Posts Tagged 'photosynthesis'

Co-culture in marine farms: macroalgae can act as chemical refuge for shell-forming molluscs under an ocean acidification scenario

With ongoing climate change, aquaculture faces environmental challenges similar to those of natural ecosystems. These include increasing stress for calcifying species, e.g. macroalgae and shellfish. In this context, ocean acidification (OA) has the potential to affect important socioeconomic activities, including shellfish aquaculture, due to changes in the seawater carbonate system. However, coastal environments are characterised by strong diurnal pH fluctuations associated with the metabolic activity of macroalgae; that is, photosynthesis and respiration. This suggests that calcifying organisms that inhabit these ecosystems are adapted to this fluctuating pH environment. Macrophyte-dominated environments may have the potential to act as an OA buffering system in the form of a photosynthetic footprint, by reducing excess of CO2 and increasing the seawater pH and Ωarg. This can support calcification and other threatened physiological processes of calcifying organisms under a reduced pH environment. Because this footprint is supportive beyond the macroalgal canopy spatial area, this chemical refuge mechanism can be applied to support shellfish aquaculture, e.g. mussels. However, this approach should be tested in commercial shellfish farms to determine critical aspects of implementation. This includes critical factors such as target species and productivity rates. The degree of OA buffering capacity caused by the metabolic activity of macroalgae might depend on community structure and hydrodynamic conditions, creating site-specific responses. This concept might aid the development of future adaptive strategies, supporting marine ecological planning for the mussel aquaculture industry in Chile.

Continue reading ‘Co-culture in marine farms: macroalgae can act as chemical refuge for shell-forming molluscs under an ocean acidification scenario’

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’

Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, scientists are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR) remains entirely unknown. Here we describe, for the first time, 2 extreme GBR mangrove lagoons (Woody Isles and Howick Island), exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (7°C) conditions. Coral cover was typically low (0.5 m diameter), with net photosynthesis and calcification rates of 2 dominant coral species (Acropora millepora, Porites lutea) reduced (20-30%), and respiration enhanced (11-35%), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic ‘strategy’) and flexibility of Symbiodiniaceae taxa associations appear crucial in supporting coral capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress tolerance.

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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.

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The combined effects of ocean acidification with fleshy macroalgae and filamentous turfs on tropical crustose coralline algae

Global climate change induces multiple stressors on tropical coral reefs that threaten their persistence. Ocean acidification decreases calcification in most dominant reef builders, such as crustose coralline algae (CCA). Climate change also has the potential to increase the biomass of fleshy macroalgae and filamentous turf in coral reef ecosystems. While fleshy macroalgae and turf may shade, abrade, and have otherwise negative consequences on CCA metabolism, their high rates of photosynthesis may mitigate OA locally through carbon uptake, resulting in a local increase in pH. This thesis explored the effects of OA, combined with the presence of either fleshy macroalgae or algal turfs, on Lithophyllum kotschyanum, an abundant species of CCA in Moorea, French Polynesia. In a mesocosm study, three canopy types, clear mimics, dark mimics, and S. pacificum, were crossed with two CO2 levels, ambient (400 μatm) and elevated (1000 μatm). The clear, dark, and S. pacificum canopies resulted in stepwise decreases in calcification of L. kotschyanum. This response suggests that shading and likely flow moderation decrease CCA calcification. To separate the effects of fleshy macroalgal metabolism from the effects of its physical structure, a subsequent mesocosm and field experiment were performed. In the mesocosm study, a header tank that provided S. pacificum-treated seawater to treatment tanks was used to determine the metabolic effect of S. pacificum on L. kotschyanum. In the field, S. pacificum canopies were attached to 20  30 cm grids, upstream from CCA samples. Data from the mesocosm study support a positive effect of carbon uptake by S. pacificum, but the metabolic effect did not translate into the field. Because S. pacificum was placed in closer proximity to CCA samples in the field than in lab, the difference in L. kotschyanum calcification between the mesocosm and field experiment may be due to physical effects of the canopy in the field, such as shading. The combined results of these two studies suggest that upstream macroalgal communities have the potential to mitigate the negative effects of OA to downstream calcifiers, but will not benefit understory calcifiers. Finally, a mesocosm experiment was conducted to address the combined effects of OA and the presence of epiphytic turf algae on host CCA. In a factorial experiment, L. kotschyanum samples with and without epiphytic turf algae were placed in flow through tanks where pCO2 was ambient (400 μatm) or elevated (1000 μatm). Results indicated a significant effect of elevated pCO2 on CCA calcification and a negative effect of turf presence, despite a higher pH in the presence of turf during light incubations. This indicates that any benefit of higher daytime pH within the DBL of L. kotschyanum was outweighed by the negative effects, such as shading, nighttime anoxia and low pH. Overall, these studies indicate that fleshy macroalgae and filamentous turfs can raise seawater pH locally, but any benefit of this effect is outweighed by the negative effects of fleshy macroalgae and turf presence. The only instance during which CCA may incur a net benefit from fleshy macroalgae occurs when calcifiers are situated downstream of a dense macroalgal community, entirely unaffected by its physical structure. Ultimately, fleshy macroalgae and turf affect CCA negatively, regardless of OA treatment.

Continue reading ‘The combined effects of ocean acidification with fleshy macroalgae and filamentous turfs on tropical crustose coralline algae’

Acidification diminishes diatom silica production in the Southern Ocean

Diatoms, large bloom-forming marine microorganisms, build frustules out of silicate, which ballasts the cells and aids their export to the deep ocean. This unique physiology forges an important link between the marine silicon and carbon cycles. However, the effect of ocean acidification on the silicification of diatoms is unclear. Here we show that diatom silicification strongly diminishes with increased acidity in a natural Antarctic community. Analyses of single cells from within the community reveal that the effect of reduced pH on silicification differs among taxa, with several species having significantly reduced silica incorporation at CO2 levels equivalent to those projected for 2100. These findings suggest that, before the end of this century, ocean acidification may influence the carbon and silicon cycle by both altering the composition of the diatom assemblages and reducing cell ballasting, which will probably alter vertical flux of these elements to the deep ocean.

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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.

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Ocean acidification in the IPCC AR5 WG II

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