Posts Tagged 'phytoplankton'

Effects of ocean acidification on calcification of the sub-Antarctic pteropod Limacina retroversa

Ocean acidification is expected to impact the high latitude oceans first, as CO2 dissolves more easily in colder waters. At the current rate of anthropogenic CO2 emissions, the sub-Antarctic Zone will start to experience undersaturated conditions with respect to aragonite within the next few decades, which will affect marine calcifying organisms. Shelled pteropods, a group of calcifying zooplankton, are considered to be especially sensitive to changes in carbonate chemistry because of their thin aragonite shells. Limacina retroversa is the most abundant pteropod in sub-Antarctic waters, and plays an important role in the carbonate pump. However, not much is known about its response to ocean acidification. In this study, we investigated differences in calcification between L. retroversa individuals exposed to ocean carbonate chemistry conditions of the past (pH 8.19; mid-1880s), present (pH 8.06), and near-future (pH 7.93; predicted for 2050) in the sub-Antarctic. After 3 days of exposure, calcification responses were quantified by calcein staining, shell weighing, and Micro-CT scanning. In pteropods exposed to past conditions, calcification occurred over the entire shell and the leading edge of the last whorl, whilst individuals incubated under present and near-future conditions mostly invested in extending their shells, rather than calcifying over their entire shell. Moreover, individuals exposed to past conditions formed larger shell volumes compared to present and future conditions, suggesting that calcification is already decreased in today’s sub-Antarctic waters. Shells of individuals incubated under near-future conditions did not increase in shell weight during the incubation, and had a lower density compared to past and present conditions, suggesting that calcification will be further compromised in the future. This demonstrates the high sensitivity of L. retroversa to relatively small and short-term changes in carbonate chemistry. A reduction in calcification of L. retroversa in the rapidly acidifying waters of the sub-Antarctic will have a major impact on aragonite-CaCO3 export from oceanic surface waters to the deep sea.

Continue reading ‘Effects of ocean acidification on calcification of the sub-Antarctic pteropod Limacina retroversa’

Calcifying phytoplankton demonstrate an enhanced role in greenhouse atmospheric CO2 regulation

The impact of calcifying phytoplankton on atmospheric CO2 concentration is determined by a number of factors, including their degree of ecological success as well as the buffering capacity of the ocean/marine sediment system. The relative importance of these factors has changed over Earth’s history and this has implications for atmospheric CO2 and climate regulation. We explore some of these implications with four “Strangelove” experiments: two in which soft-tissue production and calcification is stopped, and two in which only calcite production is forced to stop, in idealized icehouse and greenhouse climates. We find that in the icehouse climate the loss of calcifiers compensates the atmospheric CO2 impact of the loss of all phytoplankton by roughly one-sixth. But in the greenhouse climate the loss of calcifiers compensates the loss of all phytoplankton by about half. This increased impact on atmospheric CO2 concentration is due to the combination of higher rates of pelagic calcification due to warmer temperatures and weaker buffering due to widespread acidification in the greenhouse ocean. However, the greenhouse atmospheric temperature response per unit of CO2 change to removing ocean soft-tissue production and calcification is only one-fourth that in an icehouse climate, owing to the logarithmic radiative forcing dependency on atmospheric CO2 thereby reducing the climate feedback of mass extinction. This decoupling of carbon cycle and temperature sensitivities offers a mechanism to explain the dichotomy of both enhanced climate stability and destabilization of the carbonate compensation depth in greenhouse climates.

Continue reading ‘Calcifying phytoplankton demonstrate an enhanced role in greenhouse atmospheric CO2 regulation’

Interactive effects of elevated CO2 concentration and light on the picophytoplankton Synechococcus

Synechococcus is a major contributor to the primary production in tropic and subtropical oceans worldwide. Responses of this picophytoplankton to changing light and CO2 levels is of general concern to understand its ecophysiology in the context of ocean global changes. We grew Synechococcus sp. (WH7803), originally isolated from subtropic North Atlantic Ocean, under different PAR levels for about 15 generations and examined its growth, photochemical performance and the response of these parameters to elevated CO2 (1,000 μatm). The specific growth rate increased from 6 μmol m–2 s–1 to reach a maximum (0.547 ± 0.026) at 25 μmol m–2 s–1, and then became inhibited at PAR levels over 50 μmol m–2 s–1, with light use efficiency (α) and photoinhibition coefficient (β) being 0.093 and 0.002, respectively. When the cells were grown at ambient and elevated CO2 concentration (400 vs. 1,000 μatm), the high-CO2 grown cells showed significantly enhanced rates of electron transport and quantum yield as well as significant increase in specific growth rate at the limiting and inhibiting PAR levels. While the electron transport rate significantly increased at the elevated CO2 concentration under all tested light levels, the specific growth did not exhibit significant changes under the optimal growth light condition. Our results indicate that Synechococcus WH7803 grew faster under the ocean acidification (OA) treatment induced by CO2 enrichment only under limiting and inhibiting light levels, indicating the interactive effects and implying that the picophytoplankton respond differentially at different depths while exposing changing light conditions.

Continue reading ‘Interactive effects of elevated CO2 concentration and light on the picophytoplankton Synechococcus’

Physiological responses of Skeletonema costatum to the interactions of seawater acidification and the combination of photoperiod and temperature (update)

Ocean acidification (OA), which is a major environmental change caused by increasing atmospheric CO2, has considerable influences on marine phytoplankton. But few studies have investigated interactions of OA and seasonal changes in temperature and photoperiod on marine diatoms. In the present study, a marine diatom Skeletonema costatum was cultured under two different CO2 levels (LC, 400 µatm; HC, 1000 µatm) and three different combinations of temperature and photoperiod length (8:16 L:D with 5 C, 12:12 L:D with 15 C, 16:8 L:D with 25 C), simulating different seasons in typical temperate oceans, to investigate the combined effects of these factors. The results showed that specific growth rate of S. costatum increased with increasing temperature and day length. However, OA showed contrasting effects on growth and photosynthesis under different combinations of temperature and day length: while positive effects of OA were observed under spring and autumn conditions, it significantly decreased growth (11 %) and photosynthesis (21 %) in winter. In addition, OA alleviated the negative effect of low temperature and short day length on the abundance of RbcL and key photosystem II (PSII) proteins (D1 and D2). These data indicated that future ocean acidification may show differential effects on diatoms in different clusters of other factors.

Continue reading ‘Physiological responses of Skeletonema costatum to the interactions of seawater acidification and the combination of photoperiod and temperature (update)’

Springtime spatial distributions of biogenic sulfur compounds in the Yangtze river estuary and their responses to seawater acidification and dust

The spatial distributions of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethylsulfoxide (DMSO) were investigated in the Yangtze River Estuary from 9 to 23 March, 2018. The average concentrations of DMS, dissolved DMSP (DMSPd), particulate DMSP (DMSPp), dissolved DMSO (DMSOd) and particulate DMSO (DMSOp) were 3.00 ± 2.53, 1.75 ± 1.08, 10.89 ± 14.28, 9.80 ± 7.79, and 9.51 ± 8.90 nmol L‐1, respectively. The high DMS and DMSP concentrations occurred mainly in the open sea, exhibiting distribution patterns similar to chlorophyll a (Chl‐a). Due to the release of resuspended sediments, elevated DMSO concentrations were observed in the bottom waters of some stations. The three sulfur compounds were positively correlated with Chl‐a (p < 0.05), suggesting that phytoplankton played an essential role in the production of sulfur compounds. Comparisons with previous research showed that the concentrations of DMS, DMSP, and DMSOp exhibited clear seasonal variability. The average sea‐to‐air flux of DMS was 8.19 ± 12.94 μmol m‐2 d‐1 in the study area, indicating that the estuary and continental shelf sea were significant contributors to the global sulfur cycle. Ship‐based incubation experiments showed that lower pH inhibited the production of the three biogenic sulfur compounds, while the addition of dust promoted their release. Therefore, in the future, the inhibitory effect of seawater acidification on the production of phytoplankton and sulfur compounds might be offset, to some degree, by the input of nutrient‐rich dust.

Continue reading ‘Springtime spatial distributions of biogenic sulfur compounds in the Yangtze river estuary and their responses to seawater acidification and dust’

Evaluation of actin as a reference for quantitative gene expression studies in Emiliania huxleyi (Prymnesiophyceae) under ocean acidification conditions

Gene expression studies of marine phytoplankton under ocean acidification conditions are frequently based on relative measurements, with actin commonly used as a reference gene. Evidence from other organisms suggests that actin gene expression may be regulated by environmental conditions, compromising the role of actin as a reference gene. In this work the reliability of actin as a reference gene for ocean acidification experimental conditions (high CO2 vs low CO2) in two different metabolic states (acclimated metabolism vs perturbed metabolism) for the coccolithophore Emiliania huxleyi was tested. The transcriptional response of the actin (act) is compared with the expression of specific target genes associated with inorganic carbon uptake (α-carbonic anhydrase: αca1) and assimilation (RuBisCO: rbcL), which was regulated under the experimental conditions. Our results showed act expression instability in experimental conditions, evidencing that act is not a reliable reference gene for studies assessing the effect of ocean acidification on Emiliania huxleyi. Furthermore, when the act-based normalization was quantitatively tested, rbcL and αca1 expression were compromised, leading us to conclude that absolute gene expression quantification should be considered as a potentially reliable alternative for studying gene expression under ocean acidification conditions

Continue reading ‘Evaluation of actin as a reference for quantitative gene expression studies in Emiliania huxleyi (Prymnesiophyceae) under ocean acidification conditions’

Extreme levels of ocean acidification restructure the plankton community and biogeochemistry of a temperate coastal ecosystem: a mesocosm study

The oceans’ uptake of anthropogenic carbon dioxide (CO2) decreases seawater pH and alters the inorganic carbon speciation – summarized in the term ocean acidification (OA). Already today, coastal regions experience episodic pH events during which surface layer pH drops below values projected for the surface ocean at the end of the century. Future OA is expected to further enhance the intensity of these coastal extreme pH events. To evaluate the influence of such episodic OA events in coastal regions, we deployed eight pelagic mesocosms for 53 days in Raunefjord, Norway, and enclosed 56–61 m3 of local seawater containing a natural plankton community under nutrient limited post-bloom conditions. Four mesocosms were enriched with CO2 to simulate extreme pCO2 levels of 1978 – 2069 μatm while the other four served as untreated controls. Here, we present results from multivariate analyses on OA-induced changes in the phyto-, micro-, and mesozooplankton community structure. Pronounced differences in the plankton community emerged early in the experiment, and were amplified by enhanced top-down control throughout the study period. The plankton groups responding most profoundly to high CO2 conditions were cyanobacteria (negative), chlorophyceae (negative), auto- and heterotrophic microzooplankton (negative), and a variety of mesozooplanktonic taxa, including copepoda (mixed), appendicularia (positive), hydrozoa (positive), fish larvae (positive), and gastropoda (negative). The restructuring of the community coincided with significant changes in the concentration and elemental stoichiometry of particulate organic matter. Results imply that extreme CO2 events can lead to a substantial reorganization of the planktonic food web, affecting multiple trophic levels from phytoplankton to primary and secondary consumers.

Continue reading ‘Extreme levels of ocean acidification restructure the plankton community and biogeochemistry of a temperate coastal ecosystem: a mesocosm study’

Adaptive responses of free‐living and symbiotic microalgae to simulated future ocean conditions

Marine microalgae are a diverse group of microscopic eukaryotic and prokaryotic organisms capable of photosynthesis. They are important primary producers and carbon sinks but their physiology and persistence are severely affected by global climate change. Powerful experimental evolution technologies are being used to examine the potential of microalgae to respond adaptively to current and predicted future conditions, as well as to develop resources to facilitate species conservation and restoration of ecosystem functions. This review synthesizes findings and insights from experimental evolution studies of marine microalgae in response to elevated temperature and/or pCO2. Adaptation to these environmental conditions has been observed in many studies of marine dinoflagellates, diatoms and coccolithophores. An enhancement in traits such as growth and photo‐physiological performance and an increase in upper thermal limit have been shown to be possible, although the extent and rate of change differ between microalgal taxa. Studies employing multiple monoclonal replicates showed variation in responses among replicates and revealed the stochasticity of mutations. The work to date is already providing valuable information on species’ climate sensitivity or resilience to managers and policy‐makers but extrapolating these insights to ecosystem and community level impacts continues to be a challenge. We recommend future work should include in situ experiments, diurnal and seasonal fluctuations, multiple drivers and multiple starting genotypes. Fitness trade‐offs, stable versus plastic responses and the genetic bases of the changes also need investigating, and the incorporation of genome resequencing into experimental designs will be invaluable.

Continue reading ‘Adaptive responses of free‐living and symbiotic microalgae to simulated future ocean conditions’

Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean

Highlights

  • Seawater MeHg may increase in the polar oceans and decrease in the North Atlantic in 2100
  • Plankton MeHg may increase at high latitudes and decrease at mid to low latitudes
  • Ocean acidification leads to different spatial patterns compared with physical factors

Summary

Climate change-driven alterations to marine biogeochemistry will impact the formation and trophic transfer of the bioaccumulative neurotoxin methylmercury (MeHg) in the global ocean. We use a 3D model to examine how MeHg might respond to changes in primary production and plankton community driven by ocean acidification and alterations in physical factors (e.g., ocean temperature, circulation). Productivity changes lead to significant increases in seawater MeHg in the polar oceans and a decrease in the North Atlantic Ocean. Phytoplankton MeHg may increase at high latitudes and decrease in lower latitudes due to shifts in community structure. Ocean acidification might enhance phytoplankton MeHg uptake by promoting the growth of a small species that efficiently accumulate MeHg. Non-linearities in the food web structure lead to differing magnitudes of zooplankton MeHg changes relative to those for phytoplankton. Climate-driven shifts in marine biogeochemistry thus need to be considered when evaluating future trajectories in biological MeHg concentrations.

Continue reading ‘Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean’

Photosynthesis and calcification of the coccolithophore Emiliania huxleyi are more sensitive to changed levels of light and CO2 under nutrient limitation

Highlights

  • Nutrient limitation reduced the light intensity for cells to achieve the highest rates of photosynthesis and calcification.
  • Nitrate limitation enhanced calcification rate and phosphate limitation reduced photosynthetic rate.
  • Electron transport rate linearly and positively correlated with rates of photosynthesis and calcification.

Abstract

Photophysiological responses of phytoplankton to changing multiple environmental drivers are essential in understanding and predicting ecological consequences of ocean climate changes. In this study, we investigated the combined effects of two CO2 levels (410 and 925 μatm) and five light intensities (80 to 480 μmol photons m−2 s−1) on cellular pigments contents, photosynthesis and calcification of the coccolithophore Emiliania huxleyi grown under nutrient replete and limited conditions, respectively. Our results showed that high light intensity, high CO2 level and nitrate limitation acted synergistically to reduce cellular chlorophyll a and carotenoid contents. Nitrate limitation predominantly enhanced calcification rate; phosphate limitation predominantly reduced photosynthetic carbon fixation rate, with larger extent of the reduction under higher levels of CO2 and light. Reduced availability of both nitrate and phosphate under the elevated CO2 concentration decreased saturating light levels for the cells to achieve the maximal relative electron transport rate (rETRmax). Light-saturating levels for rETRmax were lower than that for photosynthetic and calcification rates under the nutrient limitation. Regardless of the culture conditions, rETR under growth light levels correlated linearly and positively with measured photosynthetic and calcification rates. Our findings imply that E. huxleyi cells acclimated to macro-nutrient limitation and elevated CO2 concentration decreased their light requirement to achieve the maximal electron transport, photosynthetic and calcification rates, indicating a photophysiological strategy to cope with CO2 rise/pH drop in shoaled upper mixing layer above the thermocline where the microalgal cells are exposed to increased levels of light and decreased levels of nutrients.

Continue reading ‘Photosynthesis and calcification of the coccolithophore Emiliania huxleyi are more sensitive to changed levels of light and CO2 under nutrient limitation’

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