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Ocean acidification (OA) is known to affect the physiology, survival, behaviour and fitness of various fish species with repercussions at the population, community and ecosystem levels. Some fish species, however, seem to acclimate rapidly to OA conditions and even thrive in acidified environments. The molecular mechanisms that enable species to successfully inhabit high CO2 environments have not been fully elucidated especially in wild fish populations. Here, we used the natural CO2 seep in Vulcano Island, Italy to study the effects of elevated CO2 exposure on the brain transcriptome of the anemone goby, a species with high population density in the CO2 seep and investigate their potential for acclimation. Compared to fish from environments with ambient CO2, gobies living in the CO2 seep showed differences in the expression of transcripts involved in ion transport and pH homeostasis, cellular stress, immune response, circadian rhythm and metabolism. We also found evidence of potential adaptive mechanisms to restore the functioning of GABAergic pathways, whose activity can be affected by exposure to elevated CO2 levels. Our findings indicate that gobies living in the CO2 seep may be capable of mitigating CO2‐induced oxidative stress and maintaining physiological pH while meeting the consequent increased energetic costs. The conspicuous difference in the expression of core circadian rhythm transcripts could provide an adaptive advantage by increasing the flexibility of physiological processes in elevated CO2 conditions thereby facilitating acclimation. Our results show potential molecular processes of acclimation to elevated CO2 in gobies enabling them to thrive in the acidified waters of Vulcano Island.

Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica , an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6–17.4 with the greatest effect at CO 2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

Marine habitat-forming species provide crucial ecosystem functions and services worldwide. Still, the individual and combined long-term effects of ocean acidification and warming on bryozoan populations, structures, and microbiomes remain unexplored. Here, we investigate the skeletal properties, microbiome shifts, and population trends of two bryozoan species living inside and outside a volcanic CO 2 vent, a natural analog to future ocean acidification conditions. We show that bryozoans can acclimatize to acidification by adjusting skeletal properties and maintaining stable microbiomes. However, we document a decrease in microbial genera playing essential functions under acidified conditions. Moreover, we show that ocean acidification exacerbates bryozoan cover loss and mortality caused by ocean warming. The observed shifts in the microbiome and cover suggest that, despite their morphological plasticity, bryozoan species will be heavily impacted by future ocean conditions, posing a threat to many benthic ecosystems in which they play a pivotal role. Ocean acidification and warming threaten bryozoan populations by altering skeletal properties, disrupting key microbial communities, and increasing mortality, with consequences for the benthic ecosystems they support.

Background Global climate change exacerbates the incidence of marine heatwaves (MHWs), which have increased in intensity and frequency over the past years, causing severe impacts on marine coastal ecosystems. MHWs have already triggered mass mortalities of habitat-forming species, including corals, sponges and gorgonians, in temperate, tropical and polar seas. In the Mediterranean, these high peaks of temperature have been shown to affect several sponge species, and likely, their symbiotic microbial communities. During the summer of 2022, populations of the sponge Petrosia ficiformis (Poiret, 1789) were conspicuously observed with signs of thermal stress linked to a MHW around the Gulf of Naples (Tyrrhenian Sea, Italy). These included depigmentation spots and tissue texture alterations, which often evolved in necrotic processes and eventual death. At the peak of the MHW, however, apparently thermoresistant sponges co-occurred with sensitive unhealthy specimens. In order to explore potential microbial drivers correlated with these divergent thermal-stress tolerances, Healthy and Unhealthy individuals were sampled along the coast of Ischia Island in early September 2022. Results Prokaryotic community characterization based on the 16 S rRNA gene revealed dissimilar compositions in Unhealthy versus apparently Healthy sponges. Increased alpha diversity richness and low evenness in thermosensitive sponges were due to an extensive presence of rare taxa, and to the introduction of potentially pathogenic groups (e.g., Vibrio spp.). Major microbial families regularly associated with P. ficiformis – SAR202, Caldilineaceae , Poribacteria or TK17, were replaced in thermosensitive specimens by professed opportunistic groups within Lentimicrobiaceae , Rhodobacteraceae or Flavobacteriaceae . In turn, conservancy of hub microbes and thermotolerant symbionts (e.g., Rhodothermaceae , Thermoanaerobaculaceae ) in Healthy sponges were observed during this disrupting event. Unhealthy microbiomes reflected lower network stability with respect to Healthy holobionts, due to the inconsistency of functional keystone taxa and prevalence of transient microbes. Conclusions Dysbiotic shifts due to colonization of scavenger groups and opportunistic microbes, and interconnectivity loss characterized thermally stressed sponges. In contrast, resistant specimens retained keystone symbionts that could have ensured functional cooperation, and maintenance of prokaryotic community cohesion under thermal stress. The existence of stress-resistant phenotypes in sponge holobionts offers a glimmer of hope for species persistence, and their study may identify potential source populations for ecosystem recovery.

Ocean acidification (OA) is known to affect the physiology, survival, behaviour and fitness of various fish species with repercussions at the population, community and ecosystem levels. Some fish species, however, seem to acclimate rapidly to OA conditions and even thrive in acidified environments. The molecular mechanisms that enable species to successfully inhabit high CO 2 environments have not been fully elucidated especially in wild fish populations. Here, we used the natural CO 2 seep in Vulcano Island, Italy to study the effects of elevated CO 2 exposure on the brain transcriptome of the anemone goby, a species with high population density in the CO 2 seep and investigate their potential for acclimation. Compared to fish from environments with ambient CO 2 , gobies living in the CO 2 seep showed differences in the expression of transcripts involved in ion transport and pH homeostasis, cellular stress, immune response, circadian rhythm and metabolism. We also found evidence of potential adaptive mechanisms to restore the functioning of GABAergic pathways, whose activity can be affected by exposure to elevated CO 2 levels. Our findings indicate that gobies living in the CO 2 seep may be capable of mitigating CO 2 ‐induced oxidative stress and maintaining physiological pH while meeting the consequent increased energetic costs. The conspicuous difference in the expression of core circadian rhythm transcripts could provide an adaptive advantage by increasing the flexibility of physiological processes in elevated CO 2 conditions thereby facilitating acclimation. Our results show potential molecular processes of acclimation to elevated CO 2 in gobies enabling them to thrive in the acidified waters of Vulcano Island.

Ocean acidification is perceived to be a major threat for many calcifying organisms, including scleractinian corals. Here, we investigate (1) whether past exposure to low pH environments associated with CO 2 vents could increase corals tolerance to low pH and (2) whether zooxanthellate corals are more tolerant to low pH than azooxanthellate corals. To test these hypotheses, two Mediterranean colonial corals Cladocora caespitosa (zooxanthellate) and Astroides calycularis (azooxanthellate) were collected from CO 2 vents and reference sites and incubated in the laboratory under present day (pH on the total scale, pH T 8.07) and low pH conditions (pH T 7.70). Rates of net calcification, dark respiration, and photosynthesis were monitored during a 6‐month experiment. Monthly net calcification was assessed every 27–35 d using the buoyant weight technique, whereas light and dark net calcification was estimated using the alkalinity anomaly technique during 1‐h incubations. Neither species showed any change in net calcification rates, respiration, and photosynthesis regardless of their environmental history, pH treatment, and trophic strategy. Our results indicate that C. caespitosa and As. calycularis could tolerate future ocean acidification conditions for at least 6 months. These results will aid in predicting species' future responses to ocean acidification, and thus improve the management and conservation of Mediterranean corals.

Ocean acidification is perceived to be a major threat for many calcifying organisms, including scleractinian corals. Here, we investigate (1) whether past exposure to low pH environments associated with CO₂ vents could increase corals tolerance to low pH and (2) whether zooxanthellate corals are more tolerant to low pH than azooxanthellate corals. To test these hypotheses, two Mediterranean colonial corals Cladocora caespitosa (zooxanthellate) and Astroides calycularis (azooxanthellate) were collected from CO₂ vents and reference sites and incubated in the laboratory under present day (pH on the total scale, pHT 8.07) and low pH conditions (pHT 7.70). Rates of net calcification, dark respiration, and photosynthesis were monitored during a 6-month experiment. Monthly net calcification was assessed every 27–35 d using the buoyant weight technique, whereas light and dark net calcification was estimated using the alkalinity anomaly technique during 1-h incubations. Neither species showed any change in net calcification rates, respiration, and photosynthesis regardless of their environmental history, pH treatment, and trophic strategy. Our results indicate that C. caespitosa and As. calycularis could tolerate future ocean acidification conditions for at least 6 months. These results will aid in predicting species’ future responses to ocean acidification, and thus improve the management and conservation of Mediterranean corals.

Aim Coralligenous reefs are the main marine bioconstructions in terms of spatial distribution within the Mediterranean basin. Two distinct reef morphologies can be distinguished based on the surface and topographical features of the seafloor: cliffs developing vertical slopes and banks found on gently steep or horizontal bottoms. Despite their importance for monitoring and conservation efforts, observations regarding the variability of biogeographical patterns are scarce. Here, we aimed to assess the differences in the composition and structure of these cliffs across ecoregions and estimate the relative role of abiotic environmental features, geographic location, and connectivity in shaping diversity patterns. Location The study was carried out in the Central‐Western Mediterranean Sea. Samples were collected at 65 sites across the Algero‐Provençal Basin, the Ionian Sea and the Tyrrhenian Sea. Methods We assessed the composition and structure of coralligenous cliffs through photographic samplings collected by scuba divers. Patterns in α‐ and β‐diversity were associated with 9 abiotic environmental variables, latitudinal and longitudinal gradients, and connectivity measures using Generalized Additive (GAM) and Conditional Autoregressive (CAR) models. Results Coralligenous cliffs were primarily composed of algae and displayed a high degree of variability. The Partition Around Medoids (PAM) clustering method successfully delineated seven distinct clusters with a non‐uniform distribution within the studied ecoregions. The α‐diversity increased in eastern and northern sites and with phosphate concentration, while decreased with temperature, chlorophyll and nitrates concentration. β‐diversity at the site level increased with temperature, while it was negatively affected by northward current speed and chlorophyll concentration. Moveover, β‐diversity increased within connected sites. Main Conclusions Coralligenous cliff diversity responds both to the physico‐chemical features of the habitat and between‐habitats connectivity. However, our findings suggest that small‐scale abiotic and biotic processes could contribute to explaining the variability observed. These findings can significantly enhance the monitoring and conservation efforts of this Mediterranean endemic ecosystem.

Naturally acidified environments, such as CO 2 vents, are important sites to evaluate the potential effects of increased ocean acidification on marine ecosystems and biota. Here we assessed the effect of high CO 2 /low pH on otolith shape and chemical composition of six coastal fish species ( Chromis chromis , Coris julis , Diplodus vulgaris , Gobius bucchichi , Sarpa salpa , Symphodus ocellatus ) in a Mediterranean shallow CO 2 vent. Taking into consideration the major and trace elements found near the vent and the gradient of dissolved inorganic carbon, we compared the otolith chemical signatures of fish exposed long-term to elevated CO 2 emissions and reduced pH (mean pH 7.8) against fish living in two control sites (mean pH 8.2). A number of element:Ca ratios (Na:Ca, Mg:Ca, Mn:Ca, Cu:Ca, Zn:Ca, Sr:Ca, Ba:Ca and Pb:Ca), along with isotope ratios, were measured in otoliths (δ 13 C and δ 18 O) and water (δ 13 C DIC ) samples. Additionally, we performed otolith outline shape and morphometric analysis to evaluate the effect of high CO 2 /low pH. We observed species-specific responses with regards to both shape and chemical signatures. Significant differences among sites were found in otolith shape (elliptical Fourier descriptors) of G. bucchichi and D. vulgaris . Elemental and isotopic signatures were also significantly different in these site attached species, though not for the other four. Overall, the carbon isotopic composition seems a good proxy to follow pH gradient in naturally acidified area. Ultimately, besides improving our knowledge of the effects of high CO 2 /low pH on otoliths, the present results contribute to our understanding on their use as natural tags.

Seagrass meadows are natural carbon sinks, yet the effect of ocean acidification on their carbon burial capacity remains poorly understood. Here we investigated natural carbon dioxide vents in Ischia, Italy to assess how seawater pH influences carbon burial in an area dominated by the seagrass Posidonia oceanica . Organic carbon burial rates (mean ± standard error) between 1954 – 2021 were low under ambient conditions (1.5 ± 0.5 g m -2 yr -1 ) but increased sharply under acidified conditions (7 ± 1 g m -2 yr -1 ), reaching sevenfold higher values under extreme acidification (10 ± 3 g m -2 yr -1 ). Stable isotopes suggest that these patterns reflect changes in the relative contribution of seagrass, macroalgae, and epiphytes to buried carbon. These findings reveal that ocean acidification can substantially alter coastal carbon cycling, potentially through shifts in community composition, with important implications for understanding past and future feedbacks between seagrass ecosystems and the marine carbon cycle.