Effet de l'acidification des océans et de concentrations plus élevées en cobalt sur la croissance et l'efficacité photosynthétique de deux espèces de coraux

Le but de cette étude était de déterminer les effets d’un enrichissement en cobalt combiné à une augmentation de la pCO2 sur la physiologie de deux espèces de coraux scléractiniaires de Calédonie, Stylophora pistillata et Acropora muricata. Les colonies de ces deux espèces ont été maintenues sous deux conditions de pHT : pHT normal (8.02) et plus faible (7.75) et deux concentrations en cobalt (0.03 μg.l-1 et 0.20 μg.l-1) durant cinq semaines. Au cours du temps, différents paramètres physiologiques des colonies coralliennes ont été suivis : leur efficacité photosynthétique grâce à un diving-PAM fluorimètre, leur croissance et leurs paramètres tissulaires (concentration en chlorophylle et densité en zooxanthelles). Les coraux exposés à une concentration plus élevée en cobalt ont diminué significativement leur croissance (de 27.9% pour S. pistillata et 28.2% pour A. muricata). Les valeurs d’ETRmax des deux espèces ont, au contraire, été augmentées par l’apport en cobalt (de 44.34 ± 2.14 à 58.11 ± 6.31 pour S. pistillata, et de 45.42 ± 5.08 à 56.48 ± 3.67 pour A. muricata). La croissance des deux espèces n’a par contre pas été modifiée par des conditions de pHT plus faible. Ces résultats montrent que le cobalt à cette concentration serait toxique pour la cellule hôte, mais pas pour les symbiontes pour lesquels il jouerait même un rôle stimulateur. Plusieurs hypothèses ont été avancées pour expliquer les effets antagonistes de ce métal sur le métabolisme de ces deux espèces

High p CO 2 promotes coral primary production

While research on ocean acidification (OA) impacts on coral reefs has focused on calcification, relatively little is known about effects on coral photosynthesis and respiration, despite these being among the most plastic metabolic processes corals may use to acclimatize to adverse conditions. Here, we present data collected between 2016 and 2018 at three natural CO2 seeps in Papua New Guinea where we measured the metabolic flexibility (i.e. in hospite photosynthesis and dark respiration) of 12 coral species. Despite some species-specific variability, metabolic rates as measured by net oxygen flux tended to be higher at high pCO(2) (ca 1200 mu atm), with increases in photosynthesis exceeding those of respiration, suggesting greater productivity of Symbiodiniaceae photosynthesis in hospite, and indicating the potential for metabolic flexibility that may enable these species to thrive in environments with high pCO(2). However, laboratory and field observations of coral mortality under high CO2 conditions associated with coral bleaching suggests that this metabolic subsidy does not result in coral higher resistance to extreme thermal stress. Therefore, the combined effects of OA and global warming may lead to a strong decrease in coral diversity despite the stimulating effect on coral productivity of OA alone

Seawater carbonate chemistry and coral primary production

While research on ocean acidification (OA) impacts on coral reefs has focused on calcification, relatively little is known about effects on coral photosynthesis and respiration, despite these being among the most plastic metabolic processes corals may use to acclimatize to adverse conditions. Here, we present data collected between 2016 and 2018 at three natural CO2 seeps in Papua New Guinea where we measured the metabolic flexibility (i.e. in hospite photosynthesis and dark respiration) of 12 coral species. Despite some species-specific variability, metabolic rates as measured by net oxygen flux tended to be higher at high pCO2 (ca 1200 µatm), with increases in photosynthesis exceeding those of respiration, suggesting greater productivity of Symbiodiniaceae photosynthesis in hospite, and indicating the potential for metabolic flexibility that may enable these species to thrive in environments with high pCO2. However, laboratory and field observations of coral mortality under high CO2 conditions associated with coral bleaching suggests that this metabolic subsidy does not result in coral higher resistance to extreme thermal stress. Therefore, the combined effects of OA and global warming may lead to a strong decrease in coral diversity despite the stimulating effect on coral productivity of OA alone

Nickel and ocean warming affect scleractinian coral growth

WOS:000407539300039International audienceThe sensitivity of corals and their Symbiodinium to warming has been extensively documented; however very few studies considered that anthropogenic inputs such as metal pollution have already an impact on many fringing reefs. Thus, today, nickel releases are common in coastal ecosystems. In this study, two major reef-building species Acropora muricata and Pocillopora damicornis were exposed in situ to ambient and moderate nickel concentrations on a short-term period (1 h) using benthic chamber experiments. Simultaneously, we tested in laboratory conditions the combined effects of a chronic exposure (8 weeks) to moderate nickel concentrations and ocean warming on A. muricata. The in situ experiment highlighted that nickel enrichment, at ambient temperature, stimulated by 27 to 47% the calcification rates of both species but not their photosynthetic performances. In contrast, an exposure to higher nickel concentration, in combination with elevated temperature simulated in aquaria, severely depressed by 30% the growth of A. muricata