The increase in extreme heat events associated to global warming threatens seagrass ecosystems, likely by affecting key plant physiological processes such as photosynthesis and respiration. Understanding species’ ability to acclimate to warming is crucial to better predict their future trends. Here, we study tolerance to warming in two key Mediterranean seagrasses, Posidonia oceanica and Cymodocea nodosa. Stress responses of shallow and deep plants were followed during and after short-term heat exposure in mesocosms by coupling photo-physiological measures with analysis of expression of photosynthesis and stress-related genes. Contrasting tolerance and capacity to heat acclimation were shown by shallow and deep P. oceanica ecotypes. While shallow plants acclimated through respiratory homeostasis and activation of photo-protective mechanisms, deep ones experienced photosynthetic injury and impaired carbon balance. This suggests that P. oceanica ecotypes are thermally adapted to local conditions and that Mediterranean warming will likely diversely affect deep and shallow meadow stands. On the other hand, contrasting mechanisms of heat-acclimation were adopted by the two species. P. oceanica regulates photosynthesis and respiration at the level of control plants while C. nodosa balances both processes at enhanced rates. These acclimation discrepancies are discussed in relation to inherent attributes of the two species.

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The Mediterranean Sea is particularly vulnerable to warming and the abrupt declines experienced by the endemic Posidonia oceanica populations after recent heatwaves have forecasted severe consequences for the ecological functions and socio‐economical services this habitat forming species provides. Nevertheless, this highly clonal and long‐lived species could be more resilient to warming than commonly thought since heat‐sensitive plants massively bloomed after a simulated heatwave, which provides the species with an opportunity to adapt to climate change. Taking advantage of this unexpected plant response, we investigated for the first time the molecular and physiological mechanisms involved in seagrass flowering through the transcriptomic analysis of bloomed plants. We also aimed to identify if flowering is a stress‐induced response as suggested from the fact that heat‐sensitive but not heat‐tolerant plants flowered. The transcriptomic profiles of flowered plants showed a strong metabolic activation of sugars and hormones and indications of an active transport of these solutes within the plant, most likely to induce flower initiation in the apical meristem. Preflowered plants also activated numerous epigenetic‐related genes commonly used by plants to regulate the expression of key floral genes and stress‐tolerance genes, which could be interpreted as a mechanism to survive and optimize reproductive success under stress conditions. Furthermore, these plants provided numerous molecular clues suggesting that the factor responsible for the massive flowering of plants from cold environments (heat‐sensitive) can be considered as a stress. Heat‐stress induced flowering may thus be regarded as an ultimate response to survive extreme warming events with potential adaptive consequences for the species. Fitness implications of this unexpected stress‐response and the potential consequences on the phenotypic plasticity (acclimation) and evolutionary (adaptation) opportunity of the species to ocean warming are finally discussed.

This study investigated the role of wild fishes associated with marine farms as potential consumers of organic waste in the water column and sediment. Experiments were carried out at a fish farm in a semi-enclosed bay in the Mediterranean Sea (SE Spain). Sediment traps collected organic waste from net-pens in the water column at different depths during June and July 2001 and benthic caging experiments were carried out on the sediment from October 2000 to January 2001. The experiment showed that about 80% of the particulate organic matter leaving the rearing net-pens may be consumed before it settles on the sediment. Significant changes in the nutrient quality of the organic matter exported are also due to consumption by wild fishes. Wild fishes thus play an important role in recycling the organic matter of the sediment, and regulate the benthic community structure. Our results indicate that the trophic role of wild fishes should be considered when evaluating the environmental impact of fish farms.