Posts Tagged 'phytoplankton'

Southern Ocean phytoplankton in a changing climate

Phytoplankton are the base of the Antarctic food web, sustain the wealth and diversity of life for which Antarctica is renowned, and play a critical role in biogeochemical cycles that mediate global climate. Over the vast expanse of the Southern Ocean (SO), the climate is variously predicted to experience increased warming, strengthening wind, acidification, shallowing mixed layer depths, increased light (and UV), changes in upwelling and nutrient replenishment, declining sea ice, reduced salinity, and the southward migration of ocean fronts. These changes are expected to alter the structure and function of phytoplankton communities in the SO. The diverse environments contained within the vast expanse of the SO will be impacted differently by climate change; causing the identity and the magnitude of environmental factors driving biotic change to vary within and among bioregions. Predicting the net effect of multiple climate-induced stressors over a range of environments is complex. Yet understanding the response of SO phytoplankton to climate change is vital if we are to predict the future state/s of the ecosystem, estimate the impacts on fisheries and endangered species, and accurately predict the effects of physical and biotic change in the SO on global climate. This review looks at the major environmental factors that define the structure and function of phytoplankton communities in the SO, examines the forecast changes in the SO environment, predicts the likely effect of these changes on phytoplankton, and considers the ramifications for trophodynamics and feedbacks to global climate change. Predictions strongly suggest that all regions of the SO will experience changes in phytoplankton productivity and community composition with climate change. The nature, and even the sign, of these changes varies within and among regions and will depend upon the magnitude and sequence in which these environmental changes are imposed. It is likely that predicted changes to phytoplankton communities will affect SO biogeochemistry, carbon export, and nutrition for higher trophic levels.

Continue reading ‘Southern Ocean phytoplankton in a changing climate’

Iron availability modulates the effects of future CO2 levels within the marine planktonic food web

Ocean acidification (OA) due to increased anthropogenic CO2 emissions is affecting marine ecosystems at an unprecedented rate, altering biogeochemical cycles. Direct empirical studies on natural communities are required to analyse the interactive effects of multiple stressors while spanning multiple trophic levels. We investigated the interactive effects of changes in CO2 and iron availability on functional plankton groups. We used mesocosms manipulating the carbonate system from the start to achieve present (low concentration, LC) and predicted future pCO2 levels (high concentration, HC). To manipulate dissolved iron (dFe), half of the mesocosms were amended with 70 nM (final concentration) of the siderophore desferoxamine B (DFB) on Day 7 (+DFB and -DFB treatments). Manipulation of both CO2 and DFB increased dFe compared to the control. During the 22 experimental days, the plankton community structure showed 2 distinct phases. In phase 1 (Days 1-10), only bacterioplankton abundances increased at elevated pCO2. In contrast, a strong community response was evident in phase 2 (Days 11-22) due to DFB addition. Biomass of the coccolithophore Emiliania huxleyi increased massively at LC+DFB. HC negatively affected E. huxleyi and Synechococcus sp., and high dFe (+DFB) had a positive effect on both. The rest of the plankton community was unaffected by the treatments. Increased dFe partially mitigated the negative effect of HC imposed on the coccolithophores, indicating that E. huxleyi was able to acclimate better to OA. This physiological iron-mediated acclimation can diminish the deleterious effects of OA on carbon export and the rain ratio, thus affecting food web dynamics and future ecosystem functioning.

Continue reading ‘Iron availability modulates the effects of future CO2 levels within the marine planktonic food web’

The uronic acid content of coccolith-associated polysaccharides provides insight into coccolithogenesis and past climate

Unicellular phytoplanktonic algae (coccolithophores) are among the most prolific producers of calcium carbonate on the planet, with a production of ∼1026 coccoliths per year. During their lith formation, coccolithophores mainly employ coccolith-associated polysaccharides (CAPs) for the regulation of crystal nucleation and growth. These macromolecules interact with the intracellular calcifying compartment (coccolith vesicle) through the charged carboxyl groups of their uronic acid residues. Here we report the isolation of CAPs from modern day coccolithophores and their prehistoric predecessors and we demonstrate that their uronic acid content (UAC) offers a species-specific signature. We also show that there is a correlation between the UAC of CAPs and the internal saturation state of the coccolith vesicle that, for most geologically abundant species, is inextricably linked to carbon availability. These findings suggest that the UAC of CAPs reports on the adaptation of coccolithogenesis to environmental changes and can be used for the estimation of past CO2 concentrations.

Continue reading ‘The uronic acid content of coccolith-associated polysaccharides provides insight into coccolithogenesis and past climate’

Integrated regulatory and metabolic networks of the marine diatom Phaeodactylum tricornutum predict the response to rising CO2 levels

Diatoms are eukaryotic microalgae that are responsible for up to 40% of the ocean’s primary productivity. How diatoms respond to environmental perturbations such as elevated carbon concentrations in the atmosphere is currently poorly understood. We developed a transcriptional regulatory network based on various transcriptome sequencing expression libraries for different environmental responses to gain insight into the marine diatom’s metabolic and regulatory interactions and provide a comprehensive framework of responses to increasing atmospheric carbon levels. This transcriptional regulatory network was integrated with a recently published genome-scale metabolic model of Phaeodactylum tricornutum to explore the connectivity of the regulatory network and shared metabolites. The integrated regulatory and metabolic model revealed highly connected modules within carbon and nitrogen metabolism. P. tricornutum’s response to rising carbon levels was analyzed by using the recent genome-scale metabolic model with cross comparison to experimental manipulations of carbon dioxide.

IMPORTANCE: Using a systems biology approach, we studied the response of the marine diatom Phaeodactylum tricornutum to changing atmospheric carbon concentrations on an ocean-wide scale. By integrating an available genome-scale metabolic model and a newly developed transcriptional regulatory network inferred from transcriptome sequencing expression data, we demonstrate that carbon metabolism and nitrogen metabolism are strongly connected and the genes involved are coregulated in this model diatom. These tight regulatory constraints could play a major role during the adaptation of P. tricornutum to increasing carbon levels. The transcriptional regulatory network developed can be further used to study the effects of different environmental perturbations on P. tricornutum’s metabolism.

Continue reading ‘Integrated regulatory and metabolic networks of the marine diatom Phaeodactylum tricornutum predict the response to rising CO2 levels’

Ocean acidification modulates expression of genes and physiological performance of a marine diatom

Ocean Acidification (OA) is known to affect various aspects of physiological performances of diatoms, but little is known about the underlining molecular mechanisms involved. Here, we show that in the model diatom Phaeodactylum tricornutum, the expression of key genes associated with photosynthetic light harvesting as well as those encoding Rubisco, carbonic anhydrase, NADH dehydrogenase and nitrite reductase, are modulated by OA (1000 μatm, pHnbs 7.83). Growth and photosynthetic carbon fixation were enhanced by elevated CO2. OA treatment decreased the expression of β-carbonic anhydrase (β-ca), which functions in balancing intracellular carbonate chemistry and the CO2 concentrating mechanism (CCM). The expression of the genes encoding fucoxanthin chlorophyll a/c protein (lhcf type (fcp)), mitochondrial ATP synthase (mtATP), ribulose-1, 5-bisphosphate carboxylase/oxygenase large subunit gene (rbcl) and NADH dehydrogenase subunit 2 (ndh2), were down-regulated during the first four days (< 8 generations) after the cells were transferred from LC (cells grown under ambient air condition; 390 μatm; pHnbs 8.19) to OA conditions, with no significant difference between LC and HC treatments with the time elapsed. The expression of nitrite reductase (nir) was up-regulated by the OA treatment. Additionally, the genes for these proteins (NiR, FCP, mtATP synthase, β-CA) showed diel expression patterns. It appeared that the enhanced photosynthetic and growth rates under OA could be attributed to stimulated nitrogen assimilation, increased CO2 availability or saved energy from down-regulation of the CCM and consequently lowered cost of protein synthesis versus that of non-nitrogenous cell components.

Continue reading ‘Ocean acidification modulates expression of genes and physiological performance of a marine diatom’

Photosynthetic responses of the marine diatom Thalassiosira pseudonana to CO2-induced seawater acidification

Ocean acidification due to atmospheric CO2 rise is expected to influence marine phytoplankton. Diatoms are responsible for about 40% of the total primary production in the ocean. In order to investigate the physiological response of marine diatom Thalassiosira pseudonana to ocean acidification, we grew the cells under ambient CO2 level (380 µatm) versus the elevated CO2 level (800 µatm) at a light level of 180 µmol m−2 s−1 for 30 generations. Our results showed that the elevated CO2 concentration caused a decrease of the effective photochemical efficiency of PSII (F′v/F′m) and increase of the dark respiration in T. pseudonana. The intracellular carbonic anhydrase activity was suppressed and the photosynthetic affinity for CO2 was lowered in the high CO2-grown cells, reflecting a downregulation of the CO2 concentrating mechanism (CCM). PSI activity was enhanced to support an increase in ATP synthesis by cyclic electron transfer as required for transport of inorganic carbon and regulation of intracellular pH. The energetic benefit from the downregulation of CCM to growth as reported in other diatom species was not observed here in T. pseudonana.

Continue reading ‘Photosynthetic responses of the marine diatom Thalassiosira pseudonana to CO2-induced seawater acidification’

Special edition of Estuarine, Coastal and Shelf Science – “Ocean acidification in the Mediterranean Sea: pelagic mesocosm experiments”

The topic of ocean acidification has received extensive attention in a recently published special edition of the journal Estuarine, Coastal and Shelf Science. Volume 186, Part A presents a series of 12 research papers focusing on pelagic mesocosm experiments conducted in the Mediterranean Sea in 2012 and 2013. Plankton plays a key role in the global carbon cycle. It is therefore important to project the evolution of plankton community structure and function in a future high-CO2 world. Several results from experiments conducted at the community level have shown increased rates of community primary production and shifts in community composition as a function of increasing pCO2. However, the great majority of these – experiments have been performed under high natural or nutrient-enriched conditions and very few data are available in areas with naturally low levels of nutrient and chlorophyll i.e. oligotrophic areas such as the Mediterranean Sea, although they represent a large and expanding part of the ocean surface. In the frame of the European Mediterranean Sea Acidification in a changing climate project (MedSeA; http://medsea-project.eu), large-scale in situ mesocosms (9 x 50 m3, 12 m deep) have been used to quantify the potential effects of CO2 enrichment in two coastal areas of the Mediterranean Sea: the bay of Calvi (Corsica, France) in June/July 2012 and the bay of Villefranche (France) in February/March 2013. These two experiments gathered the expertise of more than 25 scientists from 7 institutes and 6 countries (France, Greece, Spain, UK, Italy, Belgium, US).

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