Posts Tagged 'light'

Impact of acidification and ultraviolet radiation on the physiology of Ulva fasciata

Published 19 December 2025 Science Closed
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Ocean acidification and increased UVR exposure driven by factors such as global warming, ozone layer depletion and anthropogenic activities are impacting the physiology and ecology of macroalgae in species-specific, diverse and complex ways. This study aims to investigate the individual and combined effects of ocean acidification and ultraviolet radiation (UVR) on the physiological responses of the cosmopolitan macroalgae species Ulva fasciata. The algae samples were cultured under laboratory conditions at two different pH levels (8.2 and 7.7) and under either the presence or absence of UVR. In U. fasciata, the maximum quantum efficiency of photosystem II (Fv/Fm) decreased with low pH and UVR, and a synergistic stress response was observed when these two stressors were applied together. The relative electron transport rate (rETRmax) varied depending on pH, while UVR increased this rate. These findings indicated that U. fasciata samples were under physiological stress. The incubation period significantly affected rETRmax and showed that the organism developed time-dependent adaptation responses. Alpha, a photosynthetic efficiency indicator, was negatively affected by UVR, whereas the light saturation point (Ik) varied as a result of the interaction between incubation time, pH, and UVR. The findings suggest that UVR exerted a more pronounced inhibitory effect on the photosynthetic system and growth of U. fasciata than low pH. Furthermore, combined exposure to UVR and low pH resulted in stronger growth inhibition, and a significant interaction between the two stressors was observed. Low pH and UVR exposure caused increased carbonic anhydrase activity (CA), while high CO2 led to a decrease in nitrate reductase activity (NR). UV-absorbing compounds (UVACs) were significantly affected by low pH and culture duration, whereas the effect of UVR on these compounds became significant only through its interaction with the incubation period. This suggests that the effect of UVR emerges through temporal accumulation. The findings reveal that this species is capable of developing late-phase acclimation strategies in response to environmental stress factors and possesses a potential adaptive capacity to cope with future marine change scenarios.

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Using boron isotopes to examine calcification fluid pH changes in marine calcifiers under environmental change

Published 23 September 2025 Science Closed
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CO₂-driven ocean acidification (OA) decreases seawater pH and carbonate ion concentrations, which can impact the calcification and physiology of marine calcifiers. These organisms form calcium carbonate skeletons and shells from a specialized calcification fluid that is, to varying degrees, isolated from surrounding seawater. The carbonate structures serve as archives, preserving the chemical signature of the calcification fluid, which can be analyzed using geochemical proxies. In the following thesis, I examine how different taxa respond to future ocean changes by exposing them to predicted future acidification scenarios. Additionally, I aim to understand if an organism’s resilience to the impacts of ocean acidification is linked to their ability to regulate their calcification fluid chemistry using geochemical proxies.

In Chapter 1, I investigate the geochemistry of three reservoirs important for biomineralization – seawater, the extrapallial calcification fluid (EPF), and the shell – of two commercially important bivalve species: Crassostrea virginica and Arctica islandica to understand if the boron isotope proxy is probing calcification fluid pH. Additionally, I examined the effects of three ocean acidification conditions (ambient: 500 ppm, moderate: 900 ppm, and high: 2800 ppm CO2) on the calcification and chemistry of the calcification fluid of the same three reservoirs for C. virginica. Comparisons of seawater and extrapallial fluid geochemistry indicated that the EPF has a distinct composition that differs from seawater. Additionally, our OA experiments show that EPF chemistry is significantly affected by ocean acidification, demonstrating that the biological pathways regulating or storing these ions are impacted by ocean acidification. I also found that shell δ11B does not faithfully record seawater pH, but rather was correlated with EPF pH, despite an offset from in situ microelectrode pH measurements. However, the δ11B-calculated pH values were consistently higher than microelectrode pH measurements, indicating that the shell δ11B may reflect pH at a more localized site of calcification, rather than pH of the bulk EPF.

In Chapter 2, I investigate the effects of four different seawater pH levels (8.03, 7.93, 7.83, and 7.63) on seven complexes of temperate coralline algae collected from New Zealand. I examined the photophysiology, calcification, and geochemical proxies to probe the internal carbonate chemistry of seven different species of coralline algae under simulated end-of-century ocean acidification scenarios. Under ambient conditions we found clear physiological differences between branching and encrusting species. We found that OA treatments only had a significant effect on calcification of three of the seven species, Corallina berteroi, Corallina spp., and Jania “bottlebrush.” Additionally, OA only affected the calcification fluid pH (pHCF) of two species, decreasing pHCF for both Corallina beteroi and Jania “feather.” Nonetheless, for all species pHCF was constantly upregulated compared to seawater pH, indicating a strong control over calcifying fluid chemistry. My results underscore the high resilience of coralline algae calcification under the different end-of-century ocean acidification scenarios. This tolerance to OA is related to the species’ ability to maintain a stable carbonate chemistry to support calcification as seawater pH declined.

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Species-specific mechanisms of benthic foraminifera in response to shell dissolution

Published 31 July 2025 Science Closed
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Highlights

  • Living specimens and empty tests of two benthic foraminifera species were cultured in different pH and light conditions.
  • In acidic conditions, greater dissolution of empty tests compared to living specimens was observed.
  • No differences in the degrees of dissolution between the two species were observed.
  • Living foraminifera have active mechanism(s) to tolerate acidification.

Abstract

Ammonia confertitesta and Haynesina germanica are two common estuarine benthic foraminifera subject to sediment acidification. Nevertheless, mechanisms involved in their response to acidification are still poorly understood. Since H. germanica is kleptoplastic and photosynthetically active, unlike A. confertitesta, these species were cultured in controlled experiments to determine whether these mechanisms could mitigate acidification-induced shell dissolution. Both living and dead specimens were incubated at two pH (8.0 and 6.8) and two light conditions (0 and 24 μmol photon m-2.s-1) for 18 days. For each species, respiration and photosynthesis rates were calculated based on oxygen measurements. At the end of incubation, foraminiferal viability was assessed with CellTracker Green™ biomarker, and each test was categorised according to a dissolution scale (DS) using SEM. For both species, in acidic conditions, the tests of dead specimens were significantly more dissolved than the tests of living specimens, suggesting active mechanisms providing tolerance to acidification. For the living specimens, no significant difference in the DS distribution was observed between the two species at both conditions, suggesting that kleptoplast photosynthetic activity in H. germanica does not provide additional resistance to acidification. Until at least day 12, respiration data revealed a different biological activity for the two species, and we observed distinct behaviours (e.g., encystment and pseudopod emission). These suggest each species exhibits species-specific responses to cope with acidification. On day 18, respiration rates and binocular observations showed low biological activity, suggesting dormancy or death. Further investigation is required to identify the cellular mechanisms involved to counter acidification stress.

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Multi-trait responses in two marine diatoms to pH and irradiance reveal interactive effect of light and acidification, mediated by silicification

Published 11 March 2025 Science Closed
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Ocean ecosystem shifts in response to anthropogenic climate change are impacting marine organisms, including phytoplankton. Ocean acidification and warming represent two key threats to marine phytoplankton, causing significant changes to the upper mixed layer of the ocean, reshuffling their distribution, and reorganizing their physiology and metabolism. In this study, we investigated changes in biomolecular composition and silicification rates of the two “model” diatom species Phaeodactylum tricornutum and Thalassiosira weissflogii under low (~ 350) and projected future (~ 800) pCO2 concentrations with low (20 μmol photons m−2 s−1) and high (200 μmol photons m−2 s−1) light, simulating expected climate change-induced impacts of ocean shoaling and acidification. Specifically, our study conditions elicited changes in lipid and protein content in both species. We also found a negative effect of pCO2 on silica production under high light in T. weissflogii that was linked to improved photochemical efficiency. This interactive effect between light and pCO2 with silica production suggests a potential controlling role of the frustule in diatom photosynthesis and photoprotection (energy balance). Based on these data, ocean shoaling and acidification have the potential to influence the nutritional value and biogeochemical role of diatoms through its effect on diatom frustule synthesis and photobiology.

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Interactive effects of elevated atmospheric CO2 and UV-B radiation: a multi-level study on marine diatom Skeletonema pseudocostatum

Published 24 February 2025 Science Closed
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Highlights

  • Combined effects of UVB radiation and increased atmospheric CO2 was assessed on Skeletonema pseudocostatum.
  • Additive, synergistic and antagonistic effects were characterized based on modified independent action (IA) model.
  • The combined effects on S. pseudocostatum were dose-dependent and target-specific.
  • Additivity was most common, synergy occurred in ROS and growth, while carotenoids content reduced antagonistically.
  • An effect pathway was developed to characterize the propagation of combined effects across t biological levels.

Abstract

Climate change as a result of increases in greenhouse gas emissions, such as CO2, is causing significant alteration in global environmental conditions, including ocean acidification (OA). Although the depletion of the ozone layer has reduced, the penetration of ultraviolet-B (UVB) radiation into the oceans still remains an environmental factor that may potentially enhance the effects of OA on biota. Improved understanding of the complex interactions between multiple stressors, such as UV-B radiation and increased CO2 levels, is thus important for safeguarding ecosystems and developing effective conservation and management strategies. A 72 h experiment was carried out to investigate the combined effects of UVB irradiance (0.5 W m−2) and varying CO2 levels (350, 500, 1000 ppm) on the diatom Skeletonema pseudocostatum. The study aimed to characterize the potential combined effects at different levels of biological organization, including ROS formation, lipid peroxidation (LPO), photosynthesis, pigments, oxidative phosphorylation (OXPHOS) and growth. The findings indicate that exposure to elevated CO2 (500 ppm) alone resulted in increased total carotenoid content and growth of S. pseudocostatum, but did not significantly impact photosystem efficiency, oxidative stress, and OXPHOS. Sole UVB exposure induced oxidative stress, inhibited photosynthesis and OXPHOS processes, and suppressed growth in S. pseudocostatum. However, when co-exposed with CO2, synergistic impacts were observed for reactive oxygen species (ROS), lipid peroxidation (LPO), and growth, while carotenoids were reduced in an antagonistic manner. A putative impact pathway was proposed as an initial effort to characterize the combined effects of these stressors under proposed future marine OA scenarios involving elevated CO2.

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CO2 fertilisation counteracts the negative effect of poor water quality on the growth and photosynthesis of a Great Barrier Reef coralline alga

Published 10 February 2025 Science Closed
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The global problem of ocean acidification and localised decline in water quality are major threats to coral reefs worldwide. This study examined the individual and interactive impacts of global and local stressors by investigating the effects of increased seawater pCO2, elevated nutrient concentrations and reduced light levels on linear growth and metabolic rates of the common branching crustose coralline alga Lithophyllum cf. pygmaeum. We found complex interactions between factors on algal growth and photosynthetic rates, but overall, growth was significantly enhanced by pCO2 enrichment under all light and nutrient combinations. This is the first study to report a positive growth response in coralline algae to elevated pCO2 using linear extension methods. In contrast, the combination of reduced light levels and high nutrient concentrations simulating poor water quality conditions reduced algal growth rates by up to 67% (compared to individuals exposed to high light, low nutrients and elevated pCO2). Decreased light levels reduced linear growth, Pgross and Pnet rates by 33%, 18% and 24%, respectively, highlighting the critical role of light in coralline algal physiology. We suggest that poor water quality may counteract any CO2 fertilisation effect under ocean acidification conditions on the growth of coralline algae, and this has implications for coral reef conservation as it emphasises the importance of improving water quality to maintaining coral reef functions. These results further highlight the need for multifactorial experiments to better understand the interplay between global and local processes on coralline algae growth.

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Effects of pH, temperature, and light on the inorganic carbon uptake strategies in early life stages of Macrocystis pyrifera (Ochrophyta, Laminariales)

Published 1 January 2025 Science Closed
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The responses of seaweed species to increased CO2 and lowered pH (Ocean Acidification: OA) depend on their carbon concentrating mechanisms (CCMs) and inorganic carbon (Ci) preferences. However, few studies have described these mechanisms in the early life stages of seaweeds or assessed the effects of OA and its interactions with other environmental drivers on their functionality and photophysiology. Our study evaluated the effects of pH, light (PAR), temperature, and their interactions on the Ci uptake strategies and photophysiology in the early stages of Macrocystis pyrifera. Gametophytes were cultivated under varying pH (7.80 and 8.20), light (20 and 50 µmol photons m−2s−1), and temperature (12 and 16 °C) conditions for 25 days. We assessed photophysiological responses and CCMs (in particular, the extracellular dehydration of HCO3− to CO2 mediated by the enzyme carbonic anhydrase (CA) and direct HCO3− uptake via an anion exchange port). This study is the first to describe the Ci uptake strategies in gametophytes of M. pyrifera, demonstrating that their primary CCM is the extracellular conversion of HCO3− to CO2 mediated by CA. Additionally, our results indicate that decreased pH can positively affect their photosynthetic efficiency and maximum quantum yield; however, this response is dependent on the light and temperature conditions.

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The important role of the antioxidant stress capacity in the response of Prochlorococcus to increased CO2 under varying iron and light conditions

Published 9 December 2024 Science Closed
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Highlights

  • Low-light-adapted Prochlorococcus ecotypes have stronger low-iron adaptation capacity
  • Fe limitation in Prochlorococcus is enhanced under both low growth-limiting light and high photo-inhibitory light
  • High CO2 promotes the growth of low-light-adapted Prochlorococcus ecotypes due to a reduction in cellular oxidation stress

Abstract

Ocean acidification caused by the ongoing increase in atmospheric carbon dioxide (CO2) is expected to impact the growth of marine phytoplankton. Additionally, CO2-driven climate change influences light intensity and iron (Fe) availability in surface seawaters, two critical factors for marine phytoplankton carbon fixation and growth due to their central role in regulating photosynthesis. The cyanobacterium Prochlorococcus often dominates marine productivity in oligotrophic oceans with low but variable Fe concentrations and light intensities. However, the combined effects of light intensity, Fe availability and CO2 concentration on the growth and photosynthesis of Prochlorococcus remain unclear. In this study, we found that the high-light-adapted Prochlorococcus strain MED4, isolated from shallower depths, required much higher Fe concentrations and light intensities to grow than the low-light-adapted strain NATL1A, isolated from deeper depths. Increased CO2 had no effect on the growth of strain MED4 under any light or Fe conditions. In contrast, increased CO2 caused a 29% increase in the growth of strain NATL1A under low Fe coupled with high photo-inhibitory light condition, owing to a reduction in cellular oxidative stress. The varying antioxidant stress capacities of different Prochlorococcus strains appeared to influence their responses to increased CO2. These results indicate complex interactions among light intensity, Fe limitation, and CO2 concentration, which may affect the species distributions and productivities of marine phytoplankton, including Prochlorococcus, in a future high-CO2 ocean.

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Phytoplankton responses to changing irradiance and carbon fertilization

Published 25 November 2024 Science Closed
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Phytoplankton play a crucial role in marine ecosystems due to their innate ability to fix carbon. As atmospheric CO2 levels continue to rise from fossil fuel combustion, the resulting increase in dissolved CO2 and the concurrent decrease in ocean pH are likely to impact the phytoplankton community. The response of phytoplankton to elevated CO2 can vary significantly among species and environmental conditions (e.g. light, temperature, nutrient availability). To address these variations, an experiment was conducted using a controlled photobioreactor system, maintaining high and low light, constant temperature, nutrient levels, and two pCO2 concentrations. This study focused on two regionally relevant phytoplankton in the northern Gulf of Mexico. Skeletonema (a diatom) increased growth rates with the combination of high light and high carbon, but this was not accompanied by increases in particulate organic carbon and nitrogen (POC/N). In contrast, Micromonas commoda (a green alga) did not show changes in growth rate or POC/PON but allocated more energy towards photosynthesis. Additionally, Skeletonema displayed a decoupling between growth rate and silicification, leading to higher biogenic silica content per cell in elevated pCO2 environments. These results highlight the necessity for genera specific and regionally focused research, as the physiological plasticity among phytoplankton can vary.

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Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: a new perspective on the contribution of U. prolifera to the seawater carbon sink function

Published 21 October 2024 Science Closed
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Highlights

  • Moderate light, temperature and nitrate addition boost U. prolifera DIC absorption.
  • U. prolifera adapts carbon fixation modes to environmental changes.
  • Origin environmental conditions determine U. prolifera’s carbon sink contribution.

Abstract

The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m−2 s−1), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L−1). The results showed that moderate light (108 μmol m−2 s−1), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ13C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO3− and CO2 concentrations. Enhanced light intensity of 108 μmol m−2 s−1 could shift the carbon fixation pathway of U. prolifera towards the C4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera.

Continue reading ‘Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: a new perspective on the contribution of U. prolifera to the seawater carbon sink function’

High light intensity and CO2 enrichment synergistically mitigated the stress caused by low salinity in Pyropia yezoensis

Published 7 August 2024 Science Closed
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Macroalgae, playing a crucial role in coastal marine ecosystems, are subject to multiple environmental challenges due to tidal and seasonal alterations. In this work, we investigated the physiological responses of Pyropia yezoensis to ocean acidification (ambient CO2 (AC: 400 μatm) and elevated CO2 (HC: 1000 μatm)) under changing salinity (20, 30 psu) and light intensities (50, 100 μmol photons m−2 s−1) by measuring the growth, pigment content, chlorophyll fluorescence, and soluble sugar content. The key results are the following: (1) P. yezoensis exhibited better growth under normal salinity (30 psu) compared to hyposaline conditions (20 psu). (2) Intermediate light intensity increased phycoerythrin content, ultimately enhancing thalli growth without significant changes to the contents of chlorophyll a and carotenoids. (3) Ocean acidification alleviated hyposaline stress by enhancing pigment production in P. yezoensis only at a salinity of 20 psu, highlighting the complex interplay of these environmental factors. These findings indicate that higher light intensities and elevated pCO2 levels could mitigate the stress caused by low salinity.

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Ocean deoxygenation dampens resistance of diatoms to ocean acidification in darkness

Published 23 May 2024 Science Closed
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Respiratory activity in the oceans is declining due to the expansion of hypoxic zones and progressive deoxygenation, posing threats to marine organisms along with impacts of concurrent ocean acidification. Therefore, understanding the combined impacts of reduced pO2 and elevated pCO2 on marine primary producers is of considerable significance. Here, to simulate diatoms’ sinking into the aphotic zone of turbid coastal water, we exposed the diatoms Thalassiosira pseudonana and Thalassiosira weissflogii in darkness at 20°C to different levels of pO2 and pCO2 conditions for ~3 weeks, and monitored their biomass density, photosynthetic activity and dark respiration, and examined their recovery upon subsequent exposure to light at 20°C, simulating surface water conditions. Along with decreased cell abundance and size, measured photosynthetic capacity and dark respiration rates in these two diatoms both gradually decreased during the prolonged darkness. Reduced pO2 alone did not negatively affect the photosynthetic machinery in both the dark-survived diatom, and enhanced their subsequent recovery upon light exposure. Nevertheless, the combination of the elevated pCO2 and reduced pO2 (equivalent to hypoxia) led to the biomass loss by about 90% in T. pseudonana, and delayed the recovery of both species upon subsequent exposure to light, though it did not reduce the cell concentration of T. weissflogii during the elongated darkness. Our results suggest that reduced O2 availability diminishes the abilities of the diatoms to cope with the acidic stress associated with ocean acidification, and the expansion of hypoxic waters could delay the photosynthetic recovery of coastal diatoms when they are transported upwards through mixing from dark layers to sunlit waters.

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Adverse environmental perturbations may threaten kelp farming sustainability by exacerbating enterobacterales diseases

Published 25 March 2024 Science Closed
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Globally kelp farming is gaining attention to mitigate land-use pressures and achieve carbon neutrality. However, the influence of environmental perturbations on kelp farming remains largely unknown. Recently, a severe disease outbreak caused extensive kelp mortality in Sanggou Bay, China, one of the world’s largest high-density kelp farming areas. Here, through in situ investigations and simulation experiments, we find indications that an anomalously dramatic increase in elevated coastal seawater light penetration may have contributed to dysbiosis in the kelp Saccharina japonica’s microbiome. This dysbiosis promoted the proliferation of opportunistic pathogenic Enterobacterales, mainly including the genera Colwellia and Pseudoalteromonas. Using transcriptomic analyses, we revealed that high-light conditions likely induced oxidative stress in kelp, potentially facilitating opportunistic bacterial Enterobacterales attack that activates a terrestrial plant-like pattern recognition receptor system in kelp. Furthermore, we uncover crucial genotypic determinants of Enterobacterales dominance and pathogenicity within kelp tissue, including pathogen-associated molecular patterns, potential membrane-damaging toxins, and alginate and mannitol lysis capability. Finally, through analysis of kelp-associated microbiome data sets under the influence of ocean warming and acidification, we conclude that such Enterobacterales favoring microbiome shifts are likely to become more prevalent in future environmental conditions. Our study highlights the need for understanding complex environmental influences on kelp health and associated microbiomes for the sustainable development of seaweed farming.

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Increased light intensity enhances photosynthesis and biochemical components of red macroalga of commercial importance, Kappaphycus alvarezii, in response to ocean acidification

Published 12 March 2024 Science Closed
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Highlights

  • Effects of light availability and pCO2 on Kappaphycus alvarezii were examined.
  • Moderate increases in light intensity and pCO2 had positive effects on K. alvarezii.
  • OA and high light promoted carbon accumulation, but they had negative impacts on nitrogen.

Abstract

The concentration of atmospheric carbon dioxide (CO2) has increased drastically over the past several decades, resulting in the pH of the ocean decreasing by 0.44 ± 0.005 units, known as ocean acidification (OA). The Kappaphycus alvarezii (Rhodophyta, Solieriaceae), is a commercially and ecologically important red macroalga with significant CO2 absorption potential from seawater. The K. alvarezii also experienced light variations from self-shading and varied cultivation depths. Thus, the aim of present study was to investigate the effects of two pCO2 levels (450 and 1200 ppmv) and three light intensities (50, 100, and 150 μmol photons·m−2·s−1) on photosynthesis and the biochemical components in K. alvarezii. The results of the present study showed that a light intensity of 50 μmol photons·m−2·s−1 was optimal for K. alvarezii photosynthesis with 0.663 ± 0.030 of Fv/Fm and 0.672 ± 0.025 of Fv’/Fm’. Phycoerythrin contents at two pCO2 levels decreased significantly with an increase in light intensity by 57.14–87.76%, while phycocyanin contents only decreased from 0.0069 ± 0.001 mg g−1 FW to 0.0047 ± 0.001 mg g−1 FW with an increase in light intensity at 1200 ppmv of pCO2. Moreover, moderate increases in light intensity and pCO2 had certain positive effects on the physiological performance of K. alvarezii, specifically in terms of increasing soluble carbohydrate production. Although OA and high light levels promoted total organic carbon accumulation (21.730 ± 0.205% DW) in K. alvarezii, they had a negative impact on total nitrogen accumulation (0.600 ± 0.017% DW).

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Physiological impacts of CO2-Induced acidification and UVR on invasive alga Caulerpa racemosa

Published 11 March 2024 Science Closed
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Anthropogenically increasing atmospheric CO2 causes changes in the carbon chemistry of seawater. With these changes, the HCO3− and CO2 concentration of seawater increases, while the pH decreases. CO2-induced ocean acidification by interacting with ultraviolet radiation (UVR) affects the metabolic pathways of seaweeds such as photosynthesis, growth, and nutrient uptake in a species-specific manner. This study was designed to determine the future ecological success of Caulerpa racemosa, an invasive species in the Mediterranean. In laboratory culture, C. racemosa was exposed to CO2-induced low pH (pH: 7.7) with or without UVR (UVA: 1.2 W m−2; UVB: 0.55 W m−2) and its physiological responses were investigated. Maximum quantum yield of photosystem-II (Fv/Fm) and light utilization efficiency (α) of C. racemosa was negatively affected by low pH and UVR. However, low pH increased the rETRmax (maximum relative electron transfer rate) of C. racemosa. This increased rETRmax indicated that the photosynthesis of C. racemosa was not photosynthetically saturated at the ambient inorganic carbon pool. This could be an advantage in competing with other species in the predicted future ocean acidification. The combined effect of low pH and UVR affected the rETRmax of C. racemosa in different ways along with the incubation time. The synergistic effect observed in the first two weeks turned into an antagonistic effect in the last two weeks. The data obtained from this study suggest that incubation time is the most effective factor in the response of C. racemosa to CO2-induced low pH and moderate-level UVR. In addition, our results support the hypothesis that C. racemosa may be one of the species that will benefit from CO2-induced ocean acidification.

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The impact of extreme weather events exceeds those due to global-change drivers on coastal phytoplankton assemblages

Published 1 March 2024 Science Closed
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Highlights

  • Extreme wind and rainfall events have become frequent phenomena in coastal ecosystems.
  • We simulated these events under global change for five phytoplankton assemblages.
  • Extreme events were responsible for the bulk of variability on photosynthesis efficiency.
  • The impact of extreme events is low in assemblages with high diversity and evenness
  • Extreme events should be considered in global change studies.

Abstract

Extreme wind and rainfall events have become more frequent phenomena, impacting coastal ecosystems by inducing increased mixing regimes in the upper mixed layers (UML) and reduced transparency (i.e. browning), hence affecting phytoplankton photosynthesis. In this study, five plankton assemblages from the South Atlantic Ocean, from a gradient of environmental variability and anthropogenic exposure, were subjected to simulated extreme weather events under a global change scenario (GCS) of increased temperature and nutrients and decreased pH, and compared to ambient conditions (Control). Using multiple linear regression (MLR) analysis we determined that evenness and the ratio of diatoms/ (flagellates + dinoflagellates) significantly explained the variations (81–91 %) of the photosynthesis efficiency (i.e. Pchla/ETRchla ratio) for each site under static conditions. Mixing speed and the optical depth (i.e. attenuation coefficient * depth, kdz), as single drivers, explained 40–76 % of the variability in the Pchla/ETRchla ratio, while GCS drivers <9 %. Overall, assemblages with high diversity and evenness were less vulnerable to extreme weather events under a GCS. Extreme weather events should be considered in global change studies and conservation/management plans as even at local/regional scales, they can exceed the predicted impacts of mean global climate change on coastal primary productivity.

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Hidden cost of pH variability in seagrass beds on marine calcifiers under ocean acidification

Published 9 February 2024 Science Closed
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Highlights

  • The presence of seagrass creates variability in pH/pCO2.
  • High pCO2/low pH negatively impacts growth and calcification of sea urchin larvae.
  • The variability associated with the presence of seagrass negatively impacts growth under ocean acidification.
  • Two different calcification strategies are observed in presence and absence of seagrass.

Abstract

Coastal ecosystems experience large environmental variability leading to local adaptation. The key role of variability and adaptation in modulating the biological sensitivity to ocean acidification is increasingly acknowledged. Monitoring and understanding the ecological niche at the right spatio-temporal scale is key to understand the sensitivity of any organism and ecosystems. However, the role of the variability in relevant carbonate chemistry parameters as a driver is often overlooked. For example, the balance between photosynthesis and respiration over the day/night cycle is leading to high pH/pCO2 variability in seagrass beds. We hypothesized that (i) the calcifying larvae of the sea urchin Echinus esculentus exposed to seagrass-driven variability would have some physiological mechanisms to respond to such variability; and (ii) these mechanisms would reach their limit under ocean acidification. We compared the presence and absence of the seagrass Zostera marina in flow through mesocosms fed with seawater with 4 pHs. The carbonate chemistry was monitored and biological response of a sea urchin larvae was documented over 3 weeks. Growth and net calcification rates were measured twice a day to encompass diurnal variability. Our results show that larvae growth rate significantly decreased with decreasing average pHT in both absence and presence of seagrass. Moreover, sea urchin larvae showed a slower growth rate in presence of seagrass, only visible in the lowest pH conditions. In addition, larvae raised in presence of seagrass, maximized calcification during the day, and lower their calcification during the night. In contrast, no significant difference was observed between day and night for the net calcification rate in larvae raised in absence of seagrass. Our results demonstrate the limit of local adaptation to the present range of variability under ocean acidification conditions. It also demonstrates that photosynthetic ecosystems such as seagrass may not play a role of refuge against future ocean acidification.

Continue reading ‘Hidden cost of pH variability in seagrass beds on marine calcifiers under ocean acidification’

Future warming stimulates growth and photosynthesis in an Arctic microalga more strongly than changes in light intensity or pCO2

Published 20 December 2023 Science Closed
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We assessed the responses of solitary cells of Arctic Phaeocystis pouchetii grown under a matrix of temperature (2°C vs. 6°C), light intensity (55 vs. 160 μmol photons m−2 s−1) and pCO2 (400 vs. 1000 μatm CO2, i.e., 40.5 vs. 101.3 Pa). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, Chlorophyll a content), we measured physiological processes in vivo (electron transport rates and net photosynthesis) using fast-repetition rate fluorometry and membrane-inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth, elemental quotas and photosynthetic performance. Light stimulations manifested more prominently under 6°C, underlining temperature’s role as a “master-variable”. pCO2 was the least effective driver, exerting mostly insignificant effects. The obtained data were used for a simplistic upscaling simulation to investigate potential changes in P. pouchetii‘s bloom dynamics in the Fram Strait with increasing temperatures over the 21st century. Although solitary cells might not be fully representative of colonial cells commonly observed in the field, our results suggest that global warming accelerates bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Such a temperature-induced acceleration in the phenology of Phaeocystis and likely other Arctic phytoplankton might cause temporal mismatches, e.g., with the development of grazers, and therefore substantially affect the biogeochemistry and ecology of the Arctic.

Continue reading ‘Future warming stimulates growth and photosynthesis in an Arctic microalga more strongly than changes in light intensity or pCO2’

High light intensity and CO2 enrichment synergistically mitigated the stress caused by low salinity in Pyropia yezoensis

Published 30 November 2023 Science Closed
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Macroalgae, playing a crucial role in coastal marine ecosystems, are subject to multiple environmental challenges due to tidal and seasonal alterations. In this work, we investigated the physiological responses of Pyropia yezoensis to ocean acidification (ambient CO2 (AC: 400 μatm) and elevated CO2 (HC: 1000 μatm)) under changing salinity (20, 30 psu) and light intensities (50, 100 μmol photons m−2 s−1) by measuring the growth, pigment content, chlorophyll fluorescence, and soluble sugar content. The key results are the following: (1) P. yezoensis exhibited better growth under normal salinity (30 psu) compared to hyposaline conditions (20 psu). (2) Intermediate light intensity increased phycoerythrin content, ultimately enhancing thalli growth without significant changes to the contents of chlorophyll a and carotenoids. (3) Ocean acidification alleviated hyposaline stress by enhancing pigment production in P. yezoensis only at a salinity of 20 psu, highlighting the complex interplay of these environmental factors. These findings indicate that higher light intensities and elevated pCO2 levels could mitigate the stress caused by low salinity.

Continue reading ‘High light intensity and CO2 enrichment synergistically mitigated the stress caused by low salinity in Pyropia yezoensis’

Optimizing marine macrophyte capacity to locally ameliorate ocean acidification under variable light and flow regimes: insights from an experimental approach

Published 19 October 2023 Science Closed
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The urgent need to remediate ocean acidification has brought attention to the ability of marine macrophytes (seagrasses and seaweeds) to take up carbon dioxide (CO2) and locally raise seawater pH via primary production. This physiological process may represent a powerful ocean acidification mitigation tool in coastal areas. However, highly variable nearshore environmental conditions pose uncertainty in the extent of the amelioration effect. We developed experiments in aquaria to address two interconnected goals. First, we explored the individual capacities of four species of marine macrophytes (Ulva lactuca, Zostera marina, Fucus vesiculosus and Saccharina latissima) to ameliorate seawater acidity in experimentally elevated pCO2. Second, we used the most responsive species (i.e., S. latissima) to assess the effects of high and low water residence time on the amelioration of seawater acidity in ambient and simulated future scenarios of climate change across a gradient of irradiance. We measured changes in dissolved oxygen, pH, and total alkalinity, and derived resultant changes to dissolved inorganic carbon (DIC) and calcium carbonate saturation state (Ω). While all species increased productivity under elevated CO2, S. latissima was able to remove DIC and alter pH and Ω more substantially as CO2 increased. Additionally, the amelioration of seawater acidity by S. latissima was optimized under high irradiance and high residence time. However, the influence of water residence time was insignificant under future scenarios. Finally, we applied predictive models as a function of macrophyte biomass, irradiance, and residence time conditions in ambient and future climatic scenarios to allow projections at the ecosystem level. This research contributes to understanding the biological and physical drivers of the coastal CO2 system.

Continue reading ‘Optimizing marine macrophyte capacity to locally ameliorate ocean acidification under variable light and flow regimes: insights from an experimental approach’

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