Posts Tagged 'photosynthesis'

A story of resilience: Arctic diatom Chaetoceros gelidus exhibited high physiological plasticity to changing CO2 and light levels

Arctic phytoplankton are experiencing multifaceted stresses due to climate warming, ocean acidification, retreating sea ice, and associated changes in light availability, and that may have large ecological consequences. Multiple stressor studies on Arctic phytoplankton, particularly on the bloom-forming species, may help understand their fitness in response to future climate change, however, such studies are scarce. In the present study, a laboratory experiment was conducted on the bloom-forming Arctic diatom Chaetoceros gelidus (earlier C. socialis) under variable CO2 (240 and 900 µatm) and light (50 and 100 µmol photons m-2 s-1) levels. The growth response was documented using the pre-acclimatized culture at 2°C in a closed batch system over 12 days until the dissolved inorganic nitrogen was depleted. Particulate organic carbon and nitrogen (POC and PON), pigments, cell density, and the maximum quantum yield of photosystem II (Fv/Fm) were measured on day 4 (D4), 6 (D6), 10 (D10), and 12 (D12). The overall growth response suggested that C. gelidus maintained a steady-state carboxylation rate with subsequent conversion to macromolecules as reflected in the per-cell POC contents under variable CO2 and light levels. A substantial amount of POC buildup at the low CO2 level (comparable to the high CO2 treatment) indicated the possibility of existing carbon dioxide concentration mechanisms (CCMs) that needs further investigation. Pigment signatures revealed a high level of adaptability to variable irradiance in this species without any major CO2 effect. PON contents per cell increased initially but decreased irrespective of CO2 levels when nitrogen was limited (D6 onward) possibly to recycle intracellular nitrogen resources resulting in enhanced C: N ratios. On D12 the decreased dissolved organic nitrogen levels could be attributed to consumption under nitrogen starvation. Such physiological plasticity could make C. gelidus “ecologically resilient” in the future Arctic.

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Photorespiration in eelgrass (Zostera marina L.): a photoprotection mechanism for survival in a CO2-limited world

Photorespiration, commonly viewed as a loss in photosynthetic productivity of C3 plants, is expected to decline with increasing atmospheric CO2, even though photorespiration plays an important role in the oxidative stress responses. This study aimed to quantify the role of photorespiration and alternative photoprotection mechanisms in Zostera marina L. (eelgrass), a carbon-limited marine C3 plant, in response to ocean acidification. Plants were grown in controlled outdoor aquaria at different [CO2]aq ranging from ~55 (ambient) to ~2121 μM for 13 months and compared for differences in leaf photochemistry by simultaneous measurements of O2 flux and variable fluorescence. At ambient [CO2], photosynthesis was carbon limited and the excess photon absorption was diverted both to photorespiration and non-photochemical quenching (NPQ). The dynamic range of NPQ regulation in ambient grown plants, in response to instantaneous changes in [CO2]aq, suggested considerable tolerance for fluctuating environmental conditions. However, 60 to 80% of maximum photosynthetic capacity of ambient plants was diverted to photorespiration resulting in limited carbon fixation. The photosynthesis to respiration ratio (PE: RD) of ambient grown plants increased 6-fold when measured under high CO2 because photorespiration was virtually suppressed. Plants acclimated to high CO2 maintained 4-fold higher PE: RD than ambient grown plants as a result of a 60% reduction in photorespiration. The O2 production efficiency per unit chlorophyll was not affected by the CO2 environment in which the plants were grown. Yet, CO2 enrichment decreased the light level to initiate NPQ activity and downregulated the biomass specific pigment content by 50% and area specific pigment content by 30%. Thus, phenotypic acclimation to ocean carbonation in eelgrass, indicating the coupling between the regulation of photosynthetic structure and metabolic carbon demands, involved the downregulation of light harvesting by the photosynthetic apparatus, a reduction in the role of photorespiration and an increase in the role of NPQ in photoprotection. The quasi-mechanistic model developed in this study permits integration of photosynthetic and morphological acclimation to ocean carbonation into seagrass productivity models, by adjusting the limits of the photosynthetic parameters based on substrate availability and physiological capacity.

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Transcriptomic stability or lability explains sensitivity to climate stressors in coralline algae

Background

Crustose coralline algae (CCA) are calcifying red macroalgae that play important ecological roles including stabilisation of reef frameworks and provision of settlement cues for a range of marine invertebrates. Previous research into the responses of CCA to ocean warming (OW) and ocean acidification (OA) have found magnitude of effect to be species-specific. Response to OW and OA could be linked to divergent underlying molecular processes across species.

Results

Here we show Sporolithon durum, a species that exhibits low sensitivity to climate stressors, had little change in metabolic performance and did not significantly alter the expression of any genes when exposed to temperature and pH perturbations. In contrast, Porolithon onkodes, a major coral reef builder, reduced photosynthetic rates and had a labile transcriptomic response with over 400 significantly differentially expressed genes, with differential regulation of genes relating to physiological processes such as carbon acquisition and metabolism. The differential gene expression detected in P. onkodes implicates possible key metabolic pathways, including the pentose phosphate pathway, in the stress response of this species.

Conclusions

We suggest S. durum is more resistant to OW and OA than P. onkodes, which demonstrated a high sensitivity to climate stressors and may have limited ability for acclimatisation. Understanding changes in gene expression in relation to physiological processes of CCA could help us understand and predict how different species will respond to, and persist in, future ocean conditions predicted for 2100.

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Ocean warming and acidification affect the transitional element and macromolecular accumulation in harmful raphidophyte Heterosigma akashiwo

Despite ocean warming and acidification being expected to increase the harmful algal species worldwide, the raphidophyte Heterosigma akashiwo is reported to have decreased. However, it is unknown on the transitional scale how this species physically and metabolically modifies its elements, and macromolecular accumulation leads to such condition. With 1st,10th, and 20th culture generation under present (21℃; pCO2 400ppm [LTLC]) and projected (25℃; pCO2 1000ppm [HTHC]) ocean conditions we examined these elemental and macromolecular changes along with transcriptome sequencing. Results showed that compared to HTHC (1st generation), the (20th generation) cells showed large decreases in carbon (QC:40%), nitrogen (QN:36%), and phosphorus-quotas (QP:43%), reflected in their reduction of overall DNA and RNA quantity. Decreased metabolic pathways in photosynthesis, carbon fixation, and lipid accumulation were coincident with changes in photosynthetic efficiency, carbon, and lipid quantity with long-term (20th generation) exposure to HTHC conditions. We observed that these variations of internal metabolic changes are caused by external changes in temperature by activating the (Ca+) signaling pathway and external changes in pCO2 by altering the (proton exchange) pathways. Our results suggest that H. akashiwo in the future ocean will undergo severe changes in its elemental and macromolecular properties, leading to programmed cell death.

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Evaluation of growth, primary productivity, nutritional composition, redox state, and antimicrobial activity of red seaweeds Gracilaria debilis and Gracilaria foliifera under pCO2-induced seawater acidification

Graphical abstract.

Highlights

  • Seawater acidification improved primary productivity, pigments, and carbon storage.
  • No sign of oxidative stress under acidification
  • Improved antimicrobial activities in acidified samples
  • Possible benefits in the future high pCO2 conditions

Abstract

The genus Gracilaria is an economically important group of seaweeds as several species are utilized for various products such as agar, used in medicines, human diets, and poultry feed. Hence, it is imperative to understand their response to predicted ocean acidification conditions. In the present work, we have evaluated the response of Gracilaria foliifera and Gracilaria debilis to carbon dioxide (pCO2) induced seawater acidification (pH 7.7) for two weeks in a controlled laboratory conditions. As a response variable, we have measured growth, productivity, redox state, primary and secondary metabolites, and mineral compositions. We found a general increase in the daily growth rate, primary productivity, and tissue chemical composition (such as pigments, soluble and insoluble sugars, amino acids, and fatty acids), but a decrease in the mineral contents under the acidified condition. Under acidification, there was a decrease in malondialdehyde. However, there were no significant changes in the total antioxidant capacity and a majority of enzymatic and non-enzymatic antioxidants, except for an increase in tocopherols, ascorbate and glutathione-s-transferase in G. foliifera. These results indicate that elevated pCO2 will benefit the growth of the studied species. No sign of oxidative stress markers indicating the acclimatory response of these seaweeds towards lowered pH conditions. Besides, we also found increased antimicrobial activities of acidified samples against several of the tested food pathogens. Based on these observations, we suggest that Gracilaria spp. will be benefitted from the predicted future acidified ocean.

Continue reading ‘Evaluation of growth, primary productivity, nutritional composition, redox state, and antimicrobial activity of red seaweeds Gracilaria debilis and Gracilaria foliifera under pCO2-induced seawater acidification’

Phosphate limitation intensifies negative effects of ocean acidification on globally important nitrogen fixing cyanobacterium

Growth of the prominent nitrogen-fixing cyanobacterium Trichodesmium is often limited by phosphorus availability in the ocean. How nitrogen fixation by phosphorus-limited Trichodesmium may respond to ocean acidification remains poorly understood. Here, we use phosphate-limited chemostat experiments to show that acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. The increased phosphorus requirements were attributed primarily to elevated cellular polyphosphate contents, likely for maintaining cytosolic pH homeostasis in response to acidification. Alongside the accumulation of polyphosphate, decreased NADP(H):NAD(H) ratios and impaired chlorophyll synthesis and energy production were observed under acidified conditions. Consequently, the negative effects of acidification were amplified compared to those demonstrated previously under phosphorus sufficiency. Estimating the potential implications of this finding, using outputs from the Community Earth System Model, predicts that acidification and dissolved inorganic and organic phosphorus stress could synergistically cause an appreciable decrease in global Trichodesmium nitrogen fixation by 2100.

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The differential ability of two species of seagrass to use carbon dioxide and bicarbonate and their modelled response to rising concentrations of inorganic carbon

Seagrass meadows are one of the most productive ecosystems on the planet, but their photosynthesis rate may be limited by carbon dioxide but mitigated by exploiting the high concentration of bicarbonate in the ocean using different active processes. Seagrasses are declining worldwide at an accelerating rate because of numerous anthropogenic pressures. However, rising ocean concentrations of dissolved inorganic carbon, caused by increases in atmospheric carbon dioxide, may benefit seagrass photosynthesis. Here we compare the ability of two seagrass from the Mediterranean Sea, Posidonia oceanica (L.) Delile and Zostera marina L., to use carbon dioxide and bicarbonate at light saturation, and model how increasing concentrations of inorganic carbon affect their photosynthesis rate. pH-drift measurements confirmed that both species were able to use bicarbonate in addition to carbon dioxide, but that Z. marina was more effective than P. oceanica. Kinetic experiments showed that, compared to Z. marinaP. oceanica had a seven-fold higher affinity for carbon dioxide and a 1.6-fold higher affinity for bicarbonate. However, the maximal rate of bicarbonate uptake in Z. marina was 2.1-fold higher than in P. oceanica. In equilibrium with 410 ppm carbon dioxide in the atmosphere, the modelled rates of photosynthesis by Z. marina were slightly higher than P. oceanica, less carbon limited and depended on bicarbonate to a greater extent. This greater reliance by Z. marina is consistent with its less depleted 13C content compared to P. oceanica. Modelled photosynthesis suggests that both species would depend on bicarbonate alone at an atmospheric carbon dioxide partial pressure of 280 ppm. P. oceanica was projected to benefit more than Z. marina with increasing atmospheric carbon dioxide partial pressures, and at the highest carbon dioxide scenario of 1135 ppm, would have higher rates of photosynthesis and be more saturated by inorganic carbon than Z. marina. In both species, the proportional reliance on bicarbonate declined markedly as carbon dioxide concentrations increased and in P. oceanica carbon dioxide would become the major source of inorganic carbon.

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Quantification of the dominant drivers of acidification in the coastal Mid-Atlantic Bight

Abstract

In shallow coastal shelves like the Mid-Atlantic Bight (MAB), ocean acidification due to increased atmospheric carbon dioxide (CO2) is compounded by highly variable coastal processes including riverine freshwater inputs, nutrient loading, biogeochemical influence, coastal currents and water mass mixing, and seasonal transitions in physical parameters. Past deconstructions of carbonate system drivers in the MAB have focused on nearshore zones or single season data, and thus lack the spatial and temporal resolution required to assess impacts to important species occupying the shelf. Deconstructing highly resolved data collected during four seasonal Slocum glider deployments in the MAB, this study uses a Taylor Series decomposition to quantify the influence of temperature, salinity, biogeochemical activity, and water mass mixing on pH and aragonite saturation state from sea surface to bottom. Results show that water mass mixing and biogeochemical activity were the most significant drivers of the carbonate system in the MAB. Nearshore water was more acidic year-round due to riverine freshwater input, but photosynthesis reduced acidity at certain depths and times. Water mass mixing increased acidity in bottom water on the shelf, particularly in summer. Gulf Stream intrusions at the shelf break during fall acted to mitigate acidification on the shelf in habitats occupied by carbonate-bearing organisms. The relationships quantified here can be used to improve biogeochemical forecast models and determine habitat suitability for commercially important fin and shellfish species residing in the MAB.

Key Points

  • Water mass mixing and biogeochemical activity are the major drivers of seasonal carbonate system dynamics in the MAB
  • Water mass mixing has opposing effects on carbonate chemistry in the nearshore and at the continental shelf break

Plain Language Summary

The coastal ocean is experiencing changes in chemistry due to human activities, including carbon dioxide emissions, nutrient runoff, and seasonal changes in temperature, salinity, and coastal currents. These drivers have been studied close to shore and/or only during single seasons, leaving a gap in our understanding of seasonal changes across the entire economically important shelf region. Here, we use high-resolution data collected by a deep-sea robot that measures chemistry from ocean surface to the sea floor. We determined the importance of four key influences (temperature, salinity, water mass mixing, and biological activity) on changes in coastal chemistry over the course of a year. We found that the most important driver of shelf chemistry was mixing of freshwater at the coast and warm, salty water at the edge of the shelf. Biological activity was a secondary influence, which caused smaller scale changes in chemistry. These results can help to predict how coastal chemistry might change in the future, so that we can prepare for the effects on economically important animals and industries.

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Physical and biological effects on the carbonate system during summer in the Northern Argentine Continental Shelf (Southwestern Atlantic)

Highlights

  • New Argentine Shelf data analyzed biogeochemical mechanisms affecting the carbonate system.
  • The study region during the summer was likely an important CO2 sink.
  • Biological mechanisms affected the CO2 dynamics in the Argentine Shelf in summer.
  • Small phytoplankton (<2–3 μm) played a key role in modulating the CO2 uptake.

Abstract

The Argentine shelf and its shelf-break (Southwestern Atlantic Ocean) are known for their high biological productivity, and as an important CO2 sink region. However, many aspects of the carbonate system dynamics in the area, especially those related to the biological activity, deserve further study. Here we investigated the mechanisms affecting the carbonate system distributions, using in situ physical, chemical and biological observations collected along a section (COSTAL-AR) on the Northern Argentine Continental Shelf during two summer cruises in 2019. Our main goal was to evaluate the role of the microbial communities on the modulation of the carbonate system in the area. For that, we characterized (i) the distribution of the thermohaline properties, chlorophyll a, dissolved oxygen, carbonate system (pH, total alkalinity, dissolved inorganic carbon and high resolution underway CO2 fugacity, fCO2), dissolved inorganic nutrients, and (ii) the microbial communities (bacterioplankton, phytoplankton, and protozooplankton). Our results show that the COSTAL-AR section was likely an important CO2 sink and presented high seawater fCO2 spatial variability in both middle (272–430 μatm) or early (211–365 μatm) summer conditions. Phytoplankton played a key role in modulating the CO2 uptake and carbonate system spatial variability during summer, especially in the middle and outer shelf. The main contribution to CO2 fixation was given by small cells, since the microbial community was dominated by autotrophic picoplankton (<2 μm; e.g. Synechococcus sp. and coccal picophytoeukaryotes). Moreover, the influence of the Shelf-break front in ruling both the seawater fCO2 distribution and biological processes was evident. These findings provide new insights on the connection between the biology and the carbonate system in this sparsely sampled area of the southwestern Atlantic Ocean.

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Transcriptomic analysis reveals distinct mechanisms of adaptation of a polar picophytoplankter under ocean acidification conditions

Graphical abstract.

Highlights

  • Increase of carbon dioxide emission to the atmosphere acidifies the ocean.
  • Ocean acidification drives the growth of a small green phytoplankter (picochlorophyte).
  • Picochlorophytes exhibit distinct metabolism compared to other polar phytoplankton.
  • Genes related to ribosomal proteins, amino acid synthesis, RNA post-transcriptional modification, nitrogen assimilation, molecular chaperones, light harvesting complexes, pigment synthesis, were found to be differentially expressed under future predicted CO2 levels.

Abstract

Human emissions of carbon dioxide are causing irreversible changes in our oceans and impacting marine phytoplankton, including a group of small green algae known as picochlorophytes. Picochlorophytes grown in natural phytoplankton communities under future predicted levels of carbon dioxide have been demonstrated to thrive, along with redistribution of the cellular metabolome that enhances growth rate and photosynthesis. Here, using next-generation sequencing technology, we measured levels of transcripts in a picochlorophyte Chlorella, isolated from the sub-Antarctic and acclimated under high and current ambient CO2 levels, to better understand the molecular mechanisms involved with its ability to acclimate to elevated CO2. Compared to other phytoplankton taxa that induce broad transcriptomic responses involving multiple parts of their cellular metabolism, the changes observed in Chlorella focused on activating gene regulation involved in different sets of pathways such as light harvesting complex binding proteins, amino acid synthesis and RNA modification, while carbon metabolism was largely unaffected. Triggering a specific set of genes could be a unique strategy of small green phytoplankton under high CO2 in polar oceans.

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Elevated CO2 modulates the physiological responses of Thalassiosira pseudonana to ultraviolet radiation

Highlights

  • High CO2 exacerbated the UVR-induced inhibition on PSII activity.
  • UVR stimulated the removal rates of both PsbA and PsbD.
  • The removal of PsbD declined by high CO2 under the exposure of UVR.
  • High CO2 reversed the UVR-induced YNPQ to YNo.

Abstract

Diatoms account for a large proportion of marine primary productivity, they tend to be the predominant species in the phytoplankton communities in the surface ocean with frequent and large light fluctuations. To understand the impacts of increased CO2 on diatoms’ capacity in exploitation of variable solar radiation, we cultured a model diatom Thalassiosira pseudonana with 400 or 1000ppmv CO2 and exposed it to high photosynthetically active radiation (PAR) alone or PAR plus ultraviolet radiation (UVR) to examine its physiological performances. The results showed that the maximum photochemical efficiency (Fv/fm) was significantly reduced by high PAR and PAR + UVR in T. pseudonana, UVR-induced inhibition on PSII activity was exacerbated by high CO2. PSII activity drops coincide approximately with PsbA content in the cells exposed to high PAR or PAR + UVR, which was pronounced at high CO2. The removal of PsbD in T. pseudonana cells declined under high CO2 during UVR exposure, limiting the repair capacity of PSII. In addition, high CO2 reversed the induction of energy-dependent form of NPQ by UVR to the increase of Y(No), indicating the severe damage of the photoprotective reactions. Our findings suggest that the adverse impacts of UVR on PSII function of T. pseudonana were aggravated by the elevated CO2 through modulating its capacity in repair and protection, which thereby would influence its abundance and competitiveness in phytoplankton communities.

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A look to the future acidified ocean through the eyes of the alien and invasive alga Caulerpa cylindracea (Chlorophyta, Ulvophyceae)

Underwater CO2 vents represent natural laboratories where the responses of marine organisms to ocean acidification can be tested. In a such context, we investigated the changes in the physiology, anatomy, and ultrastructure of the non-indigenous algal species Caulerpa cylindracea growing along a natural pH/CO2 gradient, by conducting a reciprocal transplant experiment between two populations from an acidified vs a non-acidified site. Stress effects in transplants from current to lowered pH conditions resulted in a decrease in the number of active chloroplasts together with an increased number of dilatations between thylakoid membranes and a higher amount of plastoglobules. These changes were consistent with a decrease in the chlorophyll content and in photosynthetic efficiency, matched by an increase in carotenoid content and non-photochemical yields. On the opposite side, transplants from low to current pH showed a recovery to original conditions. Unexpectedly, no significant difference was recorded between wild populations living at current and lowered pH. These results suggest an ongoing acclimation process to lowered pH in the C. cylindracea populations growing in the vent area. This confirms the high plasticity of this invasive species, able to cope not only with different light and temperature conditions but even with a new acidified scenario.

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Intraspecific variation reshapes coral assemblages under elevated temperature and acidity

Insights into assemblages that can persist in extreme environments are still emerging. Ocean warming and acidification select against species with low physiological tolerance (trait-based ‘filtering’). However, intraspecific trait variation can promote species adaptation and persistence, with potentially large effects on assemblage structure. By sampling nine coral traits (four morphological, four tissue and one skeletal) along an offshore–inshore gradient in temperature and pH, we show that distantly related coral species undergo consistent intraspecific changes as they cross into warm, acidic environment. Intraspecific variation and species turnover each favoured colonies with greater tissue biomass, higher symbiont densities and reduced skeletal investments, indicating strong filtering on colony physiology within and across species. Physiological tissue traits were highly variable within species and were independent of morphology, enabling morphologically diverse species to cross into sites of elevated temperature and acidity. Widespread intraspecific change can therefore counter the loss of biodiversity and morphological structure across a steep environmental gradient.

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How does climate change affect a fishable resource? The case of the royal sea cucumber (Parastichopus regalis) in the central Mediterranean Sea

Holothurians or sea cucumbers are key organisms in marine ecosystems that, by ingesting large quantities of sediments, provide important ecosystem services. Among them, Parastichopus regalis (Cuvier, 1817) is one of the living sea cucumbers in the Mediterranean actively fished for human consumption mainly in Spain, where it is considered a gastronomic delicacy. In the Strait of Sicily (central Mediterranean Sea), this species is not exploited for commercial use even if it is used as bait by longline fishery. P. regalis is frequently caught by bottom trawling and discarded at sea by fishers after catch, and because of its capacity to resist air exposition (at least in cold months), it is reasonable to consider that it is not affected by fishing mortality. Having observed a significant decrease in abundance since 2018, the possible effects of some ecological factors related to current climate change (i.e., temperature and pH) were sought. Generalized additive models (GAMs) were applied to investigate the relationship among the abundance of P. regalis and environmental variables and fishing effort. Long time series of P. regalis densities (2008–2021) were extracted from the MEDITS bottom trawling survey and modeled as function of environmental parameters (i.e., salinity, dissolved oxygen, ammonium, pH, and chlorophyll α) and fishing effort (i.e., total number of fishing days per gross tonnage). Our results showed that this species prefers the soft bottoms (50–200 m) of the Adventure Bank and Malta Plateau, and its distribution changed over time with a slight deepening and a rarefaction of spatial distribution starting from 2011 and 2017, respectively. In addition, a positive relationship with pH concentration in surface waters during the larval dispersal phase (3-year lag before the survey) and nutrient concentration at sea bottom (1-year lag) has been found, suggesting that this species is sensitive to climate change and food availability. This study adds new knowledge about the population dynamics of an unexploited stock of P. regalis under fishing impact and environmental under climate change in fisheries management.

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Response of the green alga Ulva prolifera grown at different irradiance levels under ocean acidification at different life cycle stages

The effects of ocean acidification on macroalgae have been extensively studied. However, most studies focused on the adult stages, while other life cycle stages have been overlooked. To better understand the influence of the marine environment on macroalgae, their whole life cycle should be considered, especially the juvenile stage. In this study, Ulva prolifera was cultured under two CO2 concentrations (400 and 1000 ppmv) and at 10, 18, 30, and 55% of incident sunlight to assess the photosynthetic performance. Our results showed that the acidification treatment had a negative effect on growth at the juvenile stage, but a positive effect at the adult stage. The relative growth rate and effective quantum yield of PSII increased with decreased light levels, irrespective of the CO2 concentration. At the adult stage, the Chlorophyll (Chl) a, Chl b, and carotenoid contents declined under the high CO2 concentration. The protein content significantly increased at 18, 30%, and full sunlight levels under the high CO2 but not under the low CO2 concentration. Our results suggest that juveniles were less tolerant of the acidic stress compared with the adult stage, although the alga was able to increase cellular proteins in response to the acidic stress.

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Proton gradients across the coral calcifying cell layer: effects of light, ocean acidification and carbonate chemistry

In corals, pH regulation of the extracellular calcifying medium (ECM) by the calcifying cell layer is a crucial step in the calcification process and is potentially important to influencing how corals respond to ocean acidification. Here, we analyzed the growing edge of the reef coral Stylophora pistillata to make the first characterization of the proton gradient across the coral calcifying epithelium. At seawater pH 8 we found that while the calcifying epithelium elevates pH in the ECM on its apical side above that of seawater, pH on its basal side in the mesoglea is markedly lower, highlighting that the calcifying cells are exposed to a microenvironment distinct from the external environment. Coral symbiont photosynthesis elevates pH in the mesoglea, but experimental ocean acidification and decreased seawater inorganic carbon concentration lead to large declines in mesoglea pH relative to the ECM, which is maintained relatively stable. Together, our results indicate that the coral calcifying epithelium is functionally polarized and that environmental variation impacts pHECM regulation through its effects on the basal side of the calcifying cells.

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Elevated CO2 modulates the physiological responses of Thalassiosira pseudonana to ultraviolet radiation

Highlights

  • High CO2 exacerbated the UVR-induced inhibition on PSII activity.
  • UVR stimulated the removal rates of both PsbA and PsbD.
  • The removal of PsbD declined by high CO2 under the exposure of UVR.
  • High CO2 reversed the UVR-induced YNPQ to YNo.

Abstract

Diatoms account for a large proportion of marine primary productivity, they tend to be the predominant species in the phytoplankton communities in the surface ocean with frequent and large light fluctuations. To understand the impacts of increased CO2 on diatoms’ capacity in exploitation of variable solar radiation, we cultured a model diatom Thalassiosira pseudonana with 400 or 1000ppmv CO2 and exposed it to high photosynthetically active radiation (PAR) alone or PAR plus ultraviolet radiation (UVR) to examine its physiological performances. The results showed that the maximum photochemical efficiency (Fv/fm) was significantly reduced by high PAR and PAR + UVR in T. pseudonana, UVR-induced inhibition on PSII activity was exacerbated by high CO2. PSII activity drops coincide approximately with PsbA content in the cells exposed to high PAR or PAR + UVR, which was pronounced at high CO2. The removal of PsbD in T. pseudonana cells declined under high CO2 during UVR exposure, limiting the repair capacity of PSII. In addition, high CO2 reversed the induction of energy-dependent form of NPQ by UVR to the increase of Y(No), indicating the severe damage of the photoprotective reactions. Our findings suggest that the adverse impacts of UVR on PSII function of T. pseudonana were aggravated by the elevated CO2 through modulating its capacity in repair and protection, which thereby would influence its abundance and competitiveness in phytoplankton communities.

Continue reading ‘Elevated CO2 modulates the physiological responses of Thalassiosira pseudonana to ultraviolet radiation’

Light history modulates growth and photosynthetic responses of a diatom to ocean acidification and UV radiation

To examine the synergetic effects of ocean acidification (OA) and light intensity on the photosynthetic performance of marine diatoms, the marine centric diatom Thalassiosira weissflogii was cultured under ambient low CO2 (LC, 390 μatm) and elevated high CO2 (HC, 1000 μatm) levels under low-light (LL, 60 μmol m−2 s−1) or high-light (HL, 220 μmol m−2 s−1) conditions for over 20 generations. HL stimulated the growth rate by 128 and 99% but decreased cell size by 9 and 7% under LC and HC conditions, respectively. However, HC did not change the growth rate under LL but decreased it by 9% under HL. LL combined with HC decreased both maximum quantum yield (FV/FM) and effective quantum yield (ΦPSII), measured under either low or high actinic light. When exposed to UV radiation (UVR), LL-grown cells were more prone to UVA exposure, with higher UVA and UVR inducing inhibition of ΦPSII compared with HL-grown cells. Light use efficiency (α) and maximum relative electron transport rate (rETRmax) were inhibited more in the HC-grown cells when UVR (UVA and UVB) was present, particularly under LL. Our results indicate that the growth light history influences the cell growth and photosynthetic responses to OA and UVR.

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Environmental memory gained from exposure to extreme pCO2 variability promotes coral cellular acid–base homeostasis

Ocean acidification is a growing threat to coral growth and the accretion of coral reef ecosystems. Corals inhabiting environments that already endure extreme diel pCO2 fluctuations, however, may represent acidification-resilient populations capable of persisting on future reefs. Here, we examined the impact of pCO2 variability on the reef-building coral Pocillopora damicornis originating from reefs with contrasting environmental histories (variable reef flat versus stable reef slope) following reciprocal exposure to stable (218 ± 9) or variable (911 ± 31) diel pCO2 amplitude (μtam) in aquaria over eight weeks. Endosymbiont density, photosynthesis and net calcification rates differed between origins but not treatment, whereas primary calcification (extension) was affected by both origin and acclimatization to novel pCO2 conditions. At the cellular level, corals from the variable reef flat exhibited less intracellular pH (pHi) acidosis and faster pHi recovery rates in response to experimental acidification stress (pH 7.40) than corals originating from the stable reef slope, suggesting environmental memory gained from lifelong exposure to pCO2 variability led to an improved ability to regulate acid–base homeostasis. These results highlight the role of cellular processes in maintaining acidification resilience and suggest that prior exposure to pCO2 variability may promote more acidification-resilient coral populations in a changing climate.

Continue reading ‘Environmental memory gained from exposure to extreme pCO2 variability promotes coral cellular acid–base homeostasis’

The effects of ocean acidification on the establishment and maintenance of a model cnidarian-dinoflagellate symbiosis

Coral reefs are increasingly under threat from the effects of anthropogenic climate change, including rising sea surface temperatures and more acidified waters. At the foundation of these diverse and valuable ecosystems is the symbiotic relationship between calcifying corals and their endosymbiotic dinoflagellate algae, Symbiodiniaceae – one that is particularly sensitive to environmental stressors. Ocean acidification (OA) results in the lowering of pH and changes to carbonate chemistry and the inorganic carbon species available to marine organisms. Cnidarians such as reef-building corals may be particularly at risk from OA, as changes in pH and carbon availability can alter central physiological processes, including calcification, photosynthesis, acid-base regulation, metabolism and cell-cycle regulation. Yet, while responses to OA have been well researched at the physiological level, results have often been contradictory, and a clear understanding of the nature and extent of impacts on the cnidarian-dinoflagellate symbiosis remains equivocal. This thesis therefore aimed to provide further insights into the effects of OA on the establishment and maintenance of the cnidarian-dinoflagellate symbiosis. My research utilised the well-established model system for this symbiosis: the sea anemone Exaiptasia diaphana (‘Aiptasia’) and its native symbiont Breviolum minutum.

Continue reading ‘The effects of ocean acidification on the establishment and maintenance of a model cnidarian-dinoflagellate symbiosis’

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