Posts Tagged 'algae'

Calcium carbonate formation in coastal macroalgal ecosystems via multiple pathways

Published 24 August 2026 Science Leave a Comment
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Calcium carbonate formation in coastal macroalgal ecosystems via multiple pathways

Highlights

  • Focused on a novel inorganic carbon sequestration pathway of non-calcifying macroalgae.
  • Clarified algae-bacteria synergistic driving mechanism for calcium carbonate precipitation.
  • Systematically sorted out potential mineralizing microbial taxa within the phycosphere and their core metabolic pathways.

Abstract

Macroalgae are dominant primary producers that drive carbon sequestration in coastal ecosystems. Macroalgal carbon sequestration primarily refers to the long-term storage of macroalgae-derived organic carbon in the ocean. However, calcium carbonate (CaCO3) formation is frequently observed in non-calcifying macroalgal environments, suggesting the existence of an overlooked inorganic carbon process in macroalgal ecosystems. Here, we introduce multiple pathways that may drive CaCO3 formation in macroalgal ecosystems. These include the effects of macroalgal photosynthesis and carbon-concentrating mechanisms on the seawater carbonate system, the role of phycosphere interfacial properties in facilitating CaCO3 nucleation, and the macroalgae-bacteria synergy that promotes CaCO3 formation. We identified several current knowledge gaps—the unclear carbon sequestration or source effect of CaCO3 formation in macroalgal ecosystems and the stability of CaCO3 minerals in macroalgal ecosystems—that require further investigation. This review advances the understanding of macroalgal carbon cycling beyond organic pathways and emphasizes the importance of a comprehensive assessment of macroalgal carbon sequestration, including that of inorganic carbon.

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Giant kelp-associated variation in coastal seawater chemistry across contrasting sites in Chile and Tasmania

Published 13 August 2026 Science Leave a Comment
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Background and Aims

Widespread shifts in seawater chemistry are occurring across spatial and temporal scales, with important consequences for coastal ecosystems. Giant kelp (Macrocystis pyrifera) forests elevate seawater pH and dissolved oxygen (DO) through photosynthesis, potentially providing short-term refugia from ocean acidification and deoxygenation. However, whether these effects persist across contrasting environmental settings remains unclear. Here, we assess how biological and oceanographic conditions regulate giant kelp-mediated modification of seawater chemistry across multiple sites.

Methodology

Hourly measurements of seawater pH, DO and temperature were collected during spring–summer 2022–23 using paired deployments inside and outside giant kelp forests at one site in central Chile and four sites in Tasmania, Australia. The influence of giant kelp density was also evaluated at two sites in southern Chile and three sites in Tasmania. An upwelling index was calculated for the central Chile site to assess the influence of regional oceanographic forcing on kelp-associated seawater chemistry patterns.

Key Results

Hourly pH and DO were higher inside giant kelp forests than outside at the central Chile site and at one Tasmanian site. At these locations, stronger daytime pH–DO relationships indicated that photosynthetic carbon uptake exceeded night-time respiration, generating a net positive metabolic signal. In Tasmania, giant kelp density was positively associated with hourly pH and DO, whereas no such relationship was detected in southern Chile. At the central Chile site, kelp-associated effects intensified during a strong upwelling event, reducing the severity of low pH and DO conditions.

Conclusions

Giant kelp forests can locally buffer short-term fluctuations in seawater pH and DO, but this capacity is highly site-dependent and influenced by giant kelp density and environmental conditions. Overall, our findings suggest that continued loss of giant kelp forests in Tasmania may reduce their potential to provide short-term refugia, while in Chile the strength of kelp-mediated seawater chemistry modification is likely to remain strongly influenced by variability in upwelling and freshwater inputs.

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Invasive macroalgae exert stronger effects than elevated CO₂ on seagrass (Posidonia oceanica) seedling performance and associated microbiomes

Published 11 August 2026 Science Leave a Comment
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Highlights

  • Invasive macroalgae reduced P. oceanica seedling biomass and leaf development.
  • Macroalgal invasion depleted carbohydrate reserves in seedling roots and rhizomes.
  • Elevated CO2 increased rhizome starch but did not mitigate invasion impacts.
  • Root microbiome diversity declined markedly under macroalgal invasion.
  • Future CO2 enrichment unlikely to buffer invasion stress at recruitment stage.

Abstract

Seagrass seedlings are key to meadow recovery under global change, as they enable recolonization of degraded areas and provide genetic variability needed for adaptation. While invasive macroalgae increasingly threaten seagrass communities, elevated CO2 has been proposed to enhance seagrass performance and potentially buffer other stressors. Here, we conducted a mesocosm experiment to test the combined effects of two invasive macroalgae (Lophocladia trichoclados and Caulerpa cylindracea) and elevated CO2 on Posidonia oceanica seedlings. CO2 enrichment increased carbohydrate reserves in rhizomes and induced subtle shifts in root-associated microbiomes. In contrast, invasive macroalgae had consistently negative effects on seedling development and physiology and strongly altered both above- and belowground microbial communities. Despite its potential to stimulate seagrass productivity, elevated CO2 did not mitigate the detrimental impacts of invasive macroalgae. These findings indicate that future CO2 conditions may not offset invasion-driven stress at the recruitment stage, highlighting the need for targeted management efforts to limit macroalgal proliferation and support seagrass meadow regeneration.

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Crustose coralline algae buffer shallow reef environments from dissolution

Published 11 August 2026 Science Leave a Comment
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Ocean acidification threatens coral reefs by reducing seawater pH and carbonate saturation state. Crustose coralline algae are particularly vulnerable because their high-magnesium calcite skeletons dissolve more readily than coral aragonite skeletons. However, this dissolution may increase alkalinity and buffer reef-water chemistry. Here we show, using repeated low-tide observations and in situ incubations in a shallow reef system in the southern Great Barrier Reef, that reef-water pH varies by more than one unit over the day (7.47 – 8.61), reaching levels comparable to those projected for the end of this century. Nighttime respiration promotes dissolution of high-magnesium calcite produced by crustose coralline algae, increasing alkalinity and helping maintain seawater supersaturated with respect to aragonite. At the same time, isolated coral incubations experience a greater decline in pH and aragonite saturation state in the absence of this buffering effect. These findings suggest that high-magnesium calcite-producing communities may help partially buffer reefs against future acidification.

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The responding mechanisms of Nannochloropsis oceanica to high CO2 and the regulating functions of H+-PPase

Published 31 July 2026 Science Closed
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Highlights

  • Cytoplasmic pH homeostasis was critical for N. oceanica to tolerate 5% CO2.
  • The tolerant mechanisms involved multiple physiological-biochemical processes.
  • H+-PPase was the central regulator for alleviating cytoplasmic acidification.
  • avp1 overexpression improved cytoplasmic pH regulation by raising H+-PPase activity.

Abstract

High CO2 tolerance microalgae screen/breeding shows the urgent research priority when applying microalgae for flue gas CO2 sequestration. In this study, we chose the important resource microalgae Nannochloropsis oceanica as the target organism, the regulatory mechanisms of N. oceanica were elucidated under 5% and 20% high CO2 conditions, and the function of key regulating gene avp1 encoding H+-PPase was further explored. The results showed N. oceanica was tolerant to the 5% CO2 that maintained intracellular pH homeostasis, while severe cytoplasmic acidification was occurred under the 20% CO2 condition. Integrated physiological, biochemical, and transcriptomic analysis revealed that P-ATPase and H+-PPase activities were enhanced at 4 h and 4 d under the 5% CO2 condition, respectively. Concurrently, the reprogramming of organic acid metabolism and maintenance cellular energy supply additionally mitigated intracellular acidification. Further functional validation showed that overexpression of avp1 enhanced H+-PPase activity, increased cytoplasmic pH values, promoted pigments accumulation and growth of N. oceanica under the high CO2 condition. Therefore, this study clarified the working mode of N. oceanica to tolerant high CO2 and cytoplasmic acidification, and firstly timely proved the function of avp1, provided important data basis and gene candidates for the research of applying microalgae to CO2 sequestration.

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Exploring the multiple functions of polyphenols in Mediterranean macroalgae: insights, gaps, and future directions from a systematic review

Published 30 July 2026 Science Closed
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Macroalgae are a cornucopia of bioactive molecules whose synthesis is modulated in response to environmental variables, allowing macroalgal adaptation and survival. Among these metabolites, polyphenols are worthy of attention because they may represent an informative archive of environmental conditions, given their involvement in several ecological and biochemical functions. This systematic review synthesized the literature from the last two decades on phenolic compounds in Mediterranean macroalgae, focusing on their functional roles, taxonomic distribution, geographical occurrence, and responses to abiotic and biotic stressors, with particular attention to their potential as ‘early indicators’ of environmental change. Using a Scopus-based search strategy, nearly 6000 records were screened and studies on brown, red, and green macroalgae published between 2003 and September 2025 across the Mediterranean basin were retained.

The reviewed literature highlighted that polyphenols are widely involved in antioxidant defense, photoprotection, stress tolerance, and interspecific interactions, supporting the resilience of macroalgae in a rapidly changing Mediterranean Sea. At the same time, the review emphasizes the significant biotechnological potential of these compounds in the pharmaceutical, cosmetic, food, and agricultural sectors. Overall, the evidence indicates that fluctuations in polyphenol content often reflect rapid physiological responses to environmental stress, suggesting that these compounds may function as early indicators of ecological change in Mediterranean macroalgae.

Despite growing evidence, several important gaps remain. Research from the eastern Mediterranean basin is still limited, most studies focus on only a few model genera, and long-term field studies examining multiple environmental stressors are rare. Future research should therefore include a wider range of geographic areas and species, adopt standardized analytical methods, and investigate how polyphenols respond to combined environmental drivers. This would help clarify and validate their use as early indicators of ecological change.

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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 Closed
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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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Fish trait-based indicators of mercury bioaccumulation under natural ocean acidification: insights from a mediterranean CO₂ vent system

Published 22 July 2026 Science Closed
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Highlights

  • Low pH conditions at CO₂ vents increase THg bioavailability and fish exposure.
  • Functional traits explain THg patterns better than species identity.
  • Under low pH, less-mobile benthic fish accumulate more THg than highly-mobile fish.
  • Trophic position drives stronger THg biomagnification at low pH sites.

Abstract

Ocean acidification is not an isolated climate-related threat to marine organisms, as it may act in combination with other stressors such as trace element contamination. Here, we took advantage of naturally acidified conditions and mercury spill-out from the shallow CO2 vent of Vulcano Island (Italy) to test whether fish functional traits explain contaminant dynamics better than species identity under ocean acidification scenarios. Specifically, we investigated if trophic group and mobility influence total mercury (THg) bioaccumulation and trophic transfer in fish inhabiting Cymodocea nodosa seagrass meadows under low and ambient pH conditions. Low-mobility benthic fish, mainly represented by invertebrate feeders and small piscivores, exhibited higher THg concentration at the vent site than at reference sites, while highly mobile zooplanktivorous fish showed no significant differences, highlighting the importance of fish mobility and local trophic benthic pathways in shaping contaminant exposure and bioaccumulation. Moreover, the relationship between trophic position and THg concentration in fish was stronger in the low pH site, indicating enhanced trophic transfer and biomagnification when Hg availability is higher. In contrast, weak or absent biomagnification in the reference sites suggests that Hg transfer depends more on contaminant availability and food web structure than trophic position alone. Overall, these findings indicate that functional traits better predict Hg bioaccumulation patterns than species identity, supporting trait-based approach to assess contaminant dynamics in future ocean acidification scenarios.

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Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ13CDIC and 224Ra–228Th disequilibria

Published 21 July 2026 Science Closed
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Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO2 accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO3) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO3 dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the 224Ra–228Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ13CDIC) were employed to trace DIC sources in porewater. The results showed that CaCO3 dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO3 dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO2 accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO2 content, the TA supplied by the CaCO3 dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO3 dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO2 elevation.

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Ocean acidification exacerbates UVR-induced inhibition of photosystem II and I in Corallina officinalis

Published 16 July 2026 Science Closed
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Highlights

  • OA reduces calcification, compromising the UV shield and exacerbating photoinhibition in C. officinalis.
  • OA amplifies PSII donor-side damage, disrupts electron flow to PSI, and suppresses PSI function.
  • OA shifts PSII damage-repair balance toward irreversible photoinhibition by increasing damage and suppressing repair.

Abstract

Ocean acidification (OA) exerts diverse effects on marine macroalgae, with calcified species being particularly vulnerable. Due to calcified skeletons can contribute to physical screening against solar ultraviolet radiation (UVR), OA-driven calcification loss may increase exposure of the photosynthetic apparatus to UVR. Here, we cultured Corallina officinalis under ambient CO₂ (∼420 μatm) or elevated CO₂ (∼1000 μatm), with or without UVR, under natural solar radiation. Our results confirmed that OA reduced calcification and, under UVR, enhanced donor-side impairment of photosystem II (PSII), as evidenced by an increase in the relative K-step (Wk, an indicator of OEC damage) and a decrease in the maximum quantum yield of PSII (Fᵥ/Fₘ). This donor-side injury was accompanied by a reconfiguration of energy fluxes per PSII reaction center, particularly under combined OA and UVR. These impairments further extended to intersystem electron transport and limited the linear electron flow from PSII to the intersystem chain. Photosystem I (PSI) related electron transport was also functionally constrained, as evidenced by the reduced electron transfer probability and terminal reduction yield. This, together with the reduction of cyclic electron transport around PSI, resulted in over-reduction of the intersystem chain and making PSI the limiting photosystem. Together, these results indicate that OA amplified UVR-induced net photodamage and weakened PSII repair capacity in C. officinalis, while also constraining PSI-related electron transport. These findings highlight the potential vulnerability of calcified red algae under future high-CO₂, high-UVR coastal oceans.

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Development of hypoxia and acidification during harmful algal blooms: dynamic multi-stressor conditions in NY, USA, estuaries

Published 14 July 2026 Science Closed
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Highlights

  • Alexandrium blooms occurred with normoxic conditions and moderate pH (7.5-8.0).
  • Alexandrium blooms occurred with moderate pCO2 (400 – 1,000 µatm) and saturating conditions for aragonite.
  • Dinophysis blooms co-occurred nocturnal acidification (8.71 to 13.6 hr d−1) and hypoxia (0.52 to 3.88 hr d−1).
  • Dinophysis blooms co-occurred with high pCO2 (1,000 – 3,500 µatm) and undersaturated aragonite (Ωar < 1).
  • The co-occurrence of nocturnal hypoxia, acidification, Ωar undersaturation, and HABs coupled is a significant threat for marine life.

Abstract

While harmful algal blooms (HABs), hypoxia, and ocean acidification are common occurrences in coastal zones, research investigating the co-occurrence and interactions between these processes has been lacking. Here, we documented the initiation, peak, and demise of eight distinct HAB events caused by Alexandrium catenella and Dinophysis acuminata over a two-year period in two estuaries (Northport Harbor and Cold Spring Harbor, NY, USA). We concurrently characterized the dynamics of carbonate chemistry, including pCO2 and the saturation state of aragonite (Ωar), pH, dissolved oxygen (DO), and general environmental conditions in space and time. HABs occurred in succession and reached high densities with A. catenella blooms exceeding 104 cells L−1 being succeeded by D. acuminata blooms exceeding 106 cells L−1. A. catenella blooms occurred during spring months under generally normoxic conditions with moderate levels of pCO2 (400 – 1,000 µatm), only brief periods of acidification (pH < 7.5), mostly saturating conditions for aragonite, and an absence of hypoxia. In contrast, D. acuminata blooms, which occurred in summer, consistently co-occurred with bouts of extended nocturnal acidification (8.71 to 13.6 hr d−1) and hypoxia (0.52 to 3.88 hr d−1) coupled with higher levels of pCO2 (1,000 – 3,500 µatm), and undersaturating conditions for aragonite (Ωar < 1). During both blooms, nearshore locations hosted higher cell densities, lower pH and DO, and higher pCO2 compared to open water regions. The co-occurrence of multiple stressors including nocturnal hypoxia, acidification, Ωar undersaturation, and HABs, coupled with strong diel cycling of DO, pH, and pCO2, especially during D. acuminata blooms, represents a significant and previously unrecognized threat for marine life.

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Brown algae as winners: divergent resilience to high light under acidified conditions shapes macroalgal communities around a carbon dioxide vent

Published 14 July 2026 Science Closed
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To investigate the effects of ocean acidification (OA) on macroalgae, we conducted in situ surveys along a natural CO2 vent gradient in Shikine Island, Japan, together with complementary laboratory culture experiments. The in-situ surveys revealed that near the CO2 vent (where pH dropped by 0.37), macroalgal diversity and species-richness were less than half those at the reference sites under ambient pH conditions. Nevertheless, the rates of CO2 assimilation of several common macroalgae increased from the reference site to the areas near the CO2 vent. This enhancement coincided with decreased photosynthetic CO2 affinity, reflecting that the acidified area down-regulated CO2-concentrating mechanisms in the algae. Measured photosystem II activity revealed that macroalgae at reference sites had lower electron transport rate and light utilization efficiency. The laboratory culture experiments, in which the dominant species (Gelidium elegans and Dictyopteris undulata), were cross-exposed to ambient and elevated CO2 conditions, further demonstrated that the stress near the CO2 vent significantly exacerbated photoinhibition under high light stress. Our results demonstrate that reduced pH and high sunlight act synergistically to impair macroalgal photosynthesis through exacerbated photoinhibition. This effect was more pronounced in red algae (e.g., G. elegans) than in brown algae (e.g., D. undulata). These different physiological responses provide a mechanistic explanation for an observed community shift, from red algal dominance in ambient pCO2 areas to brown algal dominance near the vent. Our findings imply that future OA, when combined with high-light stress, may selectively disadvantage high-light-sensitive species, thereby altering macroalgal community structure in coastal waters.

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The fate of macroalgal carbon under microbial anaerobic respiration: a critical factor in macroalgae cultivation for climate change mitigation

Published 8 July 2026 Science Closed
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Highlights

  • Anoxic remineralization rates were not consistently lower than oxic rates.
  • Macroalgal degradation modulates the DIC pool, crucial for carbon sequestration.
  • Alkalinity generated by anaerobic respiration stabilizes the DIC pool.

Abstract

Macroalgae play a significant role in global carbon sequestration. Substantial macroalgal organic carbon inputs and subsequent degradation can cause deoxygenation; however, the impact of oxygen deficiency on carbon fate remains understudied, which is critical for assessing the climate mitigation role of macroalgae. Here, we investigated changes in the carbon pool and non-CO2 greenhouse gases (N2O and CH4) to assess the influence of oxygen levels on the carbon sink capacity of macroalgae. The microbial remineralization rate of macroalgal organic matter was not consistently slower under anoxic conditions (AK) compared to oxic conditions (OK). Total organic carbon (TOC) concentrations in the water column were 530 ± 94 (OK) and 282 ± 38 (AK) μmol kg−1. For dissolved inorganic carbon (DIC), concentrations on day 30 were 4585 ± 197 (OK) and 5200 ± 492 (AK) μmol kg−1, while those for total alkalinity (TA) were 2684 ± 18 (OK) and 4523 ± 671 (AK) μmol kg−1. Following a 30-day sealed incubation, the bags were opened to reach atmospheric equilibrium. Subsequently, DIC dropped to 1837 ± 79 (OK) and 3744 ± 354 (AK) μmol kg−1, and TA fell to 2059 ± 14 (OK) and 4431 ± 657 (AK) μmol kg−1. Ultimately, relative to the control group (seawater only, OS) under air-sea equilibrium, the ΔDIC values were −22 ± 76 and 1885 ± 351 μmol kg−1 in the OK and AK treatments, respectively, while ΔTA values were −57 ± 11 and 2315 ± 655 μmol kg−1. The emissions of N2O and CH4 did not substantially offset the climate effect of carbon sequestration. These results suggest that, beyond the traditional focus on organic carbon preservation, anaerobic respiration under anoxic conditions may also contribute to macroalgal carbon sequestration by generating alkalinity that enhances the retention and stabilization of DIC.

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Reassessing the climate mitigation benefits and environmental risks of coastal seaweed farming

Published 6 July 2026 Science Closed
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Seaweed farming is increasingly promoted as a nature-based solution for marine carbon dioxide removal (mCDR), offering the dual promise of climate mitigation and ecosystem enhancement. However, here we highlight a fundamental paradox: while macroalgae cultivation can significantly boost carbon sequestration and support biodiversity, it also introduces site-specific ecological risks—most notably eutrophication, hypoxia, and acidification—particularly in semi-enclosed coastal systems with limited water exchange. We synthesize current understanding of both the positive and negative impacts of large-scale macroalgae farming, examining pathways of carbon uptake, storage, and export alongside biogeochemical and food web disruptions. Critically, we identify the overlooked roles of hydrodynamic conditions and benthic-pelagic coupling in mediating ecological outcomes. To ensure that macroalgae aquaculture contributes effectively to climate goals while safeguarding coastal ecosystem resilience, we call for the development of a targeted and comprehensive evaluation framework capable of accurately assessing its impacts on adjacent waters. Such a framework should incorporate site-specific water-exchange characteristics and biogeochemical vulnerability, thereby enabling more informed and adaptive management strategies—including hydrodynamically guided site zoning—to support sustainable, long-term ecosystem benefits.

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Marine invertebrates and fishes exhibit inconsistent body size responses to ocean acidification

Published 18 June 2026 Science Closed
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Body size is a fundamental characteristic of all living organisms that determines physiological functions and life-history traits. Ecological theory predicts that ocean acidification can cause body size reductions, confirmed by several studies reporting miniaturization in ectotherms. Based on this prediction, we would expect a broad suite of species to show similar plastic body-size responses to elevated CO2. Using four natural climate change analogues of ocean acidification across the northern and southern hemispheres, we quantified body size alterations across 18 marine invertebrate and fish taxa to test for climate-driven miniaturization. Only three species consistently showed body-size reductions under ocean acidification: one urchin and two fish species. In contrast, 15 other species, ranging from highly calcified to non-calcified, displayed unchanged or increased body sizes or inconsistent miniaturization. If body-size miniaturization responses were consistently reproducible across taxa we would have observed it more frequently, suggesting that species responses to ocean acidification are more variable than previously thought and likely vary depending on a species’ physiology and life history. Thus, rather than entire communities undergoing miniaturization, species are likely to display a spectrum of responses, with some exhibiting size reductions, others demonstrating physiological resistance to elevated CO2, and others potentially benefiting from the indirect effects of ocean acidification.

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Response of HAB-forming microalgae competition to ocean acidification, warming, and changing light fields

Published 17 June 2026 Science Closed
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In recent years, the East China Sea (ECS) has experienced frequent harmful algal blooms (HABs), driven by the complex interplay of climate change—specifically ocean warming and acidification—and eutrophication-induced light attenuation. Despite their ecological significance, the interactive effects of these environmental stressors on the competitive dynamics between bloom-forming microalgae remain poorly understood. This study aimed to elucidate how warming, reduced light, and elevated CO2 influence the competition between two dominant diatoms. We conducted controlled monoculture and mixed-culture experiments using two key species: Skeletonema costatum and Chaetoceros curvisetus. The experimental design incorporated varying levels of CO2, temperature, and light intensity to simulate future coastal scenarios. Growth rates, peak cell densities, and successional patterns were monitored to assess competitive outcomes under multiple stressors. Monoculture results indicated that high temperature and low light intensity promoted the growth of both species. However, in mixed cultures, these conditions significantly accelerated the time to reach peak density and induced a definitive successional shift from S. costatum to C. curvisetus. Notably, while the general successional pattern was consistent, elevated CO2 further enhanced the competitive advantage of C. curvisetus, particularly when combined with high-temperature and low-light scenarios. These findings suggest that the synergy of future warming, declining light availability, and intensified ocean acidification in the ECS will likely favor C. curvisetus over S. costatum. This shift may increase the frequency of HAB events dominated by C. curvisetus, driving significant climate-related restructuring of phytoplankton communities in coastal ecosystems.

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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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Demonstration of an automated bioreactor for controlled acid dosing to enhance marine algae productivity

Published 26 May 2026 Science Closed
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Microalgae are an important feedstock in aquaculture with significant economic potential in generating a diversity of bioproducts. To facilitate expansion of microalgal cultivation, a continuous automated bioreactor that uses waste acid to increase carbon bioavailability in seawater for enhanced biomass production was designed and tested with Tetraselmis suecica UTEX2286. Carbon bioavailability was inferred from culture pH and bioreactor headspace CO2 concentration measurements and controlled via acidification and seawater dilution. Operating over a period of several days, the culture exhibited greater biomass productivity at a pH setpoint of 7-7.5. Outside of this range, algal activity slowed, accompanied by greater CO2 released to the headspace and lower pH during incubation. Increasing the carbon introduced to the bioreactor by increasing the dilution factor did not significantly increase the algal productivity. Importantly, acidification led to statistically significant gains in biomass productivity. Preliminary cost analysis showed while seawater is inexpensive, the acid cost drives the overall cost of the designed bioreactor system. Thus, the designed bioreactor and control scheme supports algal cultivation but requires low-cost acid to be economical, which may be achieved by strategically integrating microalgae cultivation with other coastal industries.

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Evaluating the role of seaweed farming in ocean acidification mitigation: insights from high-frequency observations

Published 5 May 2026 Science Closed
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The oceanic uptake of anthropogenic CO2 has resulted in ocean acidification (OA). Macroalgae farming has the potential to mitigate OA by removing CO2 from the surface water via photosynthesis. However, continuous in-situ observations of marine carbonate chemistry related to macroalgae farming remain limited, leaving its effectiveness in addressing OA uncertain. To address these knowledge gaps, this study examined a 2-acre Saccharina latissima, sugar kelp, farm located at Point Judith, Rhode Island, as a case study to assess the potential of sugar kelp aquaculture in mitigating local OA. Over the full growing season from December 2022 to May 2023, high-temporal-resolution (every 30–60 minutes) measurements of surface temperature, salinity, dissolved oxygen and pH were taken inside and outside the kelp farm. The results demonstrate that sugar kelp farming does not significantly impact the carbonate system, thus providing negligible OA mitigation locally. Specifically, a temporary, local-scale CO2 reduction and higher pH occurred during very early kelp growth in early February, but was reversed by a higher surface CO2, exaggerating OA, starting in mid-February. Over the entire observation period, kelp growth resulted in a 5.1 ± 11.6 μatm increase of pCO2 per week compared to the control site in the surface, a signal which is small compared to the substantial natural variability. However, the minimal pCO2 difference at the kelp farm may be reflective of the relatively small cultivation area (2 acres) or depressed growth of phytoplankton, resulting from nutrient competition between the kelp and in-situ phytoplankton. This study underscores the need for future sustained observations to evaluate the impact of seaweed cultivation on OA mitigation and the carbon cycle at the ecosystem scale.

Continue reading ‘Evaluating the role of seaweed farming in ocean acidification mitigation: insights from high-frequency observations’

Ocean acidification and harmful algal blooms combine to suppress the growth and survival of North Atlantic bivalve larvae

Published 29 April 2026 Science Closed
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While harmful algal blooms (HABs) and ocean acidification (OA) are environmental factors that can impair bivalves, the manner in which these two stressors may act and interact to impact bivalve larvae is poorly understood. This study exposed larvae of hard clams (Mercenaria mercenaria) and Eastern oysters (Crassostrea virginica) to a range of pCO2 levels found in estuaries (400–3,000 µatm) and three harmful algae, Alexandrium catenella, Dinophysis acuminata, and Margalefidinium polykrikoides, at densities found during HABs (500–7,000 cells mL-1), with one HAB species exposure per experiment. The combined OA and HAB treatment significantly reduced larval survival in all 21 experiments by 91 ± 4.6% (SE) compared to controls and reduced larval sizes in 92% of experiments by 40 ± 3.5%. Cultured M. polykrikoides had a stronger negative effect on larvae than cellular equivalent bloom populations. Densities of D. acuminata >750 cells mL-1 reduced larval survival and size (p < 0.01), but the addition of OA to D. acuminata did not suppress survival further. While the combined A. catenella and OA treatment reduced larval growth and survival at all densities (p < 0.01), A. catenella alone did not impact M. mercenaria survival or size at or below 1,000 cells mL-1 and did not impact C. virginica at any density. Oyster larvae were less impacted than hard clams by OA (33 vs. 67% of experiments) and by HABs (67 vs. 100% of experiments). Given the very low survival of bivalve larvae when exposed to combined HABs and OA in all experiments (<0.1–5%), bivalve restoration and conservation efforts should seek to avoid regions that experience these co-stressors.

Continue reading ‘Ocean acidification and harmful algal blooms combine to suppress the growth and survival of North Atlantic bivalve larvae’

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