Posts Tagged 'light'

Photosynthesis and light-dependent proton pumps increase boundary layer pH in tropical macroalgae: a proposed mechanism to sustain calcification under ocean acidification


• Tropical calcifying macroalgae raise boundary layer pH due to photosynthesis

• Different mechanisms of inorganic carbon uptake control pH increase

• Proton pumps in light without photosynthesis appear important calcification

• Photosynthesis-driven pH increase allows calcification with ocean acidification


Ocean acidification (OA) projections predict ocean pH to decline between 0.2 and 0.4 by 2100 with potential negative consequences for marine calcifiers without acclimation or adaption strategies to accomodate greater [H+] in seawater. Biotic control of calcified reef macroalgae thalli surface diffusive boundary layer (DBL) chemistry may overcome low pH in seawater as one strategy to accommodate OA conditions. To investigate this strategy, we examined surface DBL O2 and pH dynamics in five calcifying macroalgae (Halimeda, Udotea, Jania, Neogoniolithon, crustose coralline algae [CCA]) from the Florida Reef Tract under ambient (8.1) and low (7.65) pH using microsensors (100 μm) at the thalli surface in a flow-through flume. The role of photosynthesis and photosystem II (PSII)-independent proton pumps in controlling DBL pH were examined. Four of the five macroalgae exhibited a strong positive linear relationship between O2 production and increasing pH in the first 15–30 s of irradiance. Once a quasi-steady-state O2 concentration was reached (300 s), all species had DBL pH that were higher (0.02–0.32) than bulk seawater. The DBL pH increase was greatest at low pH and dependent on PSII. Some evidence was found for a light-dependent, but PSII-independent, proton pump. High DBL Δ pH upon illumination was likely in response to carbon concentrating mechanisms (CCMs) for photosynthesis. CCMs may be a HCO3−–H+ symport, OH– antiport or other DIC transport system, accompanied by proton efflux. HCO3– dehydration by external carbonic anhydrase (CAext) also produces OH– that can neutralize H+ in the DBL. CO2 or HCO3– uptake for photosynthesis may also engage H+/OH– fluxes as part of intracellular acid-base regulation changing DBL pH. A higher Δ pH within the DBL at low pH could be accounted for by greater CO2 diffusion and/or lower efficiencies in exporting cellular H+ across a lower concentration gradient, and/or a more efficient removal of H+ by CAext-driven dehydration of HCO3−. In the dark, Δ pH was less than in the light as these dynamics were primarily due to photosynthesis. We present a conceptual model of inorganic carbon uptake and ion transport pathways, as well as other processes associated with photosynthesis that drive DBL Δ pH and sustain tropical macroalgal calcification in the light under OA. In the dark, unless PSII-independent proton pumps are present, which do not appear to be ubiquitous amongst species, acidification processes likely dominate, resulting in CaCO3 net dissolution, particularly under OA conditions.

Continue reading ‘Photosynthesis and light-dependent proton pumps increase boundary layer pH in tropical macroalgae: a proposed mechanism to sustain calcification under ocean acidification’

Intercomparison of four methods to estimate coral calcification under various environmental conditions

Coral reefs are constructed by calcifiers that precipitate calcium carbonate to build their shells or skeletons through the process of calcification. Accurately assessing coral calcification rates is crucial to determine the health of these ecosystems and their response to major environmental changes such as ocean warming and acidification. Several approaches have been used to assess rates of coral calcification but there is a real need to compare these approaches in order to ascertain that high quality and intercomparable results can be produced. Here, we assessed four methods (total alkalinity anomaly, calcium anomaly, 45Ca incorporation and 13C incorporation) to determine coral calcification of the reef-building coral Stylophora pistillata. Given the importance of environmental conditions on this process, the study was performed under two pH (ambient and low level) and two light (light and dark) conditions. Under all conditions, calcification rates estimated using the alkalinity and calcium anomaly techniques as well as 45Ca incorporation were highly correlated. Such a strong correlation between the alkalinity anomaly and 45Ca incorporation techniques has not been observed in previous studies and most probably results from improvements described in the present paper. The only method which provided calcification rates significantly different from the other three techniques was 13C incorporation. Calcification rates based on this method were consistently higher than those measured using the other techniques. Although reasons for these discrepancies remain unclear, the use of this technique for assessing calcification rates in corals is not recommended without further investigations.

Continue reading ‘Intercomparison of four methods to estimate coral calcification under various environmental conditions’

Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification

Natural variability in pH in the diffusive boundary layer (DBL), the discrete layer of seawater between bulk seawater and the outer surface of organisms, could be an important factor determining the response of corals and coralline algae to ocean acidification (OA). Here, two corals with different morphologies and one coralline alga were maintained under two different regimes of flow velocities, pH, and light intensities in a 12 flumes experimental system for a period of 27 weeks. We used a combination of geochemical proxies, physiological and micro-probe measurements to assess how these treatments affected the conditions in the DBL and the response of organisms to OA. Overall, low flow velocity did not ameliorate the negative effect of low pH and therefore did not provide a refugia from OA. Flow velocity had species-specific effects with positive effects on calcification for two species. pH in the calcifying fluid (pHcf) was reduced by low flow in both corals at low light only. pHcf was significantly impacted by pH in the DBL for the two species capable of significantly modifying pH in the DBL. The dissolved inorganic carbon in the calcifying fluid (DICcf) was highest under low pH for the corals and low flow for the coralline, while the saturation state in the calcifying fluid and its proxy (FWHM) were generally not affected by the treatments. This study therefore demonstrates that the effects of OA will manifest most severely in a combination of lower light and lower flow habitats for sub-tropical coralline algae. These effects will also be greatest in lower flow habitats for some corals. Together with existing literature, these findings reinforce that the effects of OA are highly context dependent, and will differ greatly between habitats, and depending on species composition.

Continue reading ‘Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification’

The influence of paleo-seawater chemistry on foraminifera trace element proxies and their application to deep-time paleo-reconstructions

The fossilized remains of the calcite shells of foraminifera comprise one of the most continuous and reliable records of the geologic evolution of climate and ocean chemistry. The trace elemental composition of foraminiferal shells has been shown to systematically respond to seawater properties, providing a way to reconstruct oceanic conditions throughout the last 170 million years. In particular, the boron/calcium ratio of foraminiferal calcite (B/Ca) is an emerging proxy for the seawater carbonate system, which plays a major role in regulating atmospheric CO2 and thus Earth’s climate. In planktic foraminifera, previous culture studies have shown that shell B/Ca increases with seawater pH, which is hypothesized to result from increased incorporation of borate ion (B(OH)4 -) at high pH; increasing pH increases the [B(OH)4 -] of seawater. However, further experiments showed that B/Ca responds to both pH and seawater dissolved inorganic carbon concentration (DIC), leading to the hypothesis that B/Ca is driven by the [B(OH)4 -/DIC] ratio of seawater. Because pH (and thus B(OH)4 -) can be determined via the δ11B composition of foraminiferal calcite, B/Ca therefore may provide an opportunity to determine seawater DIC in the geologic past.

Continue reading ‘The influence of paleo-seawater chemistry on foraminifera trace element proxies and their application to deep-time paleo-reconstructions’

The Arctic picoeukaryote Micromonas pusilla benefits from ocean acidification under constant and dynamic light

Compared to the rest of the globe, the Arctic Ocean is affected disproportionately by climate change. Despite these fast environmental changes, we currently know little about the effects of ocean acidification (OA) on marine key species in this area. Moreover, the existing studies typically test the effects of OA under constant, hence artificial light fields. In this study, the abundant Arctic picoeukaryote Micromonas pusilla was acclimated to current (400 μatm) and future (1000 μatm) pCO2 levels under a constant as well as dynamic light, simulating natural light fields as experienced in the upper mixed layer. To describe and understand the responses to these drivers, growth, particulate organic carbon (POC) production, elemental composition, photophysiology and reactive oxygen species (ROS) production were analysed. M. pusilla was able to benefit from OA on various scales, ranging from an increase in growth rates to enhanced photosynthetic capacity, irrespective of the light regime. These beneficial effects were, however, not reflected in the POC production rates, which can be explained by energy partitioning towards cell division rather than biomass build-up. In the dynamic light regime, M. pusilla was able to optimise its photophysiology for effective light usage during both low and high light periods. This effective photoacclimation, which was achieved by modifications to photosystem II (PSII), imposed high metabolic costs leading to a reduction in growth and POC production rates when compared to constant light. There were no significant interactions observed between dynamic light and OA, indicating that M. pusilla was able maintain effective photoacclimation without increased photoinactivation under high pCO2. Based on these findings, physiologically plastic M. pusilla may exhibit a robust positive response to future Arctic Ocean conditions.

Continue reading ‘The Arctic picoeukaryote Micromonas pusilla benefits from ocean acidification under constant and dynamic light’

Effects of spectral light quality and carbon dioxide on the physiology of Micractinium inermum: growth, photosynthesis, and biochemical composition

Growth, photosynthetic parameters, and biochemical composition of the chlorophyte Micractinium inermum were determined under conditions of different light-emitting diode (LED) wavelength distributions and carbon dioxide (CO2) levels of 1, 5, and 10% v/v. Increasing the inflowing CO2 concentration from 1 to 5 to 10% had negative effects on maximum growth rate (μmax), relative growth rate (RGR), biomass production, and CO2 fixation rate. Cultures subjected to a spectral distribution containing a blue wavelength peak were less negatively affected than red as CO2 stress increased, and evidently there was a quantitative effect depending on the peak area percentage. Under non-stressful conditions (1% CO2), there were no significant differences in μmax or RGR among light treatments; however, blue light resulted in decreased lag phases (0.75 less days than red). Biomass production was significantly higher in red wavelength peak treatments at 1% CO2 (i.e., monochromatic red treatment = 0.772 ± 0.07 g L−1) which demonstrates that although blue light allows for an early growth advantage, cultures grown with red light are able to catch up and result in more biomass. At a 10% CO2 level, RGR was higher in treatments with blue peaks; red peak treatments were no longer able to overcome the stress and demonstrated lag phases 4.87 days longer than blue peak treatments. Inhibition of photosystem II function was evident as CO2 concentrations increased. Evaluation of biochemical composition revealed that protein content was significantly greater in blue peak treatments at 10% CO2, pigment content was up to 2× greater than red at all CO2 levels, and total FAME and fatty acid profiles did not exhibit major changes.

Continue reading ‘Effects of spectral light quality and carbon dioxide on the physiology of Micractinium inermum: growth, photosynthesis, and biochemical composition’

Differential photosynthetic response of a green tide alga Ulva linza to ultraviolet radiation, under short‐ and long‐term ocean acidification regimes

Both ocean acidification (OA) and solar ultraviolet (UV) radiation can bring about changes in macroalgal physiological performance. However, macroalgal responses to UV radiation when acclimatized to OA under different time scales are rare. Here, we investigate the response of Ulva linza, a green tide alga, to UV radiation in the form of photosynthetically active radiation (PAR) or PAB (PAR+UVA+UVB) radiation. Radiation exposures were assessed following long‐term (from spore to adult stage, 1 month) and short‐term (adult stage, 1 week) OA treatments. Results showed that increased CO2 decreased the damage rate (k) and repair rate (r) of thalli grown under short‐term OA conditions with PAB treatment, the ratio of r:k was not altered. Following long‐term OA conditions, r was not affected, although k was increased in thalli following PAB treatment, resulting in a reduced ratio of r:k. The relative level of UV inhibition increased and UV‐absorbing compounds decreased when algae were cultured under long‐term OA conditions. The recovery rate of thalli was enhanced when grown under long‐term OA after UV radiation treatment. These results show that blooming algae may be more sensitive to UV radiation in marine environments, but it can develop effective mechanisms to offset the negative effects, reflecting acclimation to long‐term OA conditions.

Continue reading ‘Differential photosynthetic response of a green tide alga Ulva linza to ultraviolet radiation, under short‐ and long‐term ocean acidification regimes’

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Ocean acidification in the IPCC AR5 WG II

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