ocean.

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Resilience of the macroalgae Gongolaria barbata under ocean acidification: physiological responses and restoration perspective

Published 3 February 2026 Science Closed
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The increasing CO2 concentration is a major cause of the climate change phenomenon. Concurrently, the same increase is leading to ocean acidification (OA), which is projected to decrease seawater pH by 0.4 units by 2100. Here we investigated the potential impacts of OA on the canopy-forming brown macroalga Gongolaria barbata from the Venice Lagoon. One-year-old individuals were maintained in mesocosms under two pH levels: 8.1 (current ambient value) and 7.7 (the end-of-the-century value predicted under the current scenario of anthropogenic CO2 emissions). The physiological responses of the algae were assessed during the experiment in terms of oxygen production and consumption, and maximal PSII photochemical efficiency. At the end of the experiment, we analyzed the percentage of mature receptacles, algal growth rate and the total polyphenolic content and antioxidant capacity as indicators of the stress response. The significant decrease in polyphenolic content indicates the impairment of the defence mechanisms, which could make the algae more vulnerable to grazing under acidified conditions. Yet, conversely, our results suggest that changes in pH levels do not significantly affect the physiological processes, growth or fertility of the algae. These findings suggest that while OA may weaken defence mechanisms, the preservation of physiological and reproductive functions would still support the potential of G. barbata populations from the Venice Lagoon to act as donor sources for restoration efforts, highlighting their resistance to the acidified conditions expected in the future.

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Understanding coral health from reactor engineering perspective: multiphysics modeling of coral–environment interactions

Published 30 January 2026 Science Closed
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Coral, as a bioreactor, has to continuously interact with surrounding environment to maintain a healthy state. A multi-physics reaction engineering model has been developed to capture this interaction. The coral interior is modeled as interconnected reaction units respectively for photosynthesis, respiration, and calcification, whose reaction kinetics are influenced by environmental fluctuations. Coupling between coral and environment is realized by bi-directional mass transfer at the coral-seawater interface, with consideration of the unique flow fields induced by ciliary beating. By resorting to this comprehensive model, we discover that ciliary beating demonstrates distinctively different diurnal and nocturnal functions. During daytime, beating can help reduce photosynthetic oxygen accumulation to prevent hyperoxia-induced mortality, while enhancing carbon dioxide uptake efficiency to promote nutrient production. At night, however, beating promotes oxygen acquisition for adequate respiration, while expelling carbon dioxide to inhibit symbiotic destruction under acidic stress. The model further enables mechanistic analysis of the detrimental impact of climate change on coral health, where the influences from two key factors (i.e., temperature and CO2 level) can be decoupled. It’s interesting to find out that the elevated temperature plays a dominant role during daytime, while at night the coral is dominantly influenced by rising CO2 level.

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Assessing sponge resilience to ocean acidification in natural reef environments

Published 28 January 2026 Science Closed
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Highlights

  • Sponges are key components of coral reefs globally providing a range of important functional roles.
  • We used in situ incubation chambers to measure chlorophyll concentrations, oxygen fluxes and microbial communities for two common Indo-Pacific sponge species (Melophlus sarasinorum and Neopetrosia chaliniformis) at a natural CO2 vent (pHT 7.6–7.7) and control site in Papua New Guinea.
  • We found little evidence for any physiological differences between vent and control sponges, and no differences in the overall microbial communities
  • Overall, our results support the emerging evidence that heterotrophic sponges will likely be resilient to future ocean acidification.

Abstract

Sponges are key components of coral reefs globally providing a range of important functional roles. While sponges are under threat from the impacts of global climate change, there is an emerging picture of sponge tolerance to ocean acidification (OA). However, to date all physiological studies on sponge tolerance to OA have been under ex-situ experimental conditions and only for a limited number of sponge species. Instead, here we used in situ incubation chambers to measure chlorophyll concentrations and oxygen fluxes for two common Indo-Pacific sponge species (Melophlus sarasinorum and Neopetrosia chaliniformis) at a natural CO2 vent (pHT 7.6–7.7) and control site in Papua New Guinea. We also explored differences between the sponge microbial community composition between control and vent locations for N. chaliniformis. We found very low concentrations of chlorophyll in both species, compared to other sponges, suggesting these species are largely heterotrophic. We also found little evidence for any physiological differences between vent and control sponges, and no differences in the overall microbial communities, except some specific microbes. Overall, our results support the emerging evidence that heterotrophic sponges will likely be resilient to future ocean acidification.

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Prolonged low pH reprograms carbon and nitrogen metabolism and micronutrient use in Symbiodinium kawagutii and reveals indicators for reef water quality management

Published 23 January 2026 Science Closed
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Highlights

  • Low- pH stress suppresses S. kawagutii growth by ∼50%
  • Enhanced NPQ and reduced chlorophyll indicate increased photoprotection
  • Lipid pools increase as proteins and carbohydrates are diverted to fatty acids
  • Elevated C:N ratios and Fe/Mn loss reveal nutrient limitation under acid stress
  • Multi-omics uncover upregulated CA, antioxidant enzymes, and proton pumps

Abstract

Ocean acidification is a pervasive driver of coastal and reef water-quality change. We investigated how chronic low-pH exposure representative of extreme reef scenarios (pH 7.4-7.5) reshapes the physiology and metabolism of the coral symbiont Symbiodinium kawagutii. Integrating growth assays, photophysiology, ultrastructural imaging, biochemical profiling, transcriptomics, and metabolomics, we show that low pH suppresses growth and redirects resources from biosynthesis to stress mitigation. Non-photochemical quenching increased while chlorophyll content declined, indicating photoprotective energy reallocation. Ultrastructural deterioration coincided with losses of protein and carbohydrate pools, whereas fatty-acid stores expanded, evidencing a shift in carbon storage. Elemental and trace-metal measurements revealed higher cellular C:N and significant Fe/Mn depletion, indicating micronutrient constraints under acid stress. Multi-omics analyses identified coordinated upregulation of carbonic anhydrases, vacuolar H+-ATPases, and antioxidant defenses with downregulation of nitrogen and phosphorus assimilation, forming a plastic network that maintains pH and redox homeostasis at the expense of growth. These cellular trade-offs clarify how symbiont plasticity can buffer acidified conditions while altering the quality and quantity of photosynthate available to hosts. By linking mechanistic responses to potential monitoring indicators, this study provides actionable targets to anticipate and manage acidification impacts on reef water quality and to guide restoration strategies that prioritize acid-tolerant symbiont strains and relief of micronutrient stress.

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Ocean acidification and anthropogenic carbon in the Eastern Mediterranean Sea and the effects of acidification on marine organisms

Published 15 January 2026 Science Closed
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Ocean acidification (OA), driven by rising atmospheric carbon dioxide (CO2) levels, is a critical issue affecting our oceans. The Eastern Mediterranean Sea (EMS) remains poorly understood in terms of the carbonate system and the impact of OA, despite its key role in Levantine Intermediate Water (LIW) formation and its peculiar characteristics in buffering capacity and ongoing OA. This study provides the first comprehensive spatial and temporal assessment of carbonate system in the North-Eastern Levantine Basin, in EMS, providing essential reference data for Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), and Anthropogenic Carbon (CANT). The mean TA of the measurements was 2622.11 μmol/kg, with higher surface values in summer, reflected also in the surface salinity (S) maximum caused by strong evaporation. A clear vertical gradient was observed, with TA decreasing with depth. Hot and dry meteorological conditions contribute to increased S and TA, resulting in seasonal and vertical variations in the water column. The mean DIC of the measurements was 2291.23 μmol/kg. In contrast to the observations for TA, surface DIC values were higher in winter than in summer. The higher DIC values in winter are attributed to thermodynamic equilibrium and vertical mixing in the surface waters. This study has also investigates the presence of CANT, has infiltrated deep layers, with a mean concentration of 52.07 μmol/kg, decreasing significantly throughout the water column. These findings confirms the ongoing influence of human activities on intermediate and deep layers in EMS. To reconstruct past carbonate system dynamics, the relationships of TA and DIC were determined with salinity (S) and temperature (T) data. Long-term data from METU-IMS Erdemli Time Series (ETS) stations, collected monthly for a decade, provided valuable findings into seasonal patterns and temporal shifts in TA, DIC, and pH. The coastal station displayed clear trends in the carbonate system over time, reflecting its sensitivity to local environmental changes. In contrast, the offshore station exhibited minimal variability, indicating greater stability against seasonal and long-term fluctuations. These results highlight the heightened vulnerability of coastal waters to carbonate system changes, while offshore waters remain more stable. Understanding carbonate chemistry and acidification levels is crucial for assessing impacts on marine life. In addition to the characterization of carbonate chemistry, this study also explores OA’s biological impacts on two key organisms of the Mediterranean ecosystem: phytoplankton and mussels. Firstly, effects of elevated CO₂ on phytoplankton, an essential primary producer in aquatic food webs and global biogeochemical cycles are explored. Specifically, the study explores the impacts on phytoplankton physiology, focusing on growth rates, respiration, and photopigment content in selected species from the coccolithophores, dinoflagellates, and diatoms groups. While growth rates and respiration remained relatively stable under reduced pH conditions, photopigment content was significantly influenced by changes in seawater pH, highlighting the importance of considering environmental influences on photopigment composition. The study further investigated the effects of acidification on calcifying organisms through a global program aimed at understanding the long-term effects of acidification on key seafood species and exploring adaptation strategies with a collaborative approach. This study focused on the long-term (6 months long experiment) physiological impacts of OA on marine calcifiers, specifically Mediterranean mussel, Mytilus galloprovincialis, an abundant species and one of the most consumed non-fish marine species in Türkiye. Results indicate that OA poses a substantial threat to mussel health and survival. Reduced pH levels negatively impacted survival rates, while other physiological parameters like clearance rate, condition index, respiration, and the distribution of a radionuclide, 210Po, did not significantly change. However, lipid content and immune response were affected. Oxygen consumption decreased over time, especially at lower pH. This study underscores the potential risks of OA to the fitness of the commercially important mussel species, indicating that future OA may impact both this key seafood species and its associated ecosystems. The established baseline data are crucial for future monitoring and provide valuable insights into the vulnerability of marine organisms and ecosystems to ongoing OA. By integrating chemical, biological, and ecological perspectives, this dissertation offers a comprehensive assessment of OA in EMS. It establishes baseline data for carbonate system variables, revealing distinct spatial and temporal variations influenced by S, T, and mixing processes. By linking changes in carbonate chemistry to physiological responses in primary producers and a commercially vital shellfish species, this study highlights the ecological and economic impacts of OA in EMS. The findings emphasize the need for continued research and mitigation efforts to protect marine ecosystems and commercially important species. This integrated approach provides valuable insights into the vulnerability of marine organisms and ecosystems to ongoing OA, underscoring the significance of this research for the Mediterranean Sea.

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Effects of pH on growth and anatomical characters of tapeseagrass (Enhalus acoroides (Linnaeus f.) Royle)

Published 14 January 2026 Science Closed
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Enhalus acoroides (Linnaeus f.) Royle or tape seagrass plays a vital role in tropical seagrass meadows, especially in Thailand. While ocean acidification negatively affects many marine species, it may benefit tropical seagrasses. E. acoroides relies on seed dispersal for sexual reproduction, and pH variations may influence seedling development. This study examined the effects of pH levels (6, 7, 8, and 9) on E. acoroides seedling growth over 8 weeks in controlled aquariums. All treatments showed 100% seed germination during the first week. By week 2, no significant differences in biomass were observed, but by week 8, seedlings at pH 6 had the highest dry weight (0.21±0.01 g), as well as the greatest leaf number (5.64±0.15 leaves), leaf length (23.39±2.06 mm), and leaf width (4.74±0.14 mm). One-way ANOVA revealed significant differences in growth by week 8. Lower pH levels enhanced shoot and root growth, while higher pH increased root number but reduced root length. Chlorophyll content analysis showed no correlation with pH after 8 weeks. Anatomical examination revealed tannin cells, starch granules, and thick cell walls in the mesophyll, with an enlarged exodermis in lower pH treatments, suggesting an adaptation for stabilizing in muddy, acidic conditions. These findings indicate that pH influences the growth and adaptation of E. acoroides seedlings, highlighting the species resilience to acidification. Its adaptive capacity is crucial for management, as E. acoroides can survive acidification and continue providing habitat, preserving ecosystem balance.

Continue reading ‘Effects of pH on growth and anatomical characters of tapeseagrass (Enhalus acoroides (Linnaeus f.) Royle)’

Physiological and transcriptomic responses of a harmful algal bloom-causing dinoflagellate Karenia mikimotoi to multiple environmental factors

Published 12 January 2026 Science Closed
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Highlights

  • Elevated temperature was the primary factor significantly reducing K. mikimotoi growth and photosynthesis.
  • Increased pCO₂ and high N: P ratios partially mitigated thermal stress induced by elevated temperature.
  • K. mikimotoi consistently up-regulated energy and lipid metabolism to cope with environmental stressors irrespective of treatment.
  • K. mikimotoi may persist and even thrive under multiple stressors, subsequently influencing productivity and biogeochemical cycles.

Abstract

Dinoflagellates play a crucial role in marine food webs and biogeochemical cycles, yet they are increasingly affected by global environmental changes. While there is limited understanding of their response to individual stressors projected under future oceanic conditions, their response to multiple concurrent environmental stressors remains inadequately explored. This study investigated the singular and interactive effects of elevated temperature (26 °C vs. 22 °C), increased pCO2 (1000 μatm vs. 400 μatm), and a high nitrogen-to-phosphorus ratio (N:P = 180:1 vs. 40:1) on the harmful algal bloom-forming dinoflagellate Karenia mikimotoi over a 40-day exposure period. Among these factors, elevated temperature exerted the most pronounced influence, markedly reducing the cell growth rate and photosynthesis while simultaneously increasing the particulate organic matter content and antioxidant level. Transcriptomic analyses indicated that elevated temperature enhanced the expression of genes associated with oxidative stress, suggesting a potential defense mechanism against thermal stress. Notably, increased pCO2 and a high N:P ratio appeared to mitigate thermal stress to some extent. Irrespective of the treatment, K. mikimotoi demonstrated a consistent response strategy characterized by the synergistic upregulation of energy metabolism and lipid biosynthesis pathways, coordinated by the modulation of both upstream and downstream genes in the tricarboxylic acid cycle. This metabolic reprogramming likely facilitates a more efficient allocation of energy, thereby enhancing the resilience of K. mikimotoi to environmental stress. This study underscores the interactive effects of multiple stressors on marine dinoflagellates, highlighting that elevated temperature is the most critical factor affecting dinoflagellates in future oceanic environments.

Continue reading ‘Physiological and transcriptomic responses of a harmful algal bloom-causing dinoflagellate Karenia mikimotoi to multiple environmental factors’

Compound hypoxia with heat or acidification stress induces synergistic and additive effects on coral physiology

Published 26 December 2025 Science Closed
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As climate change accelerates, coastal marine ecosystems are increasingly exposed to co-occurring stressors whose combined effects are nonlinear and difficult to predict. Deoxygenation is a rapidly intensifying yet underrecognized threat to coral reefs that interacts with heat and acidification to alter coral physiology and stress resilience. However, the effects of hypoxia-related compound events on corals are largely unknown, underscoring the need for multi-stressor studies. Here, we conducted two extended-exposure experiments (12–17 days) across the coral species Porites furcata, Porites astreoides and Siderastrea siderea, to disentangle the individual and combined effects of low dissolved oxygen (hypoxia) with either heat or acidification. We measured eight phenotypic traits related to growth, metabolism, and symbiosis health to test whether hypoxia imposes energetic constraints or other physiological stress that amplify the effects of heat or acidification. Standardized effect size analysis across 24 stressor–trait combinations revealed 13 additive, 10 synergistic, and only one antagonistic response. Hypoxia consistently suppressed dark respiration by 37–49% across species and altered photophysiology in the two Porites species, whereas acidification alone had minimal effects, particularly in S. siderea. Heat stress caused the most pronounced declines across nearly all traits, and when combined with hypoxia, it produced the highest number of synergistic interactions. In contrast, the combination of hypoxia and acidification largely resulted in additive responses, suggesting that independent physiological mechanisms underlie these effects. All corals showed strong metabolic depression under hypoxia which is likely beneficial as a short-term adaptive response but may impose energetic constraints in the long-term. These findings highlight deoxygenation as critical yet often overlooked drivers of coral reef vulnerability. More multi-stressor experiments across a range of species are urgently needed to improve predictions of reef resilience under future ocean conditions, where compound stress events are expected to become more frequent and severe.

Continue reading ‘Compound hypoxia with heat or acidification stress induces synergistic and additive effects on coral physiology’

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