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Coastal marine lagoons are environments highly vulnerable to anthropogenic pressures such as agriculture nutrient loading or runoff from metalliferous mining. Sediment microorganisms, which are key components in the biogeochemical cycles, can help attenuate these impacts by accumulating nutrients and pollutants. The Mar Menor, located in the southeast of Spain, is an example of a coastal lagoon strongly altered by anthropic pressures, but the microbial community inhabiting its sediments remains unknown. Here, we describe the sediment prokaryotic communities along a wide range of environmental conditions in the lagoon, revealing that microbial communities were highly heterogeneous among stations, although a core microbiome was detected. The microbiota was dominated by Delta- and Gammaproteobacteria and members of the Bacteroidia class. Additionally, several uncultured groups such as Asgardarchaeota were detected in relatively high proportions. Sediment texture, the presence of Caulerpa or Cymodocea, depth, and geographic location were among the most important factors structuring microbial assemblages. Furthermore, microbial communities in the stations with the highest concentrations of potentially toxic elements (Fe, Pb, As, Zn, and Cd) were less stable than those in the non-contaminated stations. This finding suggests that bacteria colonizing heavily contaminated stations are specialists sensitive to change.

Seagrass ecosystems usually respond in a nonlinear fashion to increasing pressures and environmental changes. Feedback mechanisms operating at the ecosystem level and involving multiple interactions among the seagrass meadow, its associated community and the physical environment are known to play a major role in such nonlinear responses. Phenotypic plasticity may also be important for buffering these ecological thresholds (i.e., regime shifts) as many physiological processes show nonlinear responses to gradual environmental changes, conferring the appearance of resistance before the effects at the organism and population levels are visible. However, the potential involvement of plant plasticity in driving catastrophic shifts in seagrass ecosystems has not yet been assessed. In this study, we conducted a manipulative 6-month light-gradient experiment in the field to capture nonlinearities of the physiological and population responses of the seagrass Cymodocea nodosa to gradual light reduction. The aim was to explore if and how the photo-acclimatory responses of shaded plants are translated to the population level and, hence, to the ecosystem level. Results showed that the seagrass population was rather stable under increasing shading levels through the activation of multilevel photo-acclimative responses, which are initiated with light reduction and modulated in proportion to shading intensity. The activation of photo-physiological and metabolic compensatory responses allowed shaded plants to sustain nearly constant plant productivity (metabolic carbon balance) along a range of shading levels before losing linearity and starting to decline. The species then activated plant- and meadow-scale photo-acclimative responses and drew on its energy reserves (rhizome carbohydrates) to confer additional population resilience. However, when the integration of all these buffering mechanisms failed to counterbalance the effects of extreme light limitation, the population collapse

1. The critically endangered fan mussel Pinna nobilis, endemic to the Mediterranean Sea, faces a drastic population decline all over the Mediterranean Sea owing to an infectious disease. 2. Coastal lagoons represent one of this species’ last reservoirs; thus, knowledge of the population conservation status in these ecosystems is essential. 3. An extensive monitoring programme was conducted during 2014, 2017 and 2019 in the Mar Menor lagoon, a highly eutrophicated lagoon from Spain. On average, 32,535 m2 have been surveyed each year to determine population densities, distributions and size structures. 4. The cumulative impacts produced by intensive agriculture activity around the lagoon, together with a catastrophic meteorological phenomenon that occurred in 2019, have triggered a significant decline in fan mussel population densities and in population mean shell sizes over the years. Additionally, some fan mussels were infected by the protozoan Haplosporidium pinnae. 5. Therefore, the current P. nobilis population from the Mar Menor is mainly composed of young individuals in specific areas that are facing an extreme situation. Urgent management and conservation actions focused on restoring the ecosystem and protecting P. nobilis individuals are urgently needed, together with reinforcement projects to increase the species’ stock in one of its last reservoirs.

The invasive seaweed Caulerpa cylindracea has shown a reduced ability to invade healthy Posidonia oceanica meadows by penetrating only meadow margins in early invasion stages in the western Mediterranean Sea. However, the long-term interaction with invasive seaweed could deteriorate the structure of meadows by diminishing their initial resistance to invasion as a result of potential competitive mechanisms between both macrophytes (e.g. allelopathic effects, enhanced sediment anoxia). In this study, populations of both species were monitored over a 10-year period (2007–2016) in invaded and non-invaded sites to assess meadows’ resistance evolution to the presumed long-term negative interactions between both macrophytes. The C. cylindracea biomass in the seagrass canopy was much lower (from 5- to 60-fold) than the biomass that developed just outside the seagrass meadows at all the invaded sites. The monitored seagrass populations showed stable and/or progressive trends throughout the study period, and no structural differences were observed between invaded and non-invaded meadows. To conclude, our results evidence, for the first time, the absence of a long-term competitive interaction between invasive seaweed and the structure and shoot dynamics of native P. oceanica meadows. The long-term presence of persistent gradients of algal biomass from outside to inside meadows supports the existence of highly limiting conditions for algal growth and survival in undisturbed P. oceanica canopies, what reinforces the role of healthy meadows acting as ecological barriers against the spread of C. cylindracea. This highlights the importance of conserving valuable P. oceanica meadows as a way to control bioinvasions in the Mediterranean Sea.

Global warming is emerging as one of the most critical threats to terrestrial and marine species worldwide. This study assessed the effects of simulated warming events in culture on two seagrass species, Posidonia oceanica and Cymodocea nodosa, which play a key role in coastal ecosystems of the Mediterranean Sea. Changes in fatty acids as key metabolic indicators were assessed in specimens from two geographical populations of each species adapted to different in situ temperature regimes. Total fatty acid (TFA) content and composition were compared in C. nodosa and P. oceanica from natural populations and following exposure to heat stress in culture. After heat exposure, individuals of C. nodosa and P. oceanica adapted to colder temperatures in situ accumulated significantly more TFA than controls. For both species, the proportion of polyunsaturated fatty acids (PUFA) decreased, and the percentage of saturated fatty acids (SFA) increased significantly after the heat treatment. These results highlight that populations of both species living at warmest temperatures in situ were more thermotolerant and exhibited a greater capacity to cope with heat stress by readjusting their lipid composition faster. Finally, exposure of seagrasses to warmer conditions may induce a decrease in PUFA/SFA ratio which could negatively affect their nutritional value and generate important consequences in the healthy state of next trophic levels.

Sexual reproduction in predominantly clonal marine plants increases recombination favoring adaptation and enhancing species resilience to environmental change. Recent studies of the seagrass Posidonia oceanica suggest that flowering intensity and frequency are correlated with warming events associated with global climate change, but these studies have been observational without direct experimental support. We used controlled experiments to test if warming can effectively trigger flowering in P. oceanica. A six-week heat wave was simulated under laboratory mesocosm conditions. Heating negatively impacted leaf growth rates, but by the end of the experiment most of the heated plants flowered, while controls plants did not. Heated and control plants were not genetically distinct and flowering intensity was significantly correlated with allelic richness and heterozygosity. This is an unprecedented finding, showing that the response of seagrasses to warming will be more plastic, more complex and potentially more resilient than previously imagined

The Mediterranean endemic seagrass Posidonia oceanica is generally regarded as a stenohaline species, highly sensitive to salinity increments; however, in a few particular cases, natural populations can grow under salinity levels above its normal threshold of tolerance. One such case is a population of P. oceanica in the southeastern coastal region of Spain, which is able to survive under the fluctuating influence of hypersaline waters coming from an adjacent coastal lagoon (Mar Menor). The present work examines the physiological mechanisms underlying the species’ ability to overcome this hypersaline stress and persist in the long term. To this end, the physiological, morphological, and population statuses of plants from the site of influence were compared to those from a reference site where plants grew under normal conditions P. oceanica leaves from the influenced sites showed more negative water potentials than those from the reference sites as a response to maintain a positive water balance under hypersaline conditions. However, these lower water potentials were not explained by the accumulation of intracellular solutes since their osmotic potential were similar to reference leaves and their ionic content generally lower. In addition, these leaves also accumulated higher concentrations of proline and soluble sugars but these organic osmolytes are more likely acting as osmo-protectants, rather than as osmoticums. These responses indicated that plants growing at the influenced site have developed physiological strategies to maintain lower ion concentration in their leaf tissues in order to avoid the alteration of ion homeostasis (i.e., ionic ratios), which can be toxic for plant metabolism. Photosynthesis and photochemistry were not adversely affected in leaves exposed to hypersalinity; in fact, these processes showed a tendency to become enhanced, possibly to support the assimilation of anthropogenic nitrogen coming from the lagoon waters. At the individual and p

Knowledge of the distribution and extent of seagrass habitats is currently the basis of management and conservation policies of the coastal zones in most European countries. This basic information is being requested through European directives for the establishment of monitoring programmes and the implementation of specific actions to preserve the marine environment. In addition, this information is crucial for the quantification of the ecological importance usually attributed to seagrass habitats due to, for instance, their involvement in biogeochemical cycles, marine biodiversity and quality of coastal waters or global carbon budgets. The seagrass atlas of Spain represents a huge collective effort performed by 84 authors across 30 Spanish institutions largely involved in the scientific research, management and conservation of seagrass habitats during the last three decades. They have contributed to the availability of the most precise and realistic seagrass maps for each region of the Spanish coast which have been integrated in a GIS to obtain the distribution and area of each seagrass species. Most of this information has independently originated at a regional level by regional governments, universities and public research organisations, which explain the elevated heterogeneity in criteria, scales, methods and objectives of the available information. On this basis, seagrass habitats in Spain occupy a total surface of 1,541,63 km2, 89% of which is concentrated in the Mediterranean regions; the rest is present in sheltered estuarine areas of the Atlantic peninsular regions and in the open coastal waters of the Canary Islands, which represents 50% of the Atlantic meadows. Of this surface, 71.5% corresponds to Posidonia oceanica, 19.5% to Cymodocea nodosa, 3.1% to Zostera noltii (=Nanozostera noltii), 0.3% to Zostera marina and 1.2% to Halophila decipiens. Species distribution maps are presented (including Ruppia spp.), together with maps of the main impacts and pres