Echinoderms with high levels of magnesium (Mg) in their skeletons may be especially sensitive to ocean acidification, as the solubility of calcite increases with its Mg content. However, other structural characteristics and environmental/biological factors may affect skeletal solubility. To better understand which factors can influence skeletal mineralogy , we analyzed the Mg content of Antarctic echinoderms from Deception Island, an active volcano with reduced pH and relatively warm water temperatures, and Livingston Island. We found significant interclass and inter- and intraspecific differences in the Mg content, with asteroids exhibiting the highest levels, followed by ophiuroids and echinoids. Specimens exposed to hydrothermal fluids showed lower Mg levels, which may indicate local environmental effects. These patterns suggest that environmental factors such as seawater Mg ²⁺/Ca²⁺ ratio and temperature may influence the Mg content of some echinoderms and affect their susceptibility to future environmental changes.

Posidonia oceanica is an endemic seagrass species from the Mediterranean Sea that provides critical ecological services to coastal environments. This species is distributed from the Turkish to the Spanish coast, where its westernmost record was documented in Punta Chullera, Malaga (36°18′36.45′′ N, 5°14′54.31′′ W). Nevertheless, previous studies suggested that its distribution was even further west, although these populations were never described. In this study, we documented and characterised the only known P. oceanica population on the coast of Cadiz, in Cala Sardina (36°18′38.80′′ N, 5°15′15.13′′ W). The newly documented population of P. oceanica presented a fragmented structure, consisting of nine patches found in a rocky shallow area surrounded by the invasive algae Rugulopteryx okamurae, with a total size of 61.14 m². Shoots had a relatively small size (21.0 ± 2.9 cm) in comparison with centrally-distributed populations. The relatively small size of the plants, alongside the observed low shoot density (437 ± 42 shoots m⁻²) and leaf area index (4.8 ± 0.7 m² m⁻²), may indicate that this meadow could be exposed to sub-optimal environmental conditions for plant development. By contrast, the meadow showed relatively high production rates (0.03 ± 0.01 leaf day⁻¹ shoot⁻¹) in comparison with other Mediterranean populations. The percentage of carbon in plant leaves was 38.73 ± 1.38%, while the nitrogen and C/N were 1.38 ± 0.37% and 29.93 ± 6.57, respectively. The documentation of this meadow extends the distribution of this species to the Mediterranean coast of Cadiz, making this region the place with the highest seagrass biodiversity (four species) in the Iberian Peninsula, and potentially in Europe. This exploratory study provides a baseline to examine the potential effects of climate change, anthropogenic disturbances or the presence of invasive species.

Seagrasses, which are marine flowering plants, provide numerous ecological services and goods. Zostera marina is the most widely distributed seagrass in temperate regions of the northern hemisphere, tolerant of a wide range of environmental conditions. This study aimed to (i) examine seasonal trends and correlations between key seagrass traits such as biomass production and biochemical composition, and (ii) compare seasonal adaptation of two ecotypes of Z. marina exposed to similar environmental conditions on the west coast of Ireland. During summer, plants accumulated higher levels of energetic compounds and levels of unsaturated fatty acids (FAs) decreased. Conversely, the opposite trend was observed during colder months. These findings indicate a positive seasonal correlation between the production of non-structural carbohydrates and saturated fatty acids (SFAs), suggesting that seagrasses accumulate and utilize both energetic compounds simultaneously during favorable and unfavorable environmental conditions. The two ecotypes displayed differential seasonal responses by adjusting plant morphology and production, the utilization of energetic reserves, and modulating unsaturation levels of fatty acids in seagrass leaves. These results underscore the correlated seasonal responses of key compounds, capturing ecotype-specific environmental adaptations and ecological strategies, emphasizing the robust utility of these traits as a valuable eco-physiological tool.

Global change is driving ocean warming (OW) and acidification (OA), impacting marine ecosystems worldwide, including polar regions. Seaweeds, as key primary producers in coastal ecosystems, synthesize a wide range of biochemical compounds that support higher trophic levels. Their biochemical composition is conditioned by local environmental factors, including seawater temperature, pH, and nutrient availability. However, how polar seaweeds respond to ongoing global change remains poorly understood. In this study, we examined the influence of local environmental changes on the biochemical composition - including fatty acids (FA), pigments, carbon, and nitrogen - of nine Antarctic brown and red seaweed species. Specifically, we considered a latitudinal gradient from the South Shetland Islands (⁓62°S) to Yalour Island (⁓65°S), and the presence of active fumarole vents at Deception Island. Our results reveal species-specific and location-dependent biochemical shifts in most species. While chlorophyll a concentrations tended to increase with latitude, specimens collected from fumarole vents exhibited a reduction in total FA content, PUFA:SFA (polyunsaturated to saturated fatty acid) ratios, PUFA omega-3, and pigment concentrations. These shifts under hydrothermal influence are likely driven by elevated seawater temperatures and acidic conditions, suggesting a potential decline in nutritional value under future global change scenarios. Additionally, higher magnesium content was found in the skeletons of crustose coralline algae from shallow waters than in those at 22 m depth. Our results highlight the species-specific nature of biochemical responses to environmental stressors, underlining the complexity of predicting the impacts of global change on seaweed physiology and the potential cascading effects on Antarctic food webs.

Non-structural carbohydrates (NSCs) are energetic compounds that can be accumulated in tissues and mobilized during periods of unfavorable conditions to maintain the biological functions of plants. The balance of these biochemical compounds is controlled by environmental factors such as temperature and irradiance. Zostera noltei and Zostera marina find one of their southern distribution limits in southern Spain, where relatively high seawater temperatures are reached during summer (23–24 °C). To better understand the effects of elevated temperatures on the concentration of NSCs, we conducted a seasonal study at Cadiz Bay, representing warm-adapted populations of these species. Our results showed a bimodal pattern in both species, with the highest NSC content observed in December and June, followed by a depletion in March and August. In addition, the NSC content observed in the leaves of Z. noltei (71.26 ± 30.77 mg g⁻¹ dry weight) was higher than in the rhizomes and roots (52.14 ± 38.86 mg g⁻¹ DW). The observed patterns suggest that these species accumulated NSCs to cope with periods of unfavorable environmental conditions. We also suggest that the limited concentration of NSCs in Z. noltei rhizomes and roots indicates that this population may be suffering physiological stress.

Marine macrophytes, including seagrasses and seaweeds, are major contributors to the marine carbon cycle through the release of dissolved organic carbon, a fraction of which is recalcitrant (resistant to microbial degradation for weeks to months), thereby supporting long-term carbon storage. Here we tested how warming and invasion by a non-native seagrass affect carbon dynamics in temperate macrophyte communities from southern Iberia using controlled mesocosm experiments across three temperatures. The invasive seagrass did not substantially alter carbon metabolism or dissolved organic carbon release. However, warming altered the amount and composition of released carbon. The recalcitrant fraction decreased by 28% with increasing temperature, while labile carbon (readily degradable) increased proportionally. When standardized to macrophyte carbon stocks, the recalcitrant fraction produced was comparable to sediment carbon burial rates in the same communities. These results suggest that warming restructures dissolved organic carbon composition, reducing coastal carbon storage capacity and affecting global carbon budget estimates.