Posts Tagged 'algae'

Effects of ocean acidification and eutrophication on the macroalgae Ulva spp.

Ocean acidification is the increased absorption of atmospheric CO2 in seawater and the consequent decrease in pH. This phenomenon is occurring throughout the global oceans while land use changes and large human populations near coasts are linked to increased nutrient concentrations in seawater. Ulva spp. blooms caused by nutrient enrichment occur regularly in some parts of the world and are known as green tides. There is concern that ocean acidification may increase green tides and intensify ecological and economic damages. Ulva spp. can utilize bicarbonate (HCO3-) as an inorganic carbon source, but this comes at an energetic cost as HCO3- must be converted to CO2 before it can be used for carbon fixation. Therefore, increased utilization of pCO2 with ocean acidification may benefit Ulva spp. Ocean acidification and eutrophication will occur simultaneously in many coastal ecosystems. The goal of the following investigations was to determine the effects of ocean acidification and nutrient enrichment alone and their interaction on photosynthetic, nutrient, and growth physiology of Ulva spp. In Chapter 2, the response of Ulva australis to pHT and ammonium (NH4+) enrichment were investigated in a seven day growth experiment using a range of pHT (7.56 – 7.84) and ambient and enriched NH4+ concentrations. I measured relative growth rates (RGRs), NH4+ uptake rates and pools, photosynthetic characteristics, and tissue carbon and nitrogen content. There was no interaction of pHT and NH4+ enrichment on the physiological parameters. The RGR was not affected by pHT, but was an average of two times higher in the enriched NH4+ treatment. rETRmax, total chlorophyll, and tissue nitrogen increased with both NH4+ enrichment and decreased pHT. The C:N ratio decreased with decreasing pH and with NH4+ enrichment. Although rETRmax increased and the C:N ratio decreased under decreased pH, these metabolic changes did not translate to higher growth rates. The results show that U. australis growth and physiology is more sensitive to NH4+ than it is to pH and that there is no interactive effect of NH4+ enrichment and decreasing pH. In Chapter 3, Ulva lactuca was grown for 22 days under a range of pCO2 and NH4+ concentrations and a multiple linear regression was used to analyze RGRs, NH4+ and NO3- pools, in situ NH4+ and NO3- uptake rates, tissue carbon and nitrogen content, carbohydrate and protein concentrations, and photosynthesis irradiance curves (P-I curves). The results from model selection and model-averaging techniques allowed me to make predictive models across a range of relevant ocean acidification and eutrophication scenarios and measure the effect sizes of pCO2, NH4+ enrichment, and their interaction. Overall, there was no effect of pCO2 and NH4+ on RGRs after day 5. However, there was a synergistic effect of pCO2 and NH4+ enrichment on the growth rates during days 0 – 5. When pCO2 and NH4+ concentrations increased simultaneously, NO3- uptake rates increased, which may have contributed to increased growth as seen in days 0 – 5. Maximum photosynthetic rates (Pmax) decreased with increasing pCO2 and there was a positive interaction of pCO2 and NH4+ on indicating CCMs were altered under these conditions. This shows that under high light intensities, Pmax was negatively affected by pCO2 and CCMs are not altered when nutrients are limited. Ultimately, there was no longer-term effect of ocean acidification and eutrophication on Ulva lactuca growth. Nutrient enrichment is a major cause of green tide blooms around the world and Ulva australis had the ability to enhance nutrient, photosynthetic, and growth physiology with NH4+ enrichment. Conversely, Ulva lactuca collected from a eutrophic environment, did not respond to NH4+ in terms of growth. Both chapters provided evidence that ocean acidification is unlikely to affect the growth rates of Ulva spp. However, the exception was a positive interactive effect of pCO2 and NH4+ enrichment on the growth rate of U. lactuca during the first five days, suggesting ocean acidification could play a role in initiating Ulva spp. blooms in a eutrophic environment. This could be an important consideration for determining how green tides will be affected by ocean acidification in coastal areas where nutrient enrichment occurs in pulses, resulting in transiently increased nitrogen concentrations.

Continue reading ‘Effects of ocean acidification and eutrophication on the macroalgae Ulva spp.’

The effects of ocean warming and acidification on seaweed growth and urchin grazing

Human produced carbon dioxide concentrations in the atmosphere are currently higher than previously recorded and are continuing to rise at alarming rates. This greenhouse gas is the primary driver for changing climate scenarios highlighted by an approximate 1°C increase in sea surface temperatures. In addition to driving global warming, CO2 is readily absorbed by the oceans, resulting in changes in seawater chemistry and a decrease in seawater pH (acidification). The singular effects of ocean warming and acidification are known to impact marine organisms; lesser known, however, are the combined effects of these stressors, particularly on biotic interactions. This study aimed to expand on the knowledge of how these abiotic stressors affect seaweed and seaweed-herbivore interactions by comparing seaweed growth and herbivore feeding rate and selectivity under combinations of current and modelled future temperature (18°C and 21°C) and pH (8.1 and 7.8) conditions. Growth rates of two seaweed species, a calcified red alga (Lithothrix aspergillum) and a non-calcified brown alga (giant kelp Macrocystis pyrifera), were compared among manipulated seawater conditions. In addition, the feeding rates and feeding selectivity of a common sea urchin herbivore (Strongylocentrotus purpuratus) for these two seaweeds were compared among water conditions. Lithothrix was not affected by the singular effects of pH or temperature but under combined future temperature and pH conditions, the seaweed performed poorly. While acidification is known to affect the ability of calcifying species to deposit calcium carbonate, Lithothrix appeared to only be impacted by acidification under temperature stress. Macrocystis, on the other hand, performed significantly better under future acidic conditions, regardless of temperature, as it likely experienced an increase in photosynthetic rate driven by high CO2 concentrations. Urchin herbivory rates were elevated for both seaweeds grown under acidification scenarios, possibly due to increased grazing susceptibility of Lithothrix during poor calcification/decalcification conditions and Macrocystis during new growth conditions. Feeding preference trials were inconsistent with feeding rate patterns as urchins exhibited low overall consumption and no selectivity for either seaweed under any water condition. Although the impacts of warming and acidification on growth of seaweeds and susceptibility to grazing by urchins are variable among taxa, potential future stressors are likely to alter seaweed population and seaweed-herbivore dynamics.

Continue reading ‘The effects of ocean warming and acidification on seaweed growth and urchin grazing’

Multiple phases of mg-calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralisation in Phymatolithon (Rhodophyta, Corallinales)

Magnesium content, strongly correlated with temperature, has been developed as a climate archive for the late Holocene without considering anatomical controls on Mg content. In this paper we explore the ultrastructure and cellular scale Mg-content variations within four species of North Atlantic crust-forming Phymatolithon. The cell wall has radial grains of Mg-calcite whereas the interfilament (middle lamella) has grains aligned parallel to the filament axis. The proportion of interfilament and cell wall carbonate varies by tissue and species. Three distinct primary phases of Mg-calcite are identified: interfilament Mg-calcite (mean 8.9 mol% MgCO3), perithallial cell walls Mg-calcite (mean 13.4 mol% MgCO3), and hypothallium Mg-calcite (mean 17.1 mol% MgCO3). Magnesium content for the bulk crust, an average of all phases present, shows a strongly correlated (R2= 0.975) increase of 0.31 mol% MgCO3/°C. Of concern for climate reconstructions is the potential for false warming signals from undetected post-grazing wound repair carbonate that is substantially enriched in Mg, unrelated to temperature. Within a single crust, component carbonates can range from Mg content as stable as aragonite (8 mol% MgCO3), up to 150% higher (20 mol% MgCO3) a predicted unstable high magnesium calcite. It is unlikely that existing current predictions of ocean acidification impact on coralline algae, based on saturation states calculated using average Mg contents, provide an environmentally-relevant estimate.

Continue reading ‘Multiple phases of mg-calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralisation in Phymatolithon (Rhodophyta, Corallinales)’

Species interactions can shift the response of a maerl bed community to ocean acidification and warming

Predicted ocean acidification and warming are likely to have major implications for marine organisms, especially marine calcifiers. However, little information is available on the response of marine communities as a whole to predicted changes. Here, we experimentally examined the combined effects of temperature and partial pressure of carbon dioxide (pCO2) increases on the response of maerl bed assemblages, composed of living and dead thalli of the free-living coralline alga Lithothamnion corallioides, epiphytic fleshy algae, and grazer species. Two three-month experiments were performed in the winter and summer seasons in mesocosms with four different combinations of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C). The response of maerl assemblages was assessed using metabolic measurements at the species and assemblage scales. Gross primary production and respiration of assemblages were enhanced by high pCO2 conditions in the summer. This positive effect was attributed to the increase in epiphyte biomass, which benefited from higher CO2 concentrations for growth and primary production. Conversely, high pCO2 drastically decreased the calcification rates in assemblages. This response can be attributed to the decline in calcification rates of living L. corallioides due to acidification as well as increased dissolution of dead L. corallioides. Future changes in pCO2 and temperature are likely to promote the development of non-calcifying algae to the detriment of the engineer species L. corallioides. The development of fleshy algae may be modulated by the ability of grazers to regulate epiphyte growth. However, our results suggest that predicted changes will negatively affect the metabolism of grazers and potentially their ability to control epiphyte abundance. Here, we demonstrate that the response of marine communities to climate change will depend on the direct effects on species physiology and the indirect effects due to shifts in species interactions. This double, interdependent response underlines the importance of examining community-level processes, which integrate species interactions, to better understand the impact of global change on marine ecosystems.

Continue reading ‘Species interactions can shift the response of a maerl bed community to ocean acidification and warming’

Effects of acidified seawater on calcification, photosynthetic efficiencies and the recovery processes from strong light exposure in the coral Stylophora pistillata

The aim of this study was to investigate whether coral photosynthetic efficiencies and recovery processes are affected by CO2-driven ocean acidification in symbiont photosynthesis and coral calcification. We investigated the effects of five CO2 partial pressure (pCO2) levels in adjusted seawater ranging from 300 μatm (pre-industrial) to 800 μatm (near-future) and strong and weak light intensity on maximum photosynthetic efficiency and calcification of a branching coral, Stylophora pistillata, as this species has often been used in rearing experiments to investigate the effects of acidified seawater on calcification and photosynthetic algae of corals. We found that, the photosynthetic efficiencies and recovery patterns under different light conditions did not differ among pCO2 treatments. Furthermore, calcification of S. pistillata was not affected by acidified seawater under weak or strong light conditions. Our results indicate that the photosynthetic efficiency and calcification of S. pistillata are insensitive to changes in ocean acidity.

Continue reading ‘Effects of acidified seawater on calcification, photosynthetic efficiencies and the recovery processes from strong light exposure in the coral Stylophora pistillata’

Calcifying response and recovery potential of the brown alga Padina pavonica under ocean acidification

Anthropogenic CO2 emissions are causing ocean acidification (OA), which affects calcifying organisms. Recent studies have shown that Padina pavonica investigated along a natural pCO2 gradient seems to acclimate to OA by reducing calcified structures and changing mineralogy from aragonite to calcium sulphate salts. The aim of the present study was to study the potential for acclimation of P. pavonica to OA along the same gradient and in aquaria under controlled conditions. P. pavonica was cross-transplanted for one week from a normal pH site (median value: pHTS = 8.1; pCO2 = 361 μatm) to a low pH site (median value: pHTS = 7.4; pCO2 = 1025 μatm) and vice versa. Results showed that this calcifying alga did survive under acute environmental pHTS changes but its calcification was significantly reduced. P. pavonica decalcified and changed mineralogy at pHTS = 7.4, but once brought back at pHTS = 8.1 it partially recovered the aragonite loss while preserving the calcium sulphate minerals that formed under low pHTS. These results suggest that P. pavonica could be used as a bio-indicator for monitoring OA, as well as localized anthropogenic acidity fluctuations.

Continue reading ‘Calcifying response and recovery potential of the brown alga Padina pavonica under ocean acidification’

Indirect effects of ocean acidification drive feeding and growth of juvenile crown-of-thorns starfish, Acanthaster planci

The indirect effects of changing climate in modulating trophic interactions can be as important as the direct effects of climate stressors on consumers. The success of the herbivorous juvenile stage of the crown-of-thorns starfish (COTS), Acanthaster planci, may be affected by the impacts of ocean conditions on its crustose coralline algal (CCA) food. To partition the direct effects of near future ocean acidification on juvenile COTS and indirect effects through changes in their CCA food, COTS were grown in three pHT levels (7.9, 7.8, 7.6) and fed CCA grown at similar pH levels. Consumption of CCA by COTS was bolstered when the COTS were grown in low pH and when they were fed CCA grown in low pH regardless of the pH in which the COTS were reared. COTS fed CCA grown at pH 7.6 grew fastest, but the pH/pCO2 that the COTS were reared in had no direct effect on growth. Ocean acidification conditions decreased the C : N ratio and carbonate levels in the CCA. Bolstered growth in COTS may be driven by enhanced palatability, increased nutritive state and reduced defences of their CCA food. These results indicate that near future acidification will increase the success of early juvenile COTS and boost recruitment into the coral-eating life stage.

Continue reading ‘Indirect effects of ocean acidification drive feeding and growth of juvenile crown-of-thorns starfish, Acanthaster planci’

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Ocean acidification in the IPCC AR5 WG II

OUP book