The modulating effect of light intensity on the response of the coccolithophore Gephyrocapsa oceanica to ocean acidification

Global change leads to a multitude of simultaneous modifications in the marine realm among which shoaling of the upper mixed layer, leading to enhanced surface layer light intensities, as well as increased carbon dioxide (CO2) concentration are some of the most critical environmental alterations for phytoplankton. In this study, we investigated the responses of growth, photosynthetic carbon fixation and calcification of the coccolithophore Gephyrocapsa oceanica to elevated inline image (51 Pa, 105 Pa, and 152 Pa) (1 Pa ~ 10 µatm) at a variety of light intensities (50-800 µmol photons/m**2/s). By fitting the light response curve, our results showed that rising inline image reduced the maximum rates for growth, photosynthetic carbon fixation and calcification. Increasing light intensity enhanced the sensitivity of these rate responses to inline image, and shifted the inline image optima toward lower levels. Combining the results of this and a previous study (Sett et al. 2014) on the same strain indicates that both limiting low inline image and inhibiting high inline image levels (this study) induce similar responses, reducing growth, carbon fixation and calcification rates of G. oceanica. At limiting low light intensities the inline image optima for maximum growth, carbon fixation and calcification are shifted toward higher levels. Interacting effects of simultaneously occurring environmental changes, such as increasing light intensity and ocean acidification, need to be considered when trying to assess metabolic rates of marine phytoplankton under future ocean scenarios.

Eddy covariance and environmental data of boreal bogs plant species

In boreal bogs plant species are low in number, but they differ greatly in their growth forms and photosynthetic properties. We assessed how ecosystem carbon (C) sink dynamics were affected by seasonal variations in photosynthetic rate and leaf area of different species. Photosynthetic properties (light-response parameters), leaf area development and areal cover (abundance) of the species were used to quantify species-specific net and gross photosynthesis rates (PN and PG, respectively), which were summed to express ecosystem-level PN and PG. The ecosystem-level PG was compared with a gross primary production (GPP) estimate derived from eddy covariance measurements (EC). Species areal cover rather than differences in photosynthetic properties determined the species with the highest PG of both vascular plants and Sphagna. Species-specific contributions to the ecosystem PG varied over the growing season, which in turn determined the seasonal variation in ecosystem PG. The upscaled growing-season PG estimate, 230 g C/m**2, agreed well with the GPP estimated by the EC, 243 g C/m**2. Sphagna were superior to vascular plants in ecosystem-level PG throughout the growing season but had a lower PN. PN results indicated that areal cover of the species together with their differences in photosynthetic parameters shape the ecosystem-level C balance. Species with low areal cover but high photosynthetic efficiency appear to be potentially important for the ecosystem C sink. Results imply that functional diversity may increase the stability of C sink of boreal bogs.

Winter sea surface temperature reconstruction from planktic foraminiferal transfer functions in the northeastern Arabian Sea over the last two millennia

The Indian monsoon system is an important climate feature of the northern Indian Ocean. Small variations of the wind and precipitation patterns have fundamental influence on the societal, agricultural, and economic development of India and its neighboring countries. To understand current trends, sensitivity to forcing, or natural variation, records beyond the instrumental period are needed. However, high-resolution archives of past winter monsoon variability are scarce. One potential archive of such records are marine sediments deposited on the continental slope in the NE Arabian Sea, an area where present-day conditions are dominated by the winter monsoon. In this region, winter monsoon conditions lead to distinctive changes in surface water properties, affecting marine plankton communities that are deposited in the sediment. Using planktic foraminifera as a sensitive and well-preserved plankton group, we first characterize the response of their species distribution on environmental gradients from a dataset of surface sediment samples in the tropical and sub-tropical Indian Ocean. Transfer functions for quantitative paleoenvironmental reconstructions were applied to a decadal-scale record of assemblage counts from the Pakistan Margin spanning the last 2000?years. The reconstructed temperature record reveals an intensification of winter monsoon intensity near the year 100 CE. Prior to this transition, winter temperatures were >1.5°C warmer than today. Conditions similar to the present seem to have established after 450 CE, interrupted by a singular event near 950 CE with warmer temperatures and accordingly weak winter monsoon. Frequency analysis revealed significant 75-, 40-, and 37-year cycles, which are known from decadal- to centennial-scale resolution records of Indian summer monsoon variability and interpreted as solar irradiance forcing. Our first independent record of Indian winter monsoon activity confirms that winter and summer monsoons were modulated on the same frequency bands and thus indicates that both monsoon systems are likely controlled by the same driving force.

(Table 1) Radionuclides and ESR dating of sediment cores PS1535-10 and PS1535-8

ESR-spectra of foraminifera in arctic sediment cores display the [CO2]- -signal (g=2.0006). Research on the thermal behaviour of the [CO2]- -signal shows that both natural and artificial irradiation generates a precursor and a thermal unstable component of the [CO2]- -signal. The precursor can be transfered to the stable radical, and unstable radicals can be removed by heating. The signal-change by heating depends on the irradiation dose. Because of the varying response on thermal treatment, the dose-response curves show systematic differences depending on the applied procedure (single- or multi-aliquot method with or without heating). A model for the description of the [CO2]- -signal-change is presented. The combination of two exponential saturation functions seems to be an adequate analytical description of the dose-response curve of the [CO2]- -signal in foraminifera. Due to the limited thermal stability this signal can be used for dating foraminifera with ages up to about 190 ka.

Effects of feeding and light intensity on the response of the coral Porites rus to ocean acidification

Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Nutrient availability affects the response of the calcifying chlorophyte Halimeda opuntia (L.) J.V. Lamouroux to low pH

Atmospheric carbon dioxide emissions cause a decrease in the pH and aragonite saturation state of surface ocean water. As a result, calcifying organisms are expected to suffer under future ocean conditions, but their physiological responses may depend on their nutrient status. Because many coral reefs experience high inorganic nutrient loads or seasonal changes in nutrient availability, reef organisms in localized areas will have to cope with elevated carbon dioxide and changes in inorganic nutrients. Halimeda opuntia is a dominant calcifying primary producer on coral reefs that contributes to coral reef accretion. Therefore, we investigated the carbon and nutrient balance of H. opuntia exposed to elevated carbon dioxide and inorganic nutrients. We measured tissue nitrogen, phosphorus and carbon content as well as the activity of enzymes involved in inorganic carbon uptake and nitrogen assimilation (external carbonic anhydrase and nitrate reductase, respectively). Inorganic carbon content was lower in algae exposed to high CO2, but calcification rates were not significantly affected by CO2 or inorganic nutrients. Organic carbon was positively correlated to external carbonic anhydrase activity, while inorganic carbon showed the opposite correlation. Carbon dioxide had a significant effect on tissue nitrogen and organic carbon content, while inorganic nutrients affected tissue phosphorus and N:P ratios. Nitrate reductase activity was highest in algae grown under elevated CO2 and inorganic nutrient conditions and lowest when phosphate was limiting. In general, we found that enzymatic responses were strongly influenced by nutrient availability, indicating its important role in dictating the local responses of the calcifying primary producer H. opuntia to ocean acidification.

Ocean acidification decreases the light-use efficiency in an Antarctic diatom under dynamic but not constant light, link to supplementary data

There is increasing evidence that different light intensities strongly modulate the effects of ocean acidification (OA) on marine phytoplankton. The aim of the present study was to investigate interactive effects of OA and dynamic light, mimicking natural mixing regimes. The Antarctic diatom Chaetoceros debilis was grown under two pCO2 (390 and 1000 latm) and light conditions (constant and dynamic), the latter yielding the same integrated irradiance over the day. To characterize interactive effects between treatments, growth, elemental composition, primary production and photophysiology were investigated. Dynamic light reduced growth and strongly altered the effects of OA on primary production, being unaffected by elevated pCO2 under constant light, yet significantly reduced under dynamic light. Interactive effects between OA and light were also observed for Chl production and particulate organic carbon (POC) quotas. Response patterns can be explained by changes in the cellular energetic balance. While the energy transfer efficiency from photochemistry to biomass production (Phi_e,C) was not affected by OA under constant light, it was drastically reduced under dynamic light. Contrasting responses under different light conditions need to be considered when making predictions regarding a more stratified and acidified future ocean.

Decreased light availability can amplify negative impacts of ocean acidification on calcifying coral reef organisms

Coral reef organisms are increasingly and simultaneously affected by global and local stressors such as ocean acidification (OA) and reduced light availability. However, knowledge of the interplay between OA and light availability is scarce. We exposed 2 calcifying coral reef species (the scleractinian coral Acropora millepora and the green alga Halimeda opuntia) to combinations of ambient and increased pCO2 (427 and 1073 µatm, respectively), and 2 light intensities (35 and 150 µmol photons/m**2/s) for 16 d. We evaluated the individual and combined effects of these 2 stressors on weight increase, calcification rates, O2 fluxes and chlorophyll a content for the species investigated. Weight increase of A. millepora was significantly reduced by OA (48%) and low light intensity (96%) compared to controls. While OA did not affect coral calcification in the light, it decreased calcification in the dark by 155%, leading to dissolution of the skeleton. H. opuntia weight increase was not affected by OA, but decreased (40%) at low light. OA did not affect algae calcification in the light, but decreased calcification in the dark by 164%, leading to dissolution. Low light significantly reduced gross photosynthesis (56 and 57%), net photosynthesis (62 and 60%) and respiration (43 and 48%) of A. millepora and H. opuntia, respectively. In contrast to A. millepora, H. opuntia significantly increased chlorophyll content by 15% over the course of the experiment. No interactive effects of OA and low light intensity were found on any response variable for either organism. However, A. millepora exhibited additive effects of OA and low light, while H. opuntia was only affected by low light. Thus, this study suggests that negative effects of low light and OA are additive on corals, which may have implications for management of river discharge into coastal coral reefs.

Increased CO2 modifies the carbon balance and the photosynthetic yield of two common Arctic brown seaweeds: Desmarestia aculeata and Alaria esculenta

Ocean acidification affects with special intensity Arctic ecosystems, being marine photosynthetic organisms a primary target, although the consequences of this process in the carbon fluxes of Arctic algae are still unknown. The alteration of the cellular carbon balance due to physiological acclimation to an increased CO2 concentration (1300 ppm) in the common Arctic brown seaweeds Desmarestia aculeata and Alaria esculenta from Kongsfjorden (Svalbard) was analysed. Growth rate of D. aculeata was negatively affected by CO2 enrichment, while A. esculenta was positively affected, as a result of a different reorganization of the cellular carbon budget in both species. Desmarestia aculeata showed increased respiration, enhanced accumulation of storage biomolecules and elevated release of dissolved organic carbon, whereas A. esculenta showed decreased respiration and lower accumulation of storage biomolecules. Gross photosynthesis (measured both as O2 evolution and 14C fixation) was not affected in any of them, suggesting that photosynthesis was already saturated at normal CO2 conditions and did not participate in the acclimation response. However, electron transport rate changed in both species in opposite directions, indicating different energy requirements between treatments and species specificity. High CO2 levels also affected the N-metabolism, and 13C isotopic discrimination values from algal tissue pointed to a deactivation of carbon concentrating mechanisms. Since increased CO2 has the potential to modify physiological mechanisms in different ways in the species studied, it is expected that this may lead to changes in the Arctic seaweed community, which may propagate to the rest of the food web.

Photochemical responses of the diatom Skeletonema costatum grown under elevated CO2 concentrations to short-term changes in pH

Variability in pH is a common occurrence in many aquatic environments, due to physical, chemical and biological processes. In coastal waters, lagoons, estuaries and inland waters, pH can change very rapidly (within seconds or hours) in addition to daily and seasonal changes. At the same time, progressive ocean acidification caused by anthropogenic CO2 emissions is superimposed on these spatial and temporal pH changes. Photosynthetic organisms are therefore unavoidably subject to significant pH variations at the cell surface. Whether this will affect their response to long-term ocean acidification is still unknown, nor is it known whether the short-term sensitivity to pH change is affected by the pCO2 to which the cells are acclimated. We posed the latter open question as our experimental hypothesis: Does acclimation to seawater acidification affect the response of phytoplankton to acute pH variations? The diatom Skeletonema costatum, commonly found in coastal and estuarine waters where short-term acute changes in pH frequently occur, was selected to test the hypothesis. Diatoms were grown at both 390 (pH 8.2, low CO2; LC) and 1000 (pH 7.9, high CO2; HC) µatm CO2 for at least 20 generations, and photosynthetic responses to short-term and acute changes in pH (between 8.2 and 7.6) were investigated. The effective quantum yield of LC-grown cells decreased by ca. 70% only when exposed to pH 7.6; this was not observed when exposed to pH 7.9 or 8.2. HC-grown cells did not show significant responses in any pH treatment. Non-photochemical quenching showed opposite trends. In general, our results indicate that while LC-grown cells are rather sensitive to acidification, HC-grown cells are relatively unresponsive in terms of photochemical performance.

Mineralogical, sedimentological and paleontological analyses of ODP Hole 127-797B upper Quarternary sediments

Nearly continuous cores of Quaternary fine-grained sediments with distinct dark-light colored cycles were recovered from Sites 794, 795, and 797 in the basinal parts of the Japan Sea during Leg 127. A comparison of gray value (darkness) profiles supplemented by visual inspection of core photographs between sites indicated that most of the dark and light layers were correlatable between sites, and that two of the dark layers lie close to adjacent marker ash layers. These observations indicate that deposition of dark and light layers resulted from basin-wide synchronous events. In order to understand the origin of these dark-light cycles, petrographical, mineralogical, compositional, and paleontological studies were carried out on closely spaced samples from the upper Quaternary sediments recovered from Site 797. Age model was constructed based on comparison between variation in diatom abundance and the standard oxygen isotope curve of Imbrie et al. (1984), the latter was interpolated between the five age controlled levels established at Site 797. The two curves show similar patterns which enabled us to "tune" the sediment ages to the oxygen isotope stages. We have to use variation in diatom abundance as a substitute for oxygen isotope curve since oxygen isotopic data are not available at the studied sites. Bottom water oxygenation conditions were estimated based on two criteria: (1) the degree of lamina preservation and (2) the ratio of Corg to Stot. The surface water productivity was deduced from the Corg and biogenic silica content. Results suggest that the bottom water oxygenation level and the surface water productivity varied significantly in response to the glacial-interglacial cycles. Glacio-eustatic sea-level changes and subsequent changes in water circulation in the Japan Sea appear to have been responsible for these variations and consequent changes in sediment composition throughout the Quaternary.

Seawater carbonate chemistry, growth rate and processes during experiments with Coccolithus pelagicus and Calcidiscus leptoporus, 2006

Uptake of half of the fossil fuel CO2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO2-related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO2 levels (?360 ppm CO2). With particulate organic carbon (POC) remaining constant over the range of CO2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO2, POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.

The photo-physiological response of a model cnidarian-dinoflagellate symbiosis to CO2-induced acidification at the cellular level

We measured the relationship between CO2-induced seawater acidification, photo-physiological performance and intracellular pH (pHi) in a model cnidarian-dinoflagellate symbiosis - the sea anemone Aiptasia sp. -under ambient (289.94 ± 12.54 µatm), intermediate (687.40 ± 25.10 µatm) and high (1459.92 ± 65.51 µatm) CO2 conditions. These treatments represented current CO2 levels, in addition to CO2 stabilisation scenarios IV and VI provided by the Intergovernmental Panel on Climate Change (IPCC). Anemones were exposed to each treatment for two months and sampled at regular intervals. At each time-point we measured a series of physiological responses: maximum dark-adapted fluorescent yield of PSII (Fv/Fm), gross photosynthetic rate, respiration rate, symbiont population density, and light-adapted pHi of both the dinoflagellate symbiont and isolated host anemone cell. We observed increases in all but one photo-physiological parameter (Pgross:R ratio). At the cellular level, increases in light-adapted symbiont pHi were observed under both intermediate and high CO2 treatments, relative to control conditions (pHi 7.35 and 7.46 versus pHi 7.25, respectively). The response of light-adapted host pHi was more complex, however, with no change observed under the intermediate CO2 treatment, but a 0.3 pH-unit increase under the high CO2 treatment (pHi 7.19 and 7.48, respectively). This difference is likely a result of a disproportionate increase in photosynthesis relative to respiration at the higher CO2 concentration. Our results suggest that, rather than causing cellular acidosis, the addition of CO2 will enhance photosynthetic performance, enabling both the symbiont and host cell to withstand predicted ocean acidification scenarios.

Ocean acidification mediates photosynthetic response to UV radiation and temperature increase in the diatom Phaeodactylum tricornutum

Increasing atmospheric CO2 concentration is responsible for progressive ocean acidification, ocean warming as well as decreased thickness of upper mixing layer (UML), thus exposing phytoplankton cells not only to lower pH and higher temperatures but also to higher levels of solar UV radiation. In order to evaluate the combined effects of ocean acidification, UV radiation and temperature, we used the diatom Phaeodactylum tricornutum as a model organism and examined its physiological performance after grown under two CO2 concentrations (390 and 1000 µatm) for more than 20 generations. Compared to the ambient CO2 level (390 µatm), growth at the elevated CO2 concentration increased non-photochemical quenching (NPQ) of cells and partially counteracted the harm to PS II (photosystem II) caused by UV-A and UV-B. Such an effect was less pronounced under increased temperature levels. The ratio of repair to UV-B induced damage decreased with increased NPQ, reflecting induction of NPQ when repair dropped behind the damage, and it was higher under the ocean acidification condition, showing that the increased pCO2 and lowered pH counteracted UV-B induced harm. As for photosynthetic carbon fixation rate which increased with increasing temperature from 15 to 25 °C, the elevated CO2 and temperature levels synergistically interacted to reduce the inhibition caused by UV-B and thus increase the carbon fixation.