Posts Tagged 'photosynthesis'

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Multi-interacting global-change drivers reduce photosynthetic and resource use efficiencies and prompt a microzooplankton-phytoplankton uncoupling in estuarine communities

Published 10 January 2025 Science Closed
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Highlights

  • Multi-interacting driver effects were evaluated on South Atlantic estuarine plankton
  • Warming×pH×nutrients×UVR reduced the photosynthetic and resource use efficiencies
  • A multi-driver change condition prompted a microzooplankton-phytoplankton uncoupling
  • Altered trophic interactions could reduce the energy transfer efficiency in food webs

Abstract

Plankton communities are subjected to multiple global change drivers; however, it is unknown how the interplay between them deviates from predictions based on single-driver studies, in particular when trophic interactions are explicitly considered. We investigated how simultaneous manipulation of temperature, pH, nutrient availability and solar radiation quality affects the carbon transfer from phytoplankton to herbivorous protists and their potential consequences for ecosystem functioning. Our results showed that multiple interacting global-change drivers reduced the photosynthetic (gross primary production-to-electron transport rates ratios, from 0.2 to 0.6-0.8) and resource use efficiencies (from 9 to 1 μg chlorophyll a (Chl a) μmol nitrogen-1) and prompted uncoupling between microzooplankton grazing (m) and phytoplankton growth (μ) rates (μ > m). The altered trophic interaction could be due to enhanced intra-guild predation or to microzooplankton growing at suboptimal temperatures compared to their prey. Because phytoplankton-specific loss rates to consumers grazing are the most significant uncertainty in marine biogeochemical models, we stress the need for experimental approaches quantifying it accurately to avoid bias in predicting the impacts of global change on marine ecosystems.

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Response of the photosynthetic physiology of Ulva lactuca to Cu toxicity under ocean acidification

Published 8 January 2025 Science Closed
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Highlights

  • Low Cu concentrations promote the growth of Ulva lactuca.
  • The growth of Ulva lactuca was decreased with Cu increase under low CO2 condition.
  • Ocean acidification can exacerbate the adverse effects of Cu on Ulva lactuca.

Abstract

Ocean acidification can significantly affect the physiological performance of macroalgae. While copper (Cu) is an essential element for macroalgae and has been extensively studied, the interactive effects of ocean acidification and Cu on these organisms remain less understood. In this study, we measured the photosynthetic characteristics of Ulva lactuca exposed to varying Cu concentrations at two CO2 levels (415 ppmv, low concentration; 1000 ppmv, high concentration). The results indicated that during chronic toxicity testing, the growth of juvenile U. lactuca significantly increased at Cu concentrations of 0.001 μM, 0.01 μM, and 0.1 μM regardless of low CO2 concentrations or high CO2 concentrations condition. In acute toxicity tests, elevated Cu concentrations negatively impacted the growth rate, yield, and photosynthetic rate of U. lactuca under low CO2 concentrations. Conversely, high CO2 concentrations enhanced the photosynthetic capacity of U. lactuca with increased Cu concentrations, while the growth rate significantly decreased at Cu concentration of 1.5 μM. Additionally, the activities of peroxidase (POD) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) increased, with an enhancement of malondialdehyde (MDA) content at 1.5 μM Cu under high CO2 conditions. However, the structure of the chloroplast thylakoid was disrupted by elevated Cu concentrations. These findings suggest that low Cu concentrations promote the growth of U. lactuca, whereas high Cu concentrations inhibit algal growth, and ocean acidification may exacerbate the adverse effects of Cu on U. lactuca in acute toxicity tests.

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Combined effects of pCO2 and salinity on the silicification of estuarine diatoms

Published 8 January 2025 Science Closed
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Highlights

  • Low salinity enhanced the weakening of silicification by ocean acidification (OA).
  • OA and salinity influenced the quality of diatom frustules and cell size.
  • OA could affect estuarine diatoms’ competitiveness in future environment.

Abstract

Understanding the combined effects of seawater acidification and salinity is crucial for assessing the adaptation of estuarine organisms to climate change. This study examined the physiological and nanostructural responses of two coastal diatoms, Thalassiosira pseudonana and Thalassiosira weissflogii, under different pCO2 and salinity conditions. Our results indicated that high pCO2 and low salinity decreased the biogenic silica and chlorophyll contents in both species. The weakly condensed silicon increased alongside the decrease in biogenic silica under high pCO2 conditions, with this trend being further amplified in low salinity environments. Meanwhile, the biochemical compositions and nanostructure of the diatom frustules were significantly altered by the lower salinity, leading to reduced cell size and porosity. These changes to diatom physiology and morphology may affect the diatoms’ capacity to defend against predators and viruses. This study highlights the chemical and morphological changes occurring in diatom cell walls in future acidic estuarine waters.

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Adaptation to warming and high CO2 influences diatom response norms and multi-trait variations across CO2 gradients 

Published 6 January 2025 Science Closed
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Understanding how phytoplankton response to elevated CO2 and/or warming through long-term genotypic adaptation is critical for predicting future phytoplankton distribution and community structure. In this study, we conducted a 4.5-year experimental evolution with the model marine diatom Phaeodactylum tricornutum Bohlin under four environmental regimes: ambient conditions, high CO2, warming, and combined high CO2 + warming. Following this long-term adaptation, we exposed the populations to a broad CO2 gradient in a short-term (7-day) experiment, assessing their multi-trait responses. Our results demonstrate that P. tricornutum Bohlin populations adapted to different regimes exhibit significant multi-trait variation across CO2 gradients. Notably, the variability driven by long-term adaptation exceeded that induced by short-term CO2 changes. Furthermore, both long-term adaptation and short-term CO2 exposure altered trait co-variations, highlighting the complex interplay between environmental history and immediate conditions. This study emphasizes the importance to assess long-term genetic changes in marine phytoplankton under global change, as short-term experiments alone may underestimate their capacity for adaptation and the broader implications for marine ecosystems under future climate scenarios.

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The photosynthetic quotient (PQ) of Ulva ohnoi (Chlorophyta) under future ocean conditions

Published 2 January 2025 Science Closed
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Recently there has been a rapid increase in interest regarding the CO2 removal capacity of seaweed, leading to a focus on photosynthesis research. Because direct measuring the dissolved inorganic carbon (CT) uptake rate is challenging, the use of the photosynthetic quotient (PQ), which converts oxygen evolution to carbon fixation, has proven effective. However, PQ is highly sensitive to various environmental factors, including climate change (warming and acidification). This study aimed to investigate the impact of climate change conditions on the PQ of Ulva ohnoi, namely, control (CONT: 271 µatm CO2 & 20°C), acidification (OA: 526 µatm CO2 & 20°C), warming (OW: 307 µatm CO2 & 25°C), and greenhouse (GR: 634 µatm CO2 & 25°C). The PQ was determined through an incubation experiment, where simultaneous measurements of O2 evolution and CT uptake rates were conducted in a seawater medium. The average PQ values were consistently above 1 across all treatments, with the highest PQ values observed in the CONT (1.67 ± 0.03) and the lowest in the OW (1.16 ± 0.04). While increased CO2 levels and light intensity did not affect the PQ value, higher temperatures had a significant impact on the PQ of U. ohnoi. Consequently, it can be expected that increased temperatures will lead to a decrease in PQ, resulting in increased CT uptake compared to O2 evolution. Estimating CT uptake based on O2 evolution may, therefore, lead to an overestimation of the CT uptake rate when applying theoretical PQ.

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Gene expression changes in the seagrass Cymodocea nodosa individuals in response to aquatic acidification

Published 2 January 2025 Science Closed
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Human activities have caused a rise in atmospheric carbon dioxide (CO2) levels, leading to greater absorption of CO2 by oceans and causing ocean acidification (OA). This phenomenon, marked by a reduction in pH, represents substantial risks to marine ecosystems, including seagrass meadows. Seagrasses are vital elements of coastal ecosystems, performing important functions in carbon storage, stabilizing shorelines, and preserving biodiversity; however, reactions to OA are not well understood, especially in molecular terms. This research study examined alterations in gene expression within seagrass meadows, namely Cymodocea nodosa, in reaction to simulated OA conditions. A climate chamber system was used to adjust CO2 levels to simulate future projections of OA, specifically following the RCP 8.5 scenario. Gene expression dynamics were assessed by collecting samples at different time intervals across a 36-h period. Research has demonstrated that genes related to photosynthesis are suppressed quickly after being exposed to increased amounts of CO2. Gene expression levels were found to change often over time, which is crucial for adaptation and acclimatization. However, antioxidant genes have varied responses to OA, with CAT and SOD being downregulated in distinct ways. Our findings offer valuable insights into the molecular mechanisms of seagrass responses to OA. They highlight the significance of examining short-term responses when evaluating the susceptibility of coastal ecosystems to climate change.

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Interactive effects of CO2, temperature, and nitrate limitation on the growth and physiology of strain CCMP 1334 of the marine cyanobacterium Synechococcus (Cyanophyceae)

Published 13 December 2024 Science Closed
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The marine cyanobacterium Synecococcus sp. (CCMP 1334) was grown in a continuous culture system on a 12:12 h light:dark cycle at all combinations of low and high pCO2 (400 and 1000 ppmv, respectively), nutrient availability (nitrate-limited and nutrient-replete conditions), and temperatures of 21, 24, 28, 32, and 35°C. The maximum nutrient-replete growth rate was ~1.15 day−1 at 32–35°C. Median nutrient-replete growth rates were higher at 1000 ppmv than at 400 ppmv pCO2 at all temperatures. Carbon:nitrogen ratios were independent of pCO2 at a fixed relative growth rate (i.e., growth rate ÷ nutrient-replete growth rate) but decreased with increasing temperature. Carbon:chlorophyll a ratios were decreased monotonically with increasing temperature and were higher under nitrate-limited than nutrient-replete conditions. Ratios of phycoerythrin to chlorophyll a were independent of growth conditions. Productivity indices were independent of temperature and nutrient limitation but were consistently higher at 1000 ppmv than 400 ppmv pCO2. Both growth rates and dark respiration rates were positively correlated with temperature, and the associated Q10 values were 2.2 and 2.3, respectively. A model of phytoplankton growth in which cellular carbon is allocated to structure, storage, or the light or dark reactions of photosynthesis accounted for the general patterns of cell composition and growth rate. This strain of Synechococcus appears well suited to changes in environmental conditions that are expected as the climate warms in response to anthropogenic emissions of CO2.

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Differential impacts of pH on growth, physiology, and elemental stoichiometry across three coccolithophore species

Published 10 December 2024 Science Closed
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Coccolithophores are pivotal players in ocean biogeochemistry, yet the impact of changing pH on the physiology of different species remains unclear as there has been a dominant focus on Gephyrocapsa huxleyi. Meta-analyses of existing experimental data are challenging due to the differences in multidimensional culture conditions. This study investigated the response of three species—Gephyrocapsa huxleyi, Coccolithus braarudii, and Chrysotila carterae—under varying CO2 conditions (via pH). The sensitivity to pH differed between species, but all species showed reduced growth rates under the highest CO2 (lowest pH) treatment possibly due to high [H+]-related inhibition. Low pH impacted cellular physiology and elemental stoichiometry, while the impact of high pH was less adverse. The changes in elemental production induced by low pH could exert a negative influence on the contribution of coccolithophores to nutrient and carbon export, especially for biogeochemically relevant open-ocean species. pH also affected coccolith formation, especially in C. braarudii, through CO2 limitation at high pH and low calcite saturation state at low pH. Contrasting species-specific pH sensitivities highlighted the potential for species like G. huxleyi to further outperform others like C. braarudii in an acidic ocean. Literature synthesis showed that coccolithophores show a broad CO2 optimum, although growth rates and particulate inorganic carbon to particulate organic carbon ratios consistently declined with increasing CO2. Strain-specific CO2 optima partly contributed to the variability within responses of individual species, giving the misleading perception of a broad species-level CO2 optimum. Strain-specific optima exist possibly due to their adaptation to carbonate chemistry conditions at the place of origin.

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Extreme environmental variability induces frontloading of coral biomineralisation genes to maintain calcification under pCO2 variability

Published 6 December 2024 Science Closed
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Corals residing in habitats that experience high-frequency seawater pCO2 variability may possess an enhanced capacity to cope with ocean acidification, yet we lack a clear understanding of the molecular toolkit enabling acclimatisation to environmental extremes or how life-long exposure to pCO2 variability influences biomineralisation. Here, we examined the gene expression responses and micro-skeletal characteristics of Pocillopora damicornis originating from the reef flat and reef slope of Heron Island, southern Great Barrier Reef. The reef flat and reef slope had similar mean seawater pCO2, but the reef flat experienced twice the mean daily pCO2 amplitude (range of 797 v. 399 μatm day−1, respectively). A controlled mesocosm experiment was conducted over 8 weeks, exposing P. damicornis from the reef slope and reef flat to stable (218 ± 9) or variable (911 ± 31) diel pCO2 fluctuations (μatm; mean ± SE). At the end of the exposure, P. damicornis originating from the reef flat demonstrated frontloading of 25% of the expressed genes regardless of treatment conditions, suggesting constitutive upregulation. This included higher expression of critical biomineralisation-related genes such as carbonic anhydrases, skeletal organic matrix proteins, and bicarbonate transporters. The observed frontloading corresponded with a 40% increase of the fastest deposited areas of the skeleton in reef flat corals grown under non-native, stable pCO2 conditions compared to reef slope conspecifics, suggesting a compensatory response that stems from acclimatisation to environmental extremes and/or relief from stressful pCO2 fluctuations. Under escalating ocean warming and acidification, corals acclimated to environmental variability warrant focused investigation and represent ideal candidates for active interventions to build reef resilience while societies adopt strict policies to limit climate change.

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Varying effects of climate change on the photosynthesis and calcification of crustose coralline algae: implications for settlement of coral larvae

Published 4 December 2024 Science Closed
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Highlights

  • Corals maintain settlement preferences under future climate conditions
  • Future climate conditions negatively affect crustose coralline algae physiology
  • Physiological responses to future climate conditions varied by algal species

Abstract

Coral recruitment is critical to the maintenance of healthy coral reef ecosystems. Many coral species settle preferentially on certain crustose coralline algae (CCA) (e.g., Hydrolithon boergesenii) over others (e.g., Paragoniolithon solubile). Calcifying organisms like CCA are particularly susceptible to ocean acidification (OA), and settlement behavior of larvae may be compromised as seawater temperatures increase (ocean warming; OW) and pH levels decrease as a result of climate change. Here, we examine the effects of future seawater conditions (OW and OA) on the calcification and photosynthetic efficiency of two CCA species, H. boergesenii and Pa. solubile. We also examine the effects of conditioning CCA in combined OA and OW on the settlement preferences of three coral species, Acropora palmata, A. cervicornis and Porites astreoides. Acropora palmata and Po. astreoides demonstrated a preference for H. boergesenii over Pa. solubile in choice experiments after short-term treatment (7–21 days) and this preference was not affected by future seawater conditions. A. cervicornis did not demonstrate a CCA preference under any treatment. Po. astreoides did not demonstrate a CCA preference in no-choice assays and settlement was unaffected by OW and OA even after the longest exposure (99 days). Both CCA had reduced photosynthetic efficiency after exposure to future seawater conditions. However, net calcification rate was reduced in H. boergesenii but not Pa. solubile after exposure to future seawater conditions. These results demonstrate that while climate change may differentially affect the physiological functioning of various species of CCA, coral settlement preferences are unlikely to be altered.

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Ocean acidification enhances the tolerance of dinoflagellate Prorocentrum donghaiense to nanoplastic-induced oxidative stress by modulating photosynthetic performance

Published 28 November 2024 Science Closed
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Introduction: The impact of ocean acidification (OA) and nanoplastics (NPs) on harmful algal blooms (HAB) has emerged as a major global concern. However, the combined effects of OA and NPs on the HAB species are poorly understood.

Methods: In this study, dinoflagellate Prorocentrum donghaiense, a typical HAB species, was exposed to varying concentrations of NPs (108.15 ± 8.52 nm) (0, 5, 10, and 15 mg L−1) and CO2 (low CO2: 417 ppm, pH: 8.00 and high CO2: 1045 ppm, pH: 7.73) for seven days to investigate the combined effects of OA and NPs.

Results and discussion: The findings revealed that NPs inhibited the growth of P. donghaiense by inducing oxidative stress, as indicated by elevated malondialdehyde (MDA) content and decreased carotenoid/chlorophyll-a ratio, even though photochemical efficiency (φP0, ψ0, and φE0), rETRmax and α were enhanced in response to NPs stress. However, OA promoted the growth and alleviated the adverse effects of NPs on P. donghaiense by increasing photochemical efficiency (φP0, ψ0, and φE0) and energy flux (RC/CS0, TR0/CS0, ET0/CS0) and enhancing the antioxidant ability (increased superoxide dismutase, and decreased MDA). P. donghaiense showed enhanced tolerance to NPs under simulated OA conditions. These findings enhance our knowledge of the HAB species response to NPs pollution under future OA scenarios.

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Symbiodiniaceae algal symbionts of Pocillopora damicornis larvae provide more carbon to their coral host under elevated levels of acidification and temperature

Published 25 November 2024 Science Closed
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Climate change destabilizes the symbiosis between corals and Symbiodiniaceae. The effects of ocean acidification and warming on critical aspects of coral survical such as symbiotic interactions (i.e., carbon and nitrogen assimilation and exchange) during the planula larval stage remain understudied. By combining physiological and stable isotope techniques, here we show that photosynthesis and carbon and nitrogen assimilation (H13CO3− and 15NH4+) in Pocillopora damicornis coral larvae is enhanced under acidification (1000 µatm) and elevated temperature (32 °C). Larvae maintain high survival and settlement rates under these treatment conditions with no observed decline in symbiont densities or signs of bleaching. Acidification and elevated temperature both enhance the net and gross photosynthesis of Symbiodiniaceae. This enhances light respiration and elevates C:N ratios within the holobiont. The increased carbon availability is primarily reflected in the 13C enrichment of the host, indicating a greater contribution of the algal symbionts to the host metabolism. We propose that this enhanced mutualistic symbiotic nutrient cycling may bolster coral larvae’s resistance to future ocean conditions. This research broadens our understanding of the early life stages of corals by emphasizing the significance of symbiotic interactions beyond those of adult corals.

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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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The role of rolling corals and free-living calcifying coralline algae in the management of greenhouse gas CO2 in the Colombian Caribbean

Published 25 November 2024 Science Closed
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The ongoing increase in anthropogenic CO₂ emissions since the industrial revolution has accelerated ocean acidification (OA) by introducing CO₂ into seawater, forming carbonic acid and reducing pH levels. This acidification threatens marine calcifiers by weakening their capacity to build calcium carbonate structures and promoting the dissolution of existing skeletons. Nonetheless, calcifying organisms may contribute to mitigating OA effects. This study explores the roles of corals (rolling Siderastrea radians, a seagrass dweller) and free-living calcifying coralline algae (back reef) in CO₂ mitigation in seawater. Field experiments were conducted on Isla Grande (Corales del Rosario and San Bernardo National Natural Park, Colombian Caribbean), to observe the diel variations in photosynthesis and calcification of these uncommon reef builders across different times of the day. Results demonstrate diel shifts influenced by photosynthesis/respiration and calcification/dissolution, with free-living coralline algae exhibiting higher productivity and calcification rates than corals during the day. Notably, free-living coralline algae displayed pronounced hysteresis, reflecting high sensitivity to light. These findings underscore the significant role of free-living coralline algae in marine carbon cycling, suggesting a more substantial impact on CO₂ mitigation than previously recognized. Conserving free-living coralline algae and their habitats is thus critical for supporting marine ecosystem health and resilience amidst global change, warranting further research into their metabolic responses to inform conservation strategies.

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Effects of CO2 on the nitrogen isotopic composition of marine diazotrophic cyanobacteria

Published 13 November 2024 Science Closed
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Biological N2 fixation has been crucial for sustaining early life on Earth. Very negative δ15N values detected in Archean sediments, which are not observed in present-day environments, have been attributed to the low efficiency of proto-nitrogenases. Alternatively, variations in early atmospheric CO2 may also play a role. Here we examine the effects of CO2 concentrations on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii, which utilize Mo-Fe nitrogenase (the most common form of the enzyme). Our results show that these organisms produce biomass with δ15N values up to ∼3‰ lower under both decreased and elevated CO2 concentrations compared to modern levels (∼380 μatm). These deviations from modern CO2 levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N2 fixation. This study offers an alternative explanation for the observed fluctuations in geological δ15N records and provides new insights into the past nitrogen cycle on Earth.

Key Points

  • The effects of CO2 on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii are examined
  • Both species produce biomass with δ15N values lower under both decreased and elevated CO2 concentrations compared to modern CO2 levels
  • CO2-controlled nitrogenase efficiency significantly influences organismal isotopic fractionation during N2 fixation

Plain Language Summary

The isotope effect of biological N2 fixation is crucial for understanding the nitrogen cycle, but its regulation under different atmospheric CO2 levels as appeared in Earth’s history is not well understood. Our research shows that CO2 levels significantly influence the nitrogen isotope composition in the biomass of the diazotrophic cyanobacteria Trichodesmium and Crocosphaera by affecting the nitrogenase efficiency and thus the growth rate. This study sheds light on the geological changes in the δ15N records and provides new insights into the historical nitrogen cycle on Earth.

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Variations of polyphenols and carbohydrates of Emiliania huxleyi grown under simulated ocean acidification conditions

Published 30 October 2024 Science Closed
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Cultures of the coccolithophore Emiliania huxleyi were grown under four different CO2-controlled pH conditions (7.75, 7.90, 8.10, and 8.25) to explore variations in extra- and intracellular polyphenols and carbohydrates in response to different ocean acidification scenarios. Acidification did not significantly affect final cell densities and carbohydrate contents. Intra- and extracellular phenolic compounds were identified and quantified by reverse-phase high-performance liquid chromatography (RP-HPLC), with the highest concentrations of total exuded phenolics at a pH of 8.25 (43 ± 3 nM) and 7.75 (18.0 ± 0.9 nM). Accumulation of intracellular phenolic compounds was observed in cells with decreasing pH, reaching the maximum level (9.24 ± 0.19 attomole per cell) at the lowest pH (7.75). The phenolic profiles presented significant changes in exuded epicatechin and protocatechuic acid (p < 0.05 and 0.01, respectively) and intracellular vanillic acid (p < 0.001), which play an essential role in the availability of trace metals. A significant increase in chlorophyll a content was observed in cells grown at the most acidic pH (p < 0.01), which also showed significantly higher radical inhibition activity (p < 0.01). The nature and concentration of these organic compounds present in the culture medium may influence trace metal bioavailability, affecting the biogeochemical cycling of carbon and microbial functional diversity.

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Species differences in carbon drawdown during marine phytoplankton growth

Published 9 October 2024 Science Closed
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Highlights

  • Species differences in carbon drawdown exist among marine phytoplankton species.
  • Phytoplankton species which can utilize > 1000 μM DIC may thrive under OAE.
  • Effects of OAE on phytoplankton communities depend on species differences in oceans.

Abstract

Ocean alkalinity enhancement (OAE) has been proposed as a mitigation method for negative carbon emission. Its effects on marine phytoplankton communities would depend on species differences in tolerance to high pH, which results from phytoplankton photosynthetic drawdown of dissolved inorganic carbon (DIC). In this study, 20 marine phytoplankton species were grown in sealed batch cultures and DIC, pH and chlorophyll a (Chl-a) were measured at the peaks of biomass. These results revealed a wide range of species differences. The drawdown DIC (ΔDIC) vs. increases in pH (ΔpH) graph resembled a Michaelis-Menten curve: significantly linear for ΔDIC < ~1000 μM and starting to plateau at ΔDIC > 1000 μM. This indicated that two mechanisms were operating: CO2 limitation at ΔpH < 1.41 and biologically-mediated precipitation-CO2 released carbon uptake at ΔpH > 1.41. These findings suggest that the potential effects of OAE on the phytoplankton communities would depend on the species differences in oceans.

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Effects of ocean acidification and temperature coupling on photosynthetic activity and physiological properties of Ulva fasciata and Sargassum horneri

Published 25 September 2024 Science Closed
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Simple Summary

Macroalgae in natural marine areas play an important role in mitigating ocean climate change. The complexity of natural conditions also makes it necessary to study macroalgae not only by considering the effects of changes in a single factor but also by exploring the coupled effects of different environmental conditions on macroalgae. Therefore, in this study, two species of macroalgae were used as experimental subjects to observe their growth processes under different co-treatments of temperature and CO2 concentration. The results of this study can provide a reference for how natural macroalgae can cope with future changes in ocean climate.

Abstract

To investigate the ecological impacts of macroalgae in the framework of shifting global CO2 concentrations, we conducted a study utilizing Ulva fasciata and Sargassum horneri specimens sourced from the Ma’an Archipelago in Zhejiang Province on how ocean acidification (OA) and temperature changes interact to affect the photosynthetic physiological responses of macroalgae. The results of the study showed that OA reduced the tolerance of U. fasciata to bright light at 20 °C, resulting in more pronounced photoinhibition, while 15 °C caused significant inhibition of U. fasciata, reducing its growth and photosynthetic activity, but OA alleviated the inhibition and promoted the growth of the alga to a certain extent. The tolerance of S. horneri to bright light was also reduced at 20 °C; the inhibition was relieved at 15 °C, and the OA further improved the algal growth. The Relative Growth Rate (RGR), photosynthetic pigment content, and the release of the dissolved organic carbon (DOC) of U. fasciata were mainly affected by the change in temperature; the growth of the alga and the synthesis of metabolites were more favored by 20 °C. A similar temperature dependence was observed for S. horneri, with faster growth and high metabolism at 15 °C. Our results suggest that OA reduces the tolerance of macroalgae to high light at suitable growth temperatures; however, at unsuitable growth temperatures, OA effectively mitigates this inhibitory effect and promotes algal growth.

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Key benthic species are affected by predicted warming in winter but show resistance to ocean acidification

Published 20 September 2024 Science Closed
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The effects of climate change on coastal biodiversity are a major concern because altered community compositions may change associated rates of ecosystem functioning and services. Whilst responses of single species or taxa have been studied extensively, it remains challenging to estimate responses to climate change across different levels of biological organisation. Studies that consider the effects of moderate realistic near-future levels of ocean warming and acidification are needed to identify and quantify the gradual responses of species to change. Also, studies including different levels of biological complexity may reveal opportunities for amelioration or facilitation under changing environmental conditions. To test experimentally for independent and combined effects of predicted near-future warming and acidification on key benthic species, we manipulated three levels of temperature (winter ambient, +0.8 and +2°C) and two levels of pCO2 (ambient at 450 ppm and elevated at 645 ppm) and quantified their effects on mussels and algae growing separately and together (to also test for inter-specific interactions). Warming increased mussel clearance and mortality rates simultaneously, which meant that total biomass peaked at +0.8°C. Surprisingly, however, no effects of elevated pCO2 were identified on mussels or algae. Moreover, when kept together, mussels and algae had mutually positive effects on each other’s performance (i.e. mussel survival and condition index, mussel and algal biomass and proxies for algal productivity including relative maximum electron transport rate [rETRmax], saturating light intensity [Ik] and maximum quantum yield [Fv/Fm]), independent of warming and acidification. Our results show that even moderate warming affected the functioning of key benthic species, and we identified a level of resistance to predicted ocean acidification. Importantly, we show that the presence of a second functional group enhanced the functioning of both groups (mussels and algae), independent of changing environmental conditions, which highlights the ecological and potential economic benefits of conserving biodiversity in marine ecosystems.

Continue reading ‘Key benthic species are affected by predicted warming in winter but show resistance to ocean acidification’

Effects of ocean acidification and nitrogen limitation on the growth and photophysiological performances of marine macroalgae Gracilariopsis lemaneiformis

Published 11 September 2024 Science Closed
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To investigate the effects of ocean acidification (OA) and nitrogen limitation on macroalgae growth and photophysiological responses, Gracilariopsis lemaneiformis was cultured under two main conditions: ambient (Low CO2, LC, 390 μatm) and CO2 enriched (High CO2, HC, 1000 μatm), with low (LN, 7 μmol L-1) and high (HN, 56 μmol L-1) nitrate. High CO2 levels decreased growth under both LN and HN treatments. HC reduced Chl a, carotenoids, phycoerythrin (PE), and phycocyanin (PC) under HN conditions, while only Chl a decreased under LN conditions. NO3– uptake rate was restricted under LN compared to HN, while HC enhanced it under HN. Net photosynthetic O2 evolution rates did not differ between CO2 and nitrate treatments. Dark respiration rates were higher under HN, further boosted by HC. The stimulated effective quantum yield (Y(II)) corresponded to decreased non-photochemical quenching (NPQ) under HN conditions. Nitrate, not CO2, showed significant effects on the relative electron transport rate (rETRmax), light use efficiency (α) and saturation light intensity (Ik) that with lowered rETRmax and α under LN culture. Our results indicate that OA may negatively affect Gracilariopsis lemaneiformis growth and alter its photophysiological performance under different nutrient conditions.

Continue reading ‘Effects of ocean acidification and nitrogen limitation on the growth and photophysiological performances of marine macroalgae Gracilariopsis lemaneiformis’

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