Description: The Taputapuatea marae of Ra'iatea is an emblematic landmark known throughout the world and sacred place for the Ma'ohi of Eastern Polynesia and the centre of a vast political- religious-cultural network in the Polynesian triangle. The erected stones constituting the ahu have been described as "limestone slabs" without precision by previous authors. These are in fact microatolls: corals (Porites) living in very shallow water and developing laterally, with a growth in height being limited by the lowest tide at the time of growth. A total of 38 samples were U/Th dated, of which 19 microatolls result in ages between 3 and 5 millennia. These are fossil microatolls that existed at a Holocene sea level of 0.80 m higher than today, when the Polynesians had not yet arrived. Other samples (molluscs, coral filling blocks) date back to the construction of the marae during the 17th-18th centuries. We hypothesize that the erected microatolls of the ahu were collected by Polynesians at the site and that others are still underground. Le marae Taputapuatea de Ra’iatea est un site emblématique mondialement considéré et un lieu sacré pour les Ma’ohi de la Polynésie orientale et le centre d’un vaste réseau politico-religieux-culturel du triangle polynésien. Les pierres érigées constituant l’ahu avaient été nommées « dalles calcaire » sans autre précision par les auteurs précédents. Ce sont des microatolls : coraux (Porites) vivant dans des eaux très peu profondes et se développant latéralement, la croissance en hauteur étant limitée par le bas niveau de la mer. Un total de 38 échantillons ont été datés (U/Th) sur 19 microatolls, donnant des âges de 3 et 5 millénaires. Il s’agit de microatolls fossiles dont l’existence remonte à un niveau de la mer Holocène de 0,80 m plus élevé qu’aujourd’hui, époque où les Polynésiens étaient absents. D’autres datations (mollusques, blocs de remplissage de corail) datent la construction du marae des xviie-xviiie siècles. Nous émettons l’hypothèse que les microatolls fossiles érigés de l’ahu ont été collectés par des Polynésiens sur le site et que d’autres sont toujours sous terre.

Description: The warming of our planet is changing the Arctic dramatically. The area covered by sea-ice is shrinking and the ice that is left is younger and thinner. We took part in an expedition to the Arctic, to study how these changes affect organisms living in and under the ice. Following this expedition, we found that storms can more easily break the thinner ice. Storms form cracks in the sea ice, allowing sunlight to pass into the water below, which makes algal growth possible. Algae are microscopic “plants” that grow in water or sea ice. Storms also brought thick heavy snow, which pushed the ice surface below the water. This flooded the snow and created slush. We discovered that this slush is another good habitat for algae. If Arctic sea ice continues to thin, and storms become more common, we expect that these algal habitats will become more important in the future.

Description: Diatoms are major primary producers in polar environments where they can actively grow under extremely variable conditions. Integrative modeling using a genome-scale model (GSM) is a powerful approach to decipher the complex interactions between components of diatom metabolism and can provide insights into metabolic mechanisms underlying their evolutionary success in polar ecosystems. We developed the first GSM for a polar diatom, Fragilariopsis cylindrus, which enabled us to study its metabolic robustness using sensitivity analysis. We find that the predicted growth rate was robust to changes in all model parameters (i.e., cell biochemical composition) except the carbon uptake rate. Constraints on total cellular carbon buffer the effect of changes in the input parameters on reaction fluxes and growth rate. We also show that single reaction deletion of 20% to 32% of active (nonzero flux) reactions and single gene deletion of 44% to 55% of genes associated with active reactions affected the growth rate, as well as the production fluxes of total protein, lipid, carbohydrate, DNA, RNA, and pigments by less than 1%, which was due to the activation of compensatory reactions (e.g., analogous enzymes and alternative pathways) with more highly connected metabolites involved in the reactions that were robust to deletion. Interestingly, including highly divergent alleles unique for F. cylindrus increased its metabolic robustness to cellular perturbations even more. Overall, our results underscore the high robustness of metabolism in F. cylindrus, a feature that likely helps to maintain cell homeostasis under polar conditions.