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In the Mediterranean, Cystoseira sensu lato (s.l.) (Phaeophyceae) forests have sharply declined and restoration measures are needed to compensate for the loss. Assisted regeneration through the outplanting of seedlings grown ex-situ has proven to be a sustainable option. Optimizing mesocosm culture can maximize survival of the most critical embryonic stages and reduce long-term maintenance costs. Host-microbiome interactions could also play a crucial role in seedling development and welfare. In this context, we aimed to advance a cultivation protocol that stimulates the growth and fitness of Ericaria amentacea (Phaeophyceae) seedlings and identify the associated microbial biofilm communities. Seedlings were cultured in 6 treatments [i.e., filtered seawater (SW, C, Control), von Stoch-enriched SW (VS), VS + algal extract (VSA), algal extract-enriched SW: A1< A2< A3]. After the field, A2 seedlings had the highest cover (1372 ± 53.66 mm2), which was 1.8 and 1.9 times greater than in VS and VSA, respectively. The addition of the algal extract and nutrients significantly affected the structure and composition of the microbial community that shifted over time in each culture medium. We identified a treatment-specific microbial fingerprint. After the mesocosm phase, A2 was characterized by 4 unique taxa: Postechiella, Winogradskyella, Roseovarius and Arenibacter (Bacteria). Given the success of A2 seedlings, we propose the probiotic consortia candidates characterized by the unique treatment-taxa in conjunction with the shared taxon Psychroserpens (Bacteria, present in A1, A2, VSA, VS) and the reminder community. Within the holobiont concept, the effect of algal extract or nutrients on the algae and/or biofilm could have important consequences for tuning the overall interaction networks. Our study has shown that macroalgal restoration could benefit from both the use of commercial algal extract and tailored nutrient enrichment in ex-situ cultures and the identification of probiotic consortia candidates that promote seedling growth.

Several anthropogenic factors are responsible for the decline of Cystoseira sensu lato (hereafter Cystoseira) forests along Mediterranean coasts. Some Cystoseira species are already regionally extinct, and their decline has been widely recorded. Sustainable and efficient techniques for the restoration of Cystoseira are needed. In this context, the objectives of this study were i) to analyse the reproductive traits of three populations of Gongolaria barbata from three nearby donor sites in the northern Adriatic Sea, assessing the differences in their reproductive potential and reproductive success; and ii) to evaluate the outplanting success in terms of the effectiveness of G. barbata restoration, in relation to the different donor and receiving sites (Miramare MPA and in front of the Marine Biology Station Piran - MBSP) and implemented methods (ex situ and hybrid method combining a mesocosm cultivation and a suspended culture in the field). After 2 weeks of cultivation in mesocosms, half of the tiles with germlings were transported to the receiving sites and placed on suspended lantern nets (hybrid method), which were later (after 3 months) transferred to the seabed on concrete plates with protective cages. The remaining tiles were placed on the seabed on concrete plates with protective cages after a 4-week culture (ex situ method). At both sites, lantern nets and plates were randomly placed at 3 m depth. Thallus length was measured monthly in each treatment. Seedlings in suspended culture showed lower performance at the Miramare MPA, most likely due to the unfavourable environmental conditions. The satisfactory results obtained at MBSP demonstrate the efficiency of the hybrid method and confirm its potential to reduce the cost and time required for cultivation. Since unpredictable climatic events pose the greatest threat to restoration performance, these challenges must be considered when establishing new restoration practices. Moreover, herbivore regulation is extremely urgent and should be planned and implemented on a larger regional scale.

Canopy-forming brown algae support highly productive ecosystems whose decline has been attributed to the interplay of several anthropogenic disturbances. Climate change could have disruptive effects on the biology of these species, but the role of temperature in the development of early life stages is poorly understood. The aim of this study was to assess the response of Ericaria giacconei, a winter-reproducing Southern–Mediterranean endemic species, to thermal stress by testing five temperatures (12, 15, 18, 24, and 28°C) on adults and early stages. Chlorophyll a fluorescence of adult plants was measured at 0, 24, 72, and 120 h on nine fronds in each of the three aquaria per treatment. To assess egg release, zygote settlement, and embryo growth rate, approximately 1,200 receptacles were cultured on six Petri dishes per temperature treatment, and 10 random subsections of 2 ×2 mm were examined in three Petri dishes at 0, 20, 44, and 92 h after fertilization. Adult plants showed a plastic physiological response, and thermal stress had no significant effect on PSII efficiency. Embryos fully developed only at 12 and 15°C. Mortality increased at 18 and 24°C, and no zygotes survived at 28°C. In a scenario of further increasing temperatures, the effects of warming could affect the recruitment of E. giacconei and increase its vulnerability to further stresses. These effects on the survival of early stages, which are the bottleneck for the long-term survival of the species, should be taken into account in conservation and restoration measures to maintain canopy-forming macroalgal populations and associated biodiversity and ecosystem services.

Macroalgal forests are one of the most productive and valuable marine ecosystems, but yet strongly exposed to fragmentation and loss. Detailed large-scale information on their distribution is largely lacking, hindering conservation initiatives. In this study, a systematic effort to combine spatial data on Cystoseira C. Agardh canopies (Fucales, Phaeophyta) was carried out to develop a Habitat Suitability Model (HSM) at Mediterranean scale, providing critical tools to improve site prioritization for their management, restoration and protection. A georeferenced database on the occurrence of 20 Cystoseira species was produced collecting all the available information from published and grey literature, web data portals and co-authors personal data. Data were associated to 55 predictor variable layers in the (ASCII) raster format and were used in order to develop the HSM by means of a Random Forest, a very effective Machine Learning technique. Knowledge about the distribution of Cystoseira canopies was available for about the 14% of the Mediterranean coastline. Absence data were available only for the 2% of the basin. Despite these gaps, our HSM showed high accuracy levels in reproducing Cystoseira distribution so that the first continuous maps of the habitat across the entire basin was produced. Misclassification errors mainly occurred in the eastern and southern part of the basin, where large gaps of knowledge emerged. The most relevant drivers were the geomorphological ones, followed by anthropogenic variables proxies of pollution and urbanization. Our model shows the importance of data sharing to combine a large number of spatial and environmental data, allowing to individuate areas with high probability of Cystoseira occurrence as suitable for its presence. This approach encourages the use of this modeling tool for the prediction of Cystoseira distribution and for supporting and planning conservation and management initiatives. The step forward is to refine the spatial information of presence-absence data about Cystoseira canopies and of environmental predictors in order to address species-specific assessments.