Effects of ocean acidification on growth and physiology of Ulva lactuca (Chlorophyta) in a rockpool-scenario

Rising atmospheric CO₂-concentrations will have severe consequences for a variety of biological processes. We investigated the responses of the green alga Ulva lactuca (Linnaeus) to rising CO₂-concentrations in a rockpool scenario. U. lactuca was cultured under aeration with air containing either preindustrial pCO₂ (280 μatm) or the pCO₂ predicted by the end of the 21st century (700 μatm) for 31 days. We addressed the following question: Will elevated CO₂-concentrations affect photosynthesis (net photosynthesis, maximum relative electron transport rate (rETR(max)), maximum quantum yield (Fv/Fm), pigment composition) and growth of U. lactuca in rockpools with limited water exchange? Two phases of the experiment were distinguished: In the initial phase (day 1–4) the Seawater Carbonate System (SWCS) of the culture medium could be adjusted to the selected atmospheric pCO₂ condition by continuous aeration with target pCO₂ values. In the second phase (day 4–31) the SWCS was largely determined by the metabolism of the growing U. lactuca biomass. In the initial phase, Fv/Fm and rETR(max) were only slightly elevated at high CO₂-concentrations, whereas growth was significantly enhanced. After 31 days the Chl a content of the thalli was significantly lower under future conditions and the photosynthesis of thalli grown under preindustrial conditions was not dependent on external carbonic anhydrase. Biomass increased significantly at high CO₂-concentrations. At low CO₂-concentrations most adult thalli disintegrated between day 14 and 21, whereas at high CO₂-concentrations most thalli remained integer until day 31. Thallus disintegration at low CO₂-concentrations was mirrored by a drastic decline in seawater dissolved inorganic carbon and HCO₃⁻. Accordingly, the SWCS differed significantly between the treatments. Our results indicated a slight enhancement of photosynthetic performance and significantly elevated growth of U. lactuca at future CO₂-concentrations. The accelerated thallus disintegration at high CO₂-concentrations under conditions of limited water exchange indicates additional CO₂ effects on the life cycle of U. lactuca when living in rockpools.

Sensitivity of Antarctic phytoplankton species to ocean acidification: growth, carbon acquisition, and species interaction

Despite the fact that ocean acidification is considered to be especially pronounced in the Southern Ocean, little is known about CO₂-dependent physiological processes and the interactions of Antarctic phytoplankton key species. We therefore studied the effects of CO₂ partial pressure (P_CO2) (16.2, 39.5, and 101.3 Pa) on growth and photosynthetic carbon acquisition in the bloom-forming species Chaetoceros debilis, Pseudo-nitzschia subcurvata, Fragilariopsis kerguelensis, and Phaeocystis antarctica. Using membrane-inlet mass spectrometry, photosynthetic O₂ evolution and inorganic carbon (C_i) fluxes were determined as a function of CO₂ concentration. Only the growth of C. debilis was enhanced under high P_CO2. Analysis of the carbon concentrating mechanism (CCM) revealed the operation of very efficient CCMs (i.e., high C_i affinities) in all species, but there were species-specific differences in CO₂-dependent regulation of individual CCM components (i.e., CO₂ and uptake kinetics, carbonic anhydrase activities). Gross CO₂ uptake rates appear to increase with the cell surface area to volume ratios. Species competition experiments with C. debilis and P. subcurvata under different P_CO2 levels confirmed the CO₂-stimulated growth of C. debilis observed in monospecific incubations, also in the presence of P. subcurvata. Independent of P_CO2, high initial cell abundances of P. subcurvata led to reduced growth rates of C. debilis. For a better understanding of future changes in phytoplankton communities, CO₂-sensitive physiological processes need to be identified, but also species interactions must be taken into account because their interplay determines the success of a species.

Interactive effects of ocean acidification and warming on sediment-dwelling marine calcifiers

The increase in human activities, such as the burning of fossil fuels, has elevated the concentration of atmospheric carbon dioxide and warmed the planet through the greenhouse effect. In addition, approximately 30% of the CO2 produced by human activities has dissolved into the oceans, lowering pH and reducing the abundance, and hence the availability, of carbonate ions (CO3 2-), which are essential for calcium carbonate deposition. Of great concern is the impact to photosynthetic marine calcifiers, elevated CO2 and temperature is expected to have a negative impact on the health and survivorship of calcifying marine organisms. This thesis explores the effects of elevated CO2 and temperature on the microenvironment, photosynthetic efficiency, calcification and biomechanical properties in important sediment producers on coral reefs. The reef-building and sedimentdwelling organisms, Halimeda and symbiont-bearing foraminifera are prominent, coexisting taxa in shallow coral reefs and play a vital role in tropical and subtropical ecosystems as producers of sediment and habitats and food sources for other marine organisms. However, there is limited evidence of the effects of ocean warming and acidification in these two keystone species. Irradiance alone was not found to influence photosynthetic efficiency, photoprotective mechanisms and calcification in Halimeda macroloba, Halimeda cylindracea and Halimeda opuntia (Chapter 2). There is also limited knowledge of foraminiferal biology on coral reefs, especially the symbiotic relationship between the protest host and algal symbionts. Marginopora vertebralis, the dominant tropical foraminifera, shows phototactic behavior, which is a unique mechanism for ensuring symbionts experience an ideal light environment. The diurnal photosynthetic responses of in hospite symbiont photosynthesis was linked to host movement and aided in preventing photoinhibition and bleaching by moving away from over-saturating irradiance, to more optimal light fields (Chapter 3). With this greater understanding of Halimeda and foraminiferan biology and photosynthesis, the impacts of ocean warming and acidification on photosynthesis and calcification were then tested (Chapter 4, 5 and 6). Impacts of ocean acidification and warming were investigated through exposure to a combination of four temperature (28, 30, 32, 34°C) and four pCO2 levels (380, 600, 1000, 2000 µatm; equivalent to future climate change scenarios for the current and the years 2065, 2100 and 2200 and simulating the IPCC A1F1 predictions) (Chapter 4). Elevated CO2 and temperature caused a decline in photosynthetic efficiency (FV/FM), calcification and growth in all species. After five weeks at 34°C under all CO2 levels, all species died. The elevated CO2 and temperature greatly affect the CaCO3 crystal formation with reductions in density and width. M. vertebralis experienced the greatest inhibition to crystal formation, suggesting that this high Mg-calcite depositing species is more sensitive to lower pH and higher temperature than aragonite-forming Halimeda species. Exposure to elevated temperature alone or reduced pH alone decreased photosynthesis and calcification in these species. However, there was a strong synergistic effect of elevated temperature and reduced pH, with dramatic reductions in photosynthesis and calcification in all three species. This study suggested that the elevated temperature of 32°C and the pCO2 concentration of 1000 µatm are the upper limit for survival of these species art our site of collection (Heron Island on the Great Barrier Reef, Australia). Microsensors enabled the detection of O2 surrounding specimens at high spatial and temporal resolutions and revealed a 70-80% in decrease in O2 production under elevated CO2 and temperature (1200 µatm 32°C) in Halimeda (Chapter 5) and foraminifera (Chapter 6). The results from O2 microprofiles support the photosynthetic pigment and chlorophyll fluorescence data, showing decreasing O2 production with declining chlorophyll a and b concentrations and a decrease in photosynthetic efficiency under ocean acidification and/or temperature stress. This revealed that photosynthesis and calcification are closely coupled with reductions in photosynthetic efficiency leading to reductions in calcification. Reductions in carbonate availability reduced calcification and that can lead to weakened calcified structures. Elevations in water temperature is expected to augment this weakening, resulting in decreased mechanical integrity and increased susceptibility to storm- and herbivory-induced mortality in Halimeda sp. The morphological and biomechanical properties in H. macroloba and H. cylindracea at different wave exposures were then investigated in their natural reef habitats (Chapter 7). The results showed that both species have morphological (e.g. blade surface area, holdfast volume) and biomechanical (e.g. force required to uproot, force required to break thalli) adaptations to different levels of hydrodynamic exposure. The mechanical integrity and skeletal mineralogy of Halimeda was then investigated in response to future climate change scenarios (Chapter 7). The biomechanical properties (shear strength and punch strength) significantly declined in the more heavily calcified H. cylindracea at 32ºC and 1000 µatm, whereas were variable in less heavily calcified H. macroloba, indicating different responses between Halimeda species. An increase in less-soluble low Mgcalcite was observed under elevated CO2 conditions. Significant changes in Mg:Ca and Sr:Ca ratios under elevated CO2 and temperature conditions suggested that calcification was affected at the ionic level. It is concluded that Halimeda is biomechanically sensitive to elevated temperature and more acidic oceans and may lead to increasing susceptibility to herbivory and higher risk of thallus breakage or removal from the substrate. Experimental results throughout the thesis revealed that ocean acidification and warming have negative impacts on photosynthetic efficiency, productivity, calcification and mechanical integrity, which is likely to lead to increased mortality in these species under a changing climate. A loss of these calcifying keystone species will have a dramatic impact on carbonate accumulation, sediment turnover, and coral reef community and habitat structure.

Symbiosis increases coral tolerance to ocean acidification

Increasing the acidity of ocean waters will directly threaten calcifying marine organisms such as reef-building scleractinian corals, and the myriad of species that rely on corals for protection and sustenance. Ocean pH has already decreased by around 0.1 pH units since the beginning of the industrial revolution, and is expected to decrease by another 0.2–0.4 pH units by 2100. This study mimicked the pre-industrial, present, and near-future levels of pCO² using a precise control system (±5% pCO²), to assess the impact of ocean acidification on the calcification of recently-settled primary polyps of Acropora digitifera, both with and without symbionts, and adult fragments with symbionts. The increase in pCO² of 100 μatm between the pre-industrial period and the present had more effect on the calcification rate of adult A. digitifera than the anticipated future increases of several hundreds of micro-atmospheres of pCO². The primary polyps with symbionts showed higher calcification rates than primary polyps without symbionts, suggesting that (i) primary polyps housing symbionts are more tolerant to near-future ocean acidification than organisms without symbionts, and (ii) corals acquiring symbionts from the environment (i.e. broadcasting species) will be more vulnerable to ocean acidification than corals that maternally acquire symbionts.

Production and carbonate dynamics of Halimeda incrassata (Ellis) Lamouroux altered by Thalassia testudinum Banks and Soland ex König

Ocean acidification poses a serious threat to a broad suite of calcifying organisms. Scleractinian corals and calcareous algae that occupy shallow, tropical waters are vulnerable to global changes in ocean chemistry because they already are subject to stressful and variable carbon dynamics at the local scale. For example, net heterotrophy increases carbon dioxide concentrations, and pH varies with diurnal fluctuations in photosynthesis and respiration. Few researchers, however, have investigated the possibility that carbon dioxide consumption during photosynthesis by non-calcifying photoautotrophs, such as seagrasses, can ameliorate deleterious effects of ocean acidification on sympatric calcareous algae. Naturally occurring variations in the density of seagrasses and associated calcareous algae provide an ecologically relevant test of the hypothesis that diel fluctuations in water chemistry driven by cycles of photosynthesis and respiration within seagrass beds create microenvironments that enhance macroalgal calcification. In Grape Tree Bay off Little Cayman Island BWI, we quantified net production and characterized calcification for thalli of the calcareous green alga Halimeda incrassata growing within beds of Thalassia testudinum with varying shoot densities. Results indicated that individual H. incrassata thalli were ~ 6% more calcified in dense seagrass beds. On an areal basis, however, far more calcium carbonate was produced by H. incrassata in areas where seagrasses were less dense due to higher rates of production. In addition, diel pH regimes in vegetated and unvegetated areas within the lagoon were not significantly different, suggesting a high degree of water exchange and mixing throughout the lagoon. These results suggest that, especially in well-mixed lagoons, carbonate production by calcareous algae may be more related to biotic interactions between seagrasses and calcareous algae than to seagrass-mediated changes in local water chemistry.

Ultraviolet radiation modulates the physiological responses of the calcified rhodophyte Corallina officinalis to elevated CO2

Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO₂-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO₃ content and had a miniscule effect on growth. However, UVA (4.25 W m^-2) and a moderate level of UVB (0.5 W m^-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO₂.

A coral polyp model of photosynthesis, respiration and calcification incorporating a transcellular ion transport mechanism

A numerical simulation model of coral polyp photosynthesis, respiration and calcification was developed. The model is constructed with three components (ambient seawater, coelenteron and calcifying fluid), and incorporates photosynthesis, respiration and calcification processes with transcellular ion transport by Ca-ATPase activity and passive transmembrane CO₂ transport and diffusion. The model calculates dissolved inorganic carbon and total alkalinity in the ambient seawater, coelenteron and calcifying fluid, dissolved oxygen (DO) in the seawater and coelenteron and stored organic carbon (CH₂O). To reconstruct the drastic variation between light and dark respiration, respiration rate dependency on DO in the coelenteron is incorporated. The calcification rate depends on the aragonite saturation state in the calcifying fluid (Ωa _cal). Our simulation result was a good approximation of “light-enhanced calcification.” In our model, the mechanism is expressed as follows: (1) DO in the coelenteron is increased by photosynthesis, (2) respiration is stimulated by increased DO in the light (or respiration is limited by DO depletion in the dark), then (3) calcification increases due to Ca-ATPase, which is driven by the energy generated by respiration. The model simulation results were effective in reproducing the basic responses of the internal CO₂ system and DO. The daily calcification rate, the gross photosynthetic rate and the respiration rate under a high-flow condition increased compared to those under the zero-flow condition, but the net photosynthetic rate decreased. The calculated calcification rate responses to variations in the ambient aragonite saturation state (Ωa _amb) were nonlinear, and the responses agreed with experimental results of previous studies. Our model predicted that in response to ocean acidification (1) coral calcification will decrease, but will remain at a higher value until Ωa _amb decreases to 1, by maintaining a higher Ωa _cal due to the transcellular ion transport mechanism and (2) the net photosynthetic rate will increase.

Rising CO2 interacts with growth light and growth rate to alter photosystem II photoinactivation of the coastal diatom Thalassiosira pseudonana

We studied the interactive effects of pCO₂ and growth light on the coastal marine diatom Thalassiosira pseudonana CCMP 1335 growing under ambient and expected end-of-the-century pCO₂ (750 ppmv), and a range of growth light from 30 to 380 µmol photons·m⁻²·s⁻¹. Elevated pCO₂ significantly stimulated the growth of T. pseudonana under sub-saturating growth light, but not under saturating to super-saturating growth light. Under ambient pCO₂ susceptibility to photoinactivation of photosystem II (σ_i) increased with increasing growth rate, but cells growing under elevated pCO₂ showed no dependence between growth rate and σ_i, so under high growth light cells under elevated pCO₂ were less susceptible to photoinactivation of photosystem II, and thus incurred a lower running cost to maintain photosystem II function. Growth light altered the contents of RbcL (RUBISCO) and PsaC (PSI) protein subunits, and the ratios among the subunits, but there were only limited effects on these and other protein pools between cells grown under ambient and elevated pCO₂.

Evolutionary responses of a coccolithophorid Gephyrocapsa oceanica to ocean acidification

The ongoing ocean acidification associated with a changing carbonate system may impose profound effects on marine planktonic calcifiers. Here, we show that a coccolithophore, Gephyrocapsa oceanica, evolved in response to an elevated CO2 concentration of 1000 μatm (pH reduced to 7.8) in a long term (∼ 670 generations) selection experiment. The high CO2 selected cells showed increases in photosynthetic carbon fixation, growth rate, cellular particulate organic carbon (POC) or nitrogen (PON) production and a decrease in C:N elemental ratio, indicating a greater up-regulation of PON than of POC production under the ocean acidification condition. Cells from the low CO2 selection process shifted to high CO2 exposure showed an enhanced cellular POC and PON production rates. Our data suggest that the coccolithophorid could adapt to ocean acidification with enhanced assimilations of carbon and nitrogen but decreased C:N ratios.

Dissecting the impact of CO2 and pH on the mechanisms of photosynthesis and calcification in the coccolithophore Emiliania huxleyi

Summary

• Coccolithophores are important calcifying phytoplankton predicted to be impacted by changes in ocean carbonate chemistry caused by the absorption of anthropogenic CO₂. However, it is difficult to disentangle the effects of the simultaneously changing carbonate system parameters (CO₂, bicarbonate, carbonate and protons) on the physiological responses to elevated CO₂.
• Here, we adopted a multifactorial approach at constant pH or CO₂ whilst varying dissolved inorganic carbon (DIC) to determine physiological and transcriptional responses to individual carbonate system parameters.
• We show that Emiliania huxleyi is sensitive to low CO₂ (growth and photosynthesis) and low bicarbonate (calcification) as well as low pH beyond a limited tolerance range, but is much less sensitive to elevated CO₂ and bicarbonate. Multiple up-regulated genes at low DIC bear the hallmarks of a carbon-concentrating mechanism (CCM) that is responsive to CO₂ and bicarbonate but not to pH.
• Emiliania huxleyi appears to have evolved mechanisms to respond to limiting rather than elevated CO₂. Calcification does not function as a CCM, but is inhibited at low DIC to allow the redistribution of DIC from calcification to photosynthesis. The presented data provides a significant step in understanding how E. huxleyi will respond to changing carbonate chemistry at a cellular level.