L’acidification des océans altère la faune et la flore (in French)

Des chercheurs mesurent les effets de ce phénomène en Méditerranée.

La cage en Plexiglas transparent trône sur la dalle du laboratoire d’océanographie de Villefranche-sur-Mer (UPMC/CNRS). Large de deux mètres environ, elle est prête à être déposée à une quinzaine de mètres de profondeur, à bonne distance du rivage.

Cette expérimentation est une première en France. L’objectif est de tester, au plus près de la réalité, l’impact de l’acidification des océans sur la flore – les posidonies, des plantes marines différentes des algues, notamment – et la faune des fonds méditerranéens.

Carbonate system variability in the Gulf of Trieste (North Adriatic sea)

The seasonal variability of the carbonate system in the waters of the Gulf of Trieste (GoT) was studied at PALOMA station from 2008 to 2009, in order to highlight the effects of biological processes, meteorological forcings and river loads on the dynamics of pH_T, CO₂ partial pressure (pCO₂), dissolved inorganic carbon (DIC), carbonate ion concentration (CO₃⁼), aragonite saturation state (Ω_Ar) and total alkalinity (A_T).

During winter, low seawater temperature (9.0 ± 0.4°C) and a weak biological activity (-10.7 < AOU < 15.7 μmol O₂ kg⁻¹) in a homogeneous water column led to the lowest average values of pCO₂ (328 ± 19 μatm) and Ω_Ar (2.91 ± 0.14). In summer, the water column in the area acted as a two-layer system, with production processes prevailing in the upper layer (average AOU = -29.3 μmol O₂ kg⁻¹) and respiration processes in the lower layer (average AOU = 26.8 μmol O₂ kg⁻¹). These conditions caused the decrease of DIC (50 μmol kg⁻¹) and the increase of Ω_Ar (1.0) values in the upper layer, whereas opposite trends were observed in the bottom waters. In August 2008, during a hypoxic event (dissolved oxygen DO = 86.9 μmol O₂ kg⁻¹), the intense remineralisation of organic carbon caused the rise of pCO₂ (1043 μatm) and the decreases of pH_T and Ω_Ar values down to 7.732 and 1.79 respectively.

On an annual basis, surface pCO₂ was mainly regulated by the pronounced seasonal cycle of seawater temperature. In winter, surface waters in the GoT were under-saturated with respect to atmospheric CO₂, thus acting as a sink of CO₂, in particular when strong-wind events enhanced air-sea gas exchange (FCO₂ up to -11.9 mmol m⁻²d⁻¹). During summer, the temperature-driven increase of pCO₂ was dampened by biological CO₂ uptake, as consequence a slight over-saturation (pCO₂ = 409 μatm) turned out. River plumes were generally associated to higher A_T and pCO₂ values (up to 2859 μmol kg⁻¹ and 606 μatm respectively), but their effect was highly variable in space and time. During winter, the ambient conditions that favour the formation of dense waters on this continental shelf, also favour a high absorption of CO₂ in seawater and its consequent acidification (pH_T decrease of -0.006 units during a 7-day Bora wind event). This finding indicates a high vulnerability of North Adriatic Dense Water to atmospheric CO₂ increase and ocean acidification process.

The importance of the terrestrial weathering feedback for multi-millennial coral reef habitat recovery

Modern-day coral reefs have well defined environmental envelopes for light, sea surface temperature (SST) and seawater aragonite saturation state (Ωarag). We examine the changes in global coral reef habitat on multi-millennial timescales with regard to SST and Ωarag using a climate model including a three-dimensional ocean general circulation model, a fully coupled carbon cycle, and six different parameterizations for continental weathering (the UVic Earth System Climate Model). The model is forced with emission scenarios ranging from 1,000 Pg C to 5,000 Pg C total emissions. We find that the long-term climate change response is independent of the rate at which CO2 is emitted over the next few centuries. On millennial timescales, the weathering feedback introduces a significant uncertainty even for low emission scenarios. Weathering parameterizations based on atmospheric CO2 only display a different transient response than weathering parameterizations that are dependent on temperature. Although environmental conditions for SST and Ωarag stay globally hostile for coral reefs for millennia for our high emission scenarios, some weathering parameterizations induce a near-complete recovery of coral reef habitat to current conditions after 10,000 years, while others result in a collapse of coral reef habitat throughout our simulations. We find that the multi-millennial response in sea surface temperature (SST) substantially lags the aragonite saturation recovery in all configurations. This implies that if corals can naturally adapt over millennia by selecting thermally tolerant species to match warmer ocean temperatures, prospects for long-term recovery of coral reefs are better since Ωarag recovers more quickly than SST.

Proton channels in algae: reasons to be excited

A fundamental requirement of all eukaryotes is the ability to translocate protons across membranes. This is critical in bioenergetics, for compartmentalized metabolism, and to regulate intracellular pH (pH_i) within a range that is compatible with cellular metabolism. Plants, animals, and algae utilize specialized transport machinery for membrane energization and pH homeostasis that reflects the prevailing ionic conditions in which they evolved. The recent characterization of H⁺-permeable channels in marine and freshwater algae has led to the discovery of novel functions for these transport proteins in both cellular pH homeostasis and sensory biology. Here we review the potential implications for understanding the origins and evolution of membrane excitability and the phytoplankton-based marine ecosystem responses to ocean acidification.

Host-parasite interactions: a litmus test for ocean acidification?

The effects of ocean acidification (OA) on marine species and ecosystems have received significant scientific attention in the past 10 years. However, to date, the effects of OA on hostparasite interactions have been largely ignored. As parasites play a multidimensional role in the regulation of marine population, community, and ecosystem dynamics, this knowledge gap may result in an incomplete understanding of the consequences of OA. In addition, the impact of stressors associated with OA on hostparasite interactions may serve as an indicator of future changes to the biodiversity of marine systems. This opinion article discusses the potential effects of OA on host and parasite species and proposes the use of parasites as bioindicators of OA disturbance.

The evolution of pCO2, ice volume and climate during the middle Miocene

The middle Miocene Climatic Optimum (17–15 Ma; MCO) is a period of global warmth and relatively high CO₂ and is thought to be associated with a significant retreat of the Antarctic Ice Sheet (AIS). We present here a new planktic foraminiferal δ¹¹B record from 16.6 to 11.8 Ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to CO₂. These new data demonstrate that the evolution of global climate during the middle Miocene (as reflected by changes in the cyrosphere) was well correlated to variations in the concentration of atmospheric CO₂. What is more, within our sampling resolution (∼1 sample per 300 kyr) there is no evidence of hysteresis in the response of ice volume to CO₂ forcing during the middle Miocene, contrary to what is understood about the Antarctic Ice Sheet from ice sheet modelling studies. In agreement with previous data, we show that absolute levels of CO₂ during the MCO were relatively modest (350–400 ppm) and levels either side of the MCO are similar or lower than the pre-industrial (200–260 ppm). These new data imply the presence of either a very dynamic AIS at relatively low CO₂ during the middle Miocene or the advance and retreat of significant northern hemisphere ice. Recent drilling on the Antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the AIS did occur during the middle Miocene, but the complete loss of the AIS was unlikely. Consequently, it seems that ice volume and climate variations during the middle Miocene probably involved a more dynamic AIS than the modern but also some component of land-based ice in the northern hemisphere.

Boron incorporation into calcite during growth: implications for the use of boron in carbonates as a pH proxy

Current interest in boron incorporation into carbonates arises from the observation that the isotopic composition of carbonates depends on the pH of the fluid from which they precipitated. This finding opened the possibility of using boron isotopic composition of natural carbonates as a paleo-pH proxy. In this study, coprecipitation of boron by calcite was investigated using Atomic Force Microscopy (AFM), as a function of pH, supersaturation and boron concentration. In situ AFM observations reported here provide experimental evidence of boron incorporation into calcite, which takes place to a greater extent at high pH (9.5) and under close to equilibrium conditions. Moreover, we report nanoscale observations that give indirect evidence of the incorporation of boron in non-lattice sites. Step-specific interactions of tetrahedrally-coordinated boron with calcite obtuse steps during growth are revealed as a reduction in the obtuse-step spreading rate as well as rounding and roughening of such steps. Our results suggest that, together with changes in pH, variations in the calcification rate or the calcite crystallographic form in which boron is incorporated are important factors to consider when using boron in carbonates as a pH proxy, as these factors could also influence the amount of boron incorporated during growth and possibly the boron isotopic signature.