Posts Tagged 'light'

Diel transcriptional oscillations of a plastid antiporter reflect increased resilience of Thalassiosira pseudonana in elevated CO2

Acidification of the ocean due to high atmospheric CO2 levels may increase the resilience of diatoms causing dramatic shifts in abiotic and biotic cycles with lasting implications on marine ecosystems. Here, we report a potential bioindicator of a shift in the resilience of a coastal and centric model diatom Thalassiosira pseudonana under elevated CO2. Specifically, we have discovered, through EGFP-tagging, a plastid membrane localized putative Na+(K+)/H+ antiporter that is significantly upregulated at >800 ppm CO2, with a potentially important role in maintaining pH homeostasis. Notably, transcript abundance of this antiporter gene was relatively low and constant over the diel cycle under contemporary CO2 conditions. In future acidified oceanic conditions, dramatic oscillation with >10-fold change between nighttime (high) and daytime (low) transcript abundances of the antiporter was associated with increased resilience of T. pseudonana. By analyzing metatranscriptomic data from the Tara Oceans project, we demonstrate that phylogenetically diverse diatoms express homologs of this antiporter across the globe. We propose that the differential between night- and daytime transcript levels of the antiporter could serve as a bioindicator of a shift in the resilience of diatoms in response to high CO2 conditions in marine environments.

Continue reading ‘Diel transcriptional oscillations of a plastid antiporter reflect increased resilience of Thalassiosira pseudonana in elevated CO2’

Increased light availability modulates carbon and nitrogen accumulation in the macroalga Gracilariopsis lemaneiformis (Rhodophyta) in response to ocean acidification


  • The effects of light and elevated pCO2 on Gracilariopsis were examined.
  • Ocean acidification enhanced algal biomass, photosynthesis and total C/N ratios.
  • Increasing light and elevated pCO2 lowered nutritional quality of G. lemaneiformis.


The economically important red macroalga Gracilariopsis lemaneiformis has demonstrated positive ecological functions in nutrient bioextraction efficiency and high harvestable biomass, as well as being a food and agar source owing to its richness in proteins and polysaccharides. Carbon dioxide (CO2)-induced ocean acidification has resulted in mixed nutrient compound accumulations in this marine autotroph. G. lemaneiformis also experiences light variations resulting from self-shading and varied cultivation depths. Therefore, a factorial coupling experiment was conducted to examine how growth, photosynthesis performance, soluble cell components and metabolic enzyme-driven activities respond to light availability changes and CO2 enrichment. The ocean acidification enhanced the growth characteristics, total carbon/nitrogen ratios and metabolic nutrient accumulation processes in G. lemaneiformis regardless of the light level. Photosynthetic performances, including relative electron transport rate and maximum photochemical quantum yield, were increased by high pCO2 concentrations, resulting in soluble carbohydrate accumulation. The carbon and nitrogen accumulations might result from variations in carbonic anhydrase and nitrate reductase activities under high pCO2 conditions. The soluble protein and free amino acids contents declined in response to CO2 elevation, and this effect was more pronounced as the light intensity increased. Thus, future climate changes may cause greater algal biomass accumulations, but they may negatively affect the cell composition and nutritional quality of G. lemaneiformis.

Continue reading ‘Increased light availability modulates carbon and nitrogen accumulation in the macroalga Gracilariopsis lemaneiformis (Rhodophyta) in response to ocean acidification’

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)’

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


  • 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.


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’

Ocean acidification interacts with growth light to suppress CO2 acquisition efficiency and enhance mitochondrial respiration in a coastal diatom


  • Ocean acidification (OA) enhances growth of Thalassiosira weissflogii only at limiting low light levels.
  • The energy saved from down-regulation of CCMs under OA rather than “CO2 fertilization aids in the enhancement under low levels of light energy supply.
  • Coastal diatoms can benefit from OA, especially under cloudy weather or conditions of low solar exposures.


Diatom responses to ocean acidification have been documented with variable and controversial results. We grew the coastal diatom Thalassiosira weissflogii under 410 (LC, pH 8.13) vs 1000 μatm (HC, pH 7.83) pCO2 and at different levels of light (80, 140, 220 μmol photons m−2 s−1), and found that light level alters physiological responses to OA. CO2 concentrating mechanisms (CCMs) were down-regulated in the HC-grown cells across all the light levels, as reflected by lowered activity of the periplasmic carbonic anhydrase and decreased photosynthetic affinity for CO2 or dissolved inorganic carbon. The specific growth rate was, however, enhanced significantly by 9.2% only at the limiting low light level. These results indicate that rather than CO2 “fertilization”, the energy saved from down-regulation of CCMs promoted the growth rate of the diatom when light availability is low, in parallel with enhanced respiration under OA to cope with the acidic stress by providing extra energy.

Continue reading ‘Ocean acidification interacts with growth light to suppress CO2 acquisition efficiency and enhance mitochondrial respiration in a coastal diatom’

Irradiance, photosynthesis and elevated pCO2 effects on net calcification in tropical reef macroalgae


  • Most species from high-light environments are not able to calcifying under OA at night
  • Low-light species may be more susceptible to OA compared to high-light
  • Some species exhibit light-triggered calcification independent of photosystem II
  • Photosystem II independent calcification not sustained under OA


Calcifying tropical macroalgae produce sediment, build three-dimensional habitats, and provide substrate for invertebrate larvae on reefs. Thus, lower calcification rates under declining pH and increasing ocean pCO2, or ocean acidification, is a concern. In the present study, calcification rates were examined experimentally under predicted end-of-the-century seawater pCO2 (1116 μatm) and pH (7.67) compared to ambient controls (pCO2 409 μatm; pH 8.04). Nine reef macroalgae with diverse calcification locations, calcium carbonate structure, photophysiology, and site-specific irradiance were examined under light and dark conditions. Species included five from a high light patch reef on the Florida Keys Reef Tract (FKRT) and four species from low light reef walls on Little Cayman Island (LCI). Experiments on FKRT and LCI species were conducted at 500 and 50 μmol photons m−2 s−1 in situ irradiance, respectively. Calcification rates independent of photosystem-II (PSII) were also investigated for FKRT species. The most consistent negative effect of elevated pCO2 on calcification rates in the tropical macroalgae examined occurred in the dark. Most species (89%) had net calcification rates of zero or net dissolution in the dark at low pH. Species from the FKRT that sustained positive net calcification rates in the light at low pH also maintained ~30% of their net calcification rates without PSII at ambient pH. However, calcification rates in the light independent of PSII were not sustained at low pH. Regardless of these low pH effects, most FKRT species daily net calcification rates, integrating light/dark rates over a 24h period, were not significantly different between low and ambient pH. This was due to a 10-fold lower dark, compared to light, calcification rate, and a strong correspondence between calcification and photosynthetic rates. Interestingly, low-light species sustained calcification rates on par with high-light species without high rates of photosynthesis. Low-light species’ morphology and physiology that promote high calcification rates at ambient pH, may increase their vulnerability to low pH. Our data indicate that the negative effect of elevated pCO2 and low pH on tropical macroalgae at the organismal level is their impact on dark net calcification, probably enhanced dissolution. However, elevated pCO2 and low pH effects on macroalgae daily calcification rates are greatest in species with lower net calcification rates in the light. Thus, macroalgae able to maintain high calcification rates in the light (high and low irradiance) at low pH, and/or sustain strong biotic control with high [H+] in the bulk seawater, are expected to dominate under global change.

Continue reading ‘Irradiance, photosynthesis and elevated pCO2 effects on net calcification in tropical reef macroalgae’

Diffusive boundary layers and ocean acidification: implications for sea urchin settlement and growth

Chemical changes in the diffusive boundary layer (DBL) generated by photosynthesising macroalgae are expected to play an important role in modulating the effects of ocean acidification (OA), but little is known about the effects on early life stages of marine invertebrates in modified DBLs. Larvae that settle to macroalgal surfaces and remain within the DBL will experience pH conditions markedly different from the bulk seawater. We investigated the interactive effects of seawater pH and DBL thickness on settlement and early post-settlement growth of the sea urchin Pseudechinus huttoni, testing whether coralline-algal DBLs act as an environmental buffer to OA. DBL thickness and pH levels (estimated from well-established relationships with oxygen concentration) above the crustose coralline algal surfaces varied with light availability (with photosynthesis increasing pH to as high as pH 9.0 and respiration reducing pH to as low as pH 7.4 under light and dark conditions, respectively), independent of bulk seawater pH (7.5, 7.7, and 8.1). Settlement success of P. huttoni increased over time for all treatments, irrespective of estimated pH in the DBL. Juvenile test growth was similar in all DBL manipulations, showing resilience to variable and low seawater pH. Spine development, however, displayed greater variance with spine growth being negatively affected by reduced seawater pH in the DBL only in the dark treatments. Scanning electron microscopy revealed no observable differences in structural integrity or morphology of the sea urchin spines among pH treatments. Our results suggest that early juvenile stages of P. huttoni are well adapted to variable pH regimes in the DBL of macroalgae across a range of bulk seawater pH treatments.

Continue reading ‘Diffusive boundary layers and ocean acidification: implications for sea urchin settlement and growth’

Coral-macroalgal competition under ocean warming and acidification


  • Study investigates a common coral-macroalgal interaction under a low end emission scenario.
  • Light calcification is negatively influenced by an interaction of macroalgal contact and scenario.
  • Protein content, zooxanthellae density and Chlorophyll a were enhanced under scenario conditions.
  • Negative impacts of macroalgae on corals were observed, but not enhanced by scenario conditions.
  • More research on the impacts of climate change on the dynamics of coral-algal interactions is needed.


Competition between corals and macroalgae is frequently observed on reefs with the outcome of these interactions affecting the relative abundance of reef organisms and therefore reef health. Anthropogenic activities have resulted in increased atmospheric CO2 levels and a subsequent rise in ocean temperatures. In addition to increasing water temperature, elevated CO2 levels are leading to a decrease in oceanic pH (ocean acidification). These two changes have the potential to alter ecological processes within the oceans, including the outcome of competitive coral-macroalgal interactions. In our study, we explored the combined effect of temperature increase and ocean acidification on the competition between the coral Porites lobata and on the Great Barrier Reef abundant macroalga Chlorodesmis fastigiata. A temperature increase of +1 °C above present temperatures and CO2 increase of +85 ppm were used to simulate a low end emission scenario for the mid- to late 21st century, according to the Representative Concentration Pathway 2.6 (RCP2.6). Our results revealed that the net photosynthesis of P. lobata decreased when it was in contact with C. fastigiata under ambient conditions, and that dark respiration increased under RCP2.6 conditions. The Photosynthesis to Respiration (P:R) ratios of corals as they interacted with macroalgal competitors were not significantly different between scenarios. Dark calcification rates of corals under RCP2.6 conditions, however, were negative and significantly decreased compared to ambient conditions. Light calcification rates were negatively affected by the interaction of macroalgal contact in the RCP2.6 scenario, compared to algal mimics and to coral under ambient conditions. Chlorophyll a, and protein content increased in the RCP2.6 scenario, but were not influenced by contact with the macroalga. We conclude that the coral host was negatively affected by RCP2.6 conditions, whereas the productivity of its symbionts (zooxanthellae) was enhanced. While a negative effect of the macroalga (C. fastigiata) on the coral (P. lobata) was observed for the P:R ratio under control conditions, it was not enhanced under RCP2.6 conditions.

Continue reading ‘Coral-macroalgal competition under ocean warming and acidification’

Effects of warming and CO2 enrichment on O2 consumption, porewater oxygenation and pH of subtidal silt sediment

We investigated the effects of seawater warming and CO2 enrichment on the microbial community metabolism (using O2 consumption as a proxy) in subtidal silt sediment. Intact sediment cores, without large dwelling infauna, were incubated for 24 days at 12 (in situ) and 18 °C to confirm the expected temperature response. We then enriched the seawater overlying a subset of cold and warm-incubated cores with CO2 (+ ΔpCO2: 253–396 µatm) for 16 days and measured the metabolic response. Warming increased the depth-integrated volume-specific O2 consumption (Rvol), the maximum in the volume-specific O2 consumption at the bottom of the oxic zone (Rvol,bmax) and the volume-specific net O2 production (Pn,vol), and decreased the O2 penetration depth (O2-pd) and the depth of Rvol,bmax (depthbmax). Benthic photosynthesis oscillated the pH in the upper 2 mm of the sediment. CO2 enrichment of the warm seawater did not alter this oscillation but shifted the pH profile towards acidity; the effect was greatest at the surface and decreased to a depth of 12 mm. Confoundment rendered the CO2 treatment of the cold seawater inconclusive. In warm seawater, we found no statistically clear effect of CO2 enrichment on RvolRvol,bmaxPn,vol, O2-pd, or depthbmax and therefore suspect that this perturbation did not alter the microbial community metabolism. This confirms the conclusion from experiments with other, contrasting types of sediment.

Continue reading ‘Effects of warming and CO2 enrichment on O2 consumption, porewater oxygenation and pH of subtidal silt sediment’

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Ocean acidification in the IPCC AR5 WG II

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