Jellyfish are increasingly recognized as a significant contributor to marine organic matter (OM) on a global scale, with implications for ecosystem dynamics. While the role of jellyfish detritus in microbial nutrient cycling has been explored, the contribution of OM released by live jellyfish—primarily as mucus (hereinafter referred to as mucus-associated OM, or MAOM)—remains understudied. This study investigates the release of organic and inorganic nutrients through MAOM from live jellyfish and their effects on ambient microbial communities in the northern Adriatic Sea using a series of leaching and short-term microcosm experiments. Our results show that per gram of MAOM dry weight from the jellyfish Aurelia spp, approximatively 2 µmol of phosphate, 4 µmol of dissolved inorganic nitrogen, 18 µmol dissolved organic nitrogen, 134 µmol of dissolved organic carbon and 15 µmol of dissolved free amino acids can be released in the ambient seawater in 24 h. Almost half of the OM is released as dissolved OM (DOM), of which a substantial part is low molecular weight (<1 kDa) molecules. During the first 20 h, the DOM fraction of MAOM was rapidly consumed by the ambient microbial community without a corresponding increase in biomass, likely due to nitrogen limitation. In the subsequent 22 h, microbial growth accelerated to 0.19 ± 0.03 h−1 until phosphate became limiting, leading to a sharp decline in microbial production. Our metagenomics analysis revealed that the MAOM-degrading microbial community, dominated by Gammaproteobacteria opportunistic copiotrophs, exhibited increased functional capacity for nutrient assimilation and OM degradation, particularly in the transport and metabolism of amino acids (particularly glycine and taurine) and phosphorus. These traits mirror those found in detritus-degrading microbial communities, suggesting that jellyfish blooms promote the emergence of specialized microbial consortia with shared metabolic capabilities. Taken together, our findings highlight that live jellyfish, through the release of OM, play an active and previously underappreciated role in shaping ambient microbial community dynamics and nutrient fluxes in marine systems affected by jellyfish blooms.

Machine learning models provide a scalable approach for predicting the diversity of eukaryotic microbial plankton from environmental predictors. However, the extent to which these models generalize to data outside the training set remains poorly quantified. In this study, XGBoost was used to predict the 18S rRNA gene Shannon Diversity Index (SDI) from seven environmental predictors derived from satellite and model data. Surface samples were collected between 2001 and 2025 at two fixed stations in the northwestern Mediterranean (BBMO and SOLA), one fixed station in the northern Adriatic Sea (VIDA), and during the HOTMIX expedition, which sampled an east-west open-sea transect across the Mediterranean Sea. Model performance was assessed using standard repeated K-fold cross-validation (CV), Leave-One-Dataset-Out CV (LODO-CV), and a blocked spatiotemporal CV that combined LODO with temporal forward chaining. Under standard K-fold CV, the model showed moderate performance (R² = 0.44, RMSE = 0.59). In contrast, performance declined substantially under LODO-CV (R² = 0.09, RMSE = 0.73), with uniformly low per-dataset generalization, a pattern also observed with blocked spatiotemporal CV. VIDA and HOTMIX sampled environmental regimes distinct from those at BBMO and SOLA, which may partly explain their poor transferability. Additionally, BBMO and SOLA, despite similar environmental conditions, exhibited poor transferability, indicating that technical differences among independently collected 18S rRNA datasets likely constrain transferability, although their effects cannot be disentangled from environmental variation. Overall, these results highlight the limitations of imbalanced training data and underscore the importance of spatially explicit evaluation, protocol standardization, and environmentally representative coverage.

Bacterial extracellular vesicles (BEVs) are nanoscale membranous structures released by diverse types of bacteria, and are capable of transporting and delivering biological compounds between cells. Experimental investigation of BEVs in laboratory model systems indicates that these nanoparticles may play a number of roles in the ecophysiology of marine bacterial communities, but their functional potential in the environment remains unclear. Here we describe the proteomic composition of BEV populations over more than 5000 nautical miles of surface waters in the South Pacific, linking BEV cargoes to the bacterial communities producing them. The presence of marine BEVs was consistently observed across a range of biogeochemical conditions, with an overall abundance comparable to that of bacterial cells (up to 108 BEVs L-1). The protein cargo of marine BEVs, however, differed significantly among ocean regions. The BEV populations were enriched in carbohydrate transporters under phytoplankton bloom conditions, and contained iron uptake-related proteins in nutrient-limited waters. These data suggest that BEVs could enable cells to perform key extracellular functions in the marine environment. Our observations highlight the ubiquity of marine BEVs and biogeographic patterns in their ecological potential across oceanic scales.

Marine prokaryotic communities are major contributors to oceanic food webs and global biogeochemical cycles. However, basin-scale diversity patterns and environmental drivers remain poorly understood. In this study, we applied a machine-learning framework to model the diversity of marine prokaryotic communities across the Mediterranean Sea. Diversity was quantified using the Shannon Diversity Index (SDI) derived from 16S rRNA gene sequencing. The in situ dataset included similar to 600 samples collected year-round from 2001 to 2023 at coastal and open-water sites, providing broad temporal coverage and multisite spatial sampling. We trained an XGBoost model using satellite-derived and modeled oceanographic variables matched to the SDI observations. The model achieved robust predictive performance (R-2 = 0.78 for training and 0.70 for testing, with RMSE = 0.31 and MAPE = 0.05 across both) and captured broad basin spatial and seasonal patterns in prokaryotic community diversity, with greater uncertainty in less-represented regions. Diversity was highest in nutrient-rich coastal areas and during winter mixing, and lowest in summer-stratified or oligotrophic waters. SHAP analysis identified photoperiod as the most significant predictor, underscoring the central role of seasonal light cycles in shaping prokaryotic community diversity. Other predictors exhibited significant season- and region-dependent effects, each contributing positively within specific environmental thresholds. Climatological diversity maps revealed consistent spatiotemporal patterns, highlighting a notable west-to-east decrease in diversity and coastal hotspots. These results demonstrate that machine learning can identify major environmental drivers of prokaryotic diversity and upscale discrete observations to basin-wide predictions. This approach is transferable to other planktonic groups and supports scalable ecosystem monitoring across environmental gradients.

Gelatinous zooplankton (hereinafter cnidarian Medusozoa and ctenophores or "jellyfish") are widespread in marine ecosystems and can form blooms, releasing large amounts of labile, protein-rich organic matter (jelly-OM) upon decay. This material fuels intense bacterial activity, yet its ecological consequences remain poorly understood. We conducted a two-stage microcosm experiment simulating a bloom decay of the invasive ctenophore Mnemiopsis leidyi to examine microbial processing of jelly-OM and its effect on primary production (PP). In the first stage, over the course of 3 days, we observed jelly-OM stimulating rapid growth of opportunistic bacterial community. The community was dominated by Pseudoalteromonadaceae-key degraders of diverse jellyfish, which exhibited enhanced metabolism of amino acids, lipids, and carbohydrates and elevated extracellular enzymatic activities, including leucine aminopeptidase, lipase, chitinase, and alkaline phosphatase. These processes led to marked ammonium accumulation. In the second stage, exposure of a fresh microbial assemblage to residues from jelly-OM degradation resulted in a significant increase of PP and phytoplankton biomass over a period of five days. This was dominated by diatoms and was fueled by accumulated ammonium. Concurrently, the bacterial community shifted toward taxa typically associated with phytoplankton blooms. Together, these results, further supported by in situ observations, reveal a likely coupling between jellyfish decay and phytoplankton growth, suggesting that jellyfish blooms act as transient but powerful nutrient sources capable of triggering ecosystem shifts. As jellyfish are projected to thrive under future ocean conditions, our findings underscore the need to re-evaluate their role in biogeochemical cycles-particularly as overlooked drivers of phytoplankton dynamics.

Diatom blooms influence carbon cycling through organic matter production and its deposition or remineralization - processes mediated by the microbial community. Viruses can influence diatom bloom dynamics and even terminate blooms, yet interactions between diatoms, their viruses, and associated bacteria remain poorly resolved. Here, we examined how infection of the toxigenic diatom Pseudo-nitzschia galaxiae by its ssRNA virus PnGalRNAV reshapes host physiology, microbiome structure, and organic-matter processing in non-axenic batch cultures. Using epi-fluorescence microscopy, 16S rRNA amplicon sequencing, and metatranscriptomics, we linked microbial composition, localisation, and functional activity during viral lysis. Infection rapidly collapsed diatom growth and induced a senescence-like host state, with broad repression of photosynthesis, silicon metabolism, and core biosynthetic pathways, alongside induction of heat-shock and other stress-related genes. Concurrently, phycosphere-associated bacteria declined, detritosphere-associated bacteria increased, and community composition shifted from Marinobacter (Gammaproteobacteria) dominated, towards Flavobacteriaceae (Bacteroidetes) dominated, especially by Polaribacter. In non-infected controls Alphaproteobacteria proved to benefit from the stable healthy phycospheres with a distinct DOM pool. Bacterial metatranscriptomes showed significant upregulation of polysaccharide-degradation-associated genes in infected cultures, indicating active utilisation of lysis-derived diatom glycans. Similar compositional and metabolic profiles in infected cultures and later-stage senescent controls suggest infection accelerated senescence-associated microbial processes. Overall, viral lysis converted a productive diatom culture into a detrital, DOM-rich environment that selects for specialised polysaccharide degraders, redirects carbon through the viral shunt and may accelerate nutrient recycling in coastal systems. Extending this approach to natural microbial communities and diverse diatom-virus systems will help determine whether these mechanisms are broadly conserved.

Once gelatinous zooplankton (GZ) organisms die, they begin to sink. During the sinking process they decay, with decay rates strongly dependent on ambient ocean temperature - warmer temperatures accelerate mass decay rates in the upper water column during the presence of a marine heatwave, leading to reduced GZ carbon flux into the deep ocean. We leverage this temperature dependence of the decay rates to quantify marine heatwave (MHW) related inhibition of vertical GZ mass fluxes out of the euphotic zone (at 200 m depth). We use established methodologies for MHW detection and quantification to isolate some of the strongest MHW events in the northwest Mediterranean in the past 20 years, specifically June 2003, July 2006 and July 2019 events. We present a new Lagrangian tracking class CarbonDrift for the OpenDrift environment, which couples mass decay and organism sinking rates while allowing for horizontal advection during sinking. We use this Lagrangian model for vertical tracking of the sinking organisms and compute the fraction of the sinking organism mass reaching the bottom of the euphotic zone under i) climatological temperature field and ii) during the three mentioned MHWs. The difference between climatological and MHW simulations allows quantification of the impact of MHW on the vertical carbon flux out of the euphotic zone. We show that during each of these marine heatwaves, carbon export out of the euphotic zone (at 200 m) decreases by 2 - 6 % in comparison to exports in climatological conditions. The accumulated effect of this inhibition proportionally diminishes Mediterranean's capacity to act as a deep ocean carbon sink.

Canopy-forming brown macroalgae (Fucales) offer numerous key ecosystem services in Mediterranean coastal areas. However, anthropogenic pressures and climate change have significantly impacted their habitats, leading to an extensive population decline. Interactions between algae and microbiota are a major ecological aspect, yet they represent a significant knowledge gap. In our baseline study, we describe the diversity and host specificity of the microbiome of two genetically identical but morphologically distinct populations of Gongolaria barbata from anthropogenically impacted northern Adriatic Sea. Our preliminary results showed that the microbiomes of G. barbata exhibited low host specificity, with 75% of the algae-associated amplicon sequence variants (ASVs) being part of the core coastal ecosystem microbiome. However, microbiomes of specific algal parts, ambient seawater, and sediment differed significantly in terms of alpha diversity and composition. In contrast, the holdfast and axis show higher similarity with sediment microbiomes, indicating potential horizontal transmission pathways. Microbiomes associated with deciduous parts of morphologically distinct G. barbata populations showed no difference in alpha diversity and composition. In contrast, higher variation in alpha diversity and lower sequence proportion of shared ASVs were observed in the holdfast and axis of the two distinct populations. Our observational study provides valuable new insights and baseline for future hypothesis-driven research on the interactions between algae and associated microbiota-a knowledge gap that needs to be addressed in the future for better understanding of the ecological and evolutionary dynamics of coastal ecosystems.IMPORTANCEOur study focuses on the microbiomes of canopy-forming brown macroalgae from the Fucales order, essential habitat builders in Mediterranean coastal areas. These habitats, offering key ecosystem services, face significant declines due to anthropogenic pressures and climate change. We used next-generation 16S rRNA amplicon sequencing to reveal novel insights into the diversity and host specificity of Gongolaria barbata populations in impacted ecosystems. Our findings suggest environmental factors influence the structure of the algae microbiome, with potential recruitment from adjacent sediment communities. This research enhances the understanding of marine ecosystems' ecological and evolutionary dynamics, providing valuable insights for conservation and management efforts.

Diatom blooms influence carbon cycling through organic matter production and its subsequent deposition or remineralization – processes that are all tightly mediated by interactions with the microbial community. Viruses, as integral part of microbial communities, are known to influence diatom bloom dynamics and can even terminate blooms. However, the three-way interactions between diatoms, their viruses and associated bacteria remain poorly resolved. In this study we examined how infection of the toxigenic diatom Pseudo-nitzschia galaxiae by its ssRNA virus PnGalRNAV reshapes host physiology, microbiome structure and organic-matter processing in non-axenic batch cultures. With an integrated transcriptomics and microscopy-based approach, we investigated the response of the bacterial community to dissolved organic matter (DOM) released by viral lysis of diatoms and observed a significant increase of Flavobacteriaceae (Bacteriodetes) suggesting a specialized role in utilizing DOM released due to viral lysis. Despite overwhelming viral RNA, bacterial metatranscriptomes revealed upregulation of polysaccharide-degradation associated genes, indicating active utilisation of virus-derived diatom glycans. Host transcripts showed broad repression of photosynthesis, silicon metabolism and core biosynthetic pathways, alongside induction of heat-shock and other stress-related genes, consistent with a senescence-like state. Our results demonstrate that PnGalRNAV infection speeds the termination of P. galaxiae growth, rapidly converting a productive diatom culture into a detrital DOM-rich environment that selects for specialised polysaccharide degraders and redirects carbon through the viral shunt. Infected cultures reached senescence much sooner than uninfected ones, suggesting that the ssRNA virus of Pseudo-nitzschia galaxiae can shorten bloom duration and accelerate nutrient recycling, with implications for coastal biogeochemistry. By integrating spatially resolved microscopy with community and metatranscriptomic profiling, this study links microbial composition, localization, and functional activity during diatom viral lysis. Our comprehensive approach can be extended to diverse diatom–virus systems in the future to better predict when viral outbreaks will favour recycling versus export of phytoplankton-derived carbon. ### Competing Interest Statement The authors have declared no competing interest. The Slovenian Research and Innovation Agency, https://ror.org/059bp8k51, 01Z14396, P1–0237 LifeWatch (Slovenia), RI-SI-2 European Marine Biological Resource Centre, Italy, 39124 European Biodiversity Partnership, European Commission, Spanish Ministry for the Ecological Transition and the Demographic Challenge (MITECO), the Spanish Fundación Biodiversidad, the Italian Ministry for Education, University and Research, the Slovenian Ministry of Higher Education, Science and Innovation, and the French National Research Agency., 101052342

Decaying gelatinous zooplankton (GZ) originating from surface waters has been proposed as a possible major contributor to the biological carbon pump. However, studies arrived at largely diverging conclusions concerning the role of decaying GZ as organic matter supply for the deep-sea heterotrophic biota. We complement previous approaches to GZ sinking by proposing the first dynamically consistent physical model coupling GZ sinking speed and its mass. We evaluate GZ contribution to deep-ocean carbon sequestration and to the soft-tissue carbon pump by solving the model equations on the global ocean grid employing monthly climatological temperature fields and published exponential and linear temperature dependencies of mass decay rates. We present the global ocean distribution of the fraction of GZ-mass sinking out of the euphotic zone (200 m depth), twilight zone (1000 m depth) and the fraction of mass reaching the global ocean floor. Solutions in the upper water column are strongly dependent on the mass decay rate. Since most of the decay happens in the initial phase of the sinking process, the sinking-decay coupling exerts a substantial impact on sinking rates but has limited effect on the fraction of mass reaching the bathypelagic and abyssal ocean. Our model approach indicates that there are substantial latitudinal differences in the potential supply of GZ detrital matter to the deep sea. While at low latitudes only negligible amounts of GZ biomass are deposited at the ocean floor, high latitudes allow for substantial GZ detrital mass transport to depths below 1000 m.

ABSTRACT Gelatinous zooplankton (GZ) represents an important component of marine food webs, capable of generating massive blooms with severe environmental impact. When these blooms collapse, considerable amounts of organic matter (GZ-OM) either sink to the seafloor or can be introduced into the ocean’s interior, promoting bacterial growth and providing a colonizable surface for microbial interactions. We hypothesized that GZ-OM is an overlooked marine hotspot for transmitting antimicrobial resistance genes (ARGs). To test this, we first re-analyzed metagenomes from two previous studies that experimentally evolved marine microbial communities in the presence and absence of OM from Aurelia aurita and Mnemiopsis leidyi recovered from bloom events and thereafter performed additional time-resolved GZ-OM degradation experiments to improve sample size and statistical power of our analysis. We analyzed these communities for composition, ARG, and mobile genetic element (MGE) content. Communities exposed to GZ-OM displayed up to fourfold increased relative ARG and up to 10-fold increased MGE abundance per 16S rRNA gene copy compared to the controls. This pattern was consistent across ARG and MGE classes and independent of the GZ species, indicating that nutrient influx and colonizable surfaces drive these changes. Potential ARG carriers included genera containing potential pathogens raising concerns of ARG transfer to pathogenic strains. Vibrio was pinpointed as a key player associated with elevated ARGs and MGEs. Whole-genome sequencing of a Vibrio isolate revealed the genetic capability for ARG mobilization and transfer. This study establishes the first link between two emerging issues of marine coastal zones, jellyfish blooms and ARG spread, both likely increasing with future ocean change. Hence, jellyfish blooms are a quintessential “One Health” issue where decreasing environmental health directly impacts human health.IMPORTANCEJellyfish blooms are, in the context of human health, often seen as mainly problematic for oceanic bathing. Here we demonstrate that they may also play a critical role as marine environmental hotspots for the transmission of antimicrobial resistance (AMR). This study employed (re-)analyses of microcosm experiments to investigate how particulate organic matter introduced to the ocean from collapsed jellyfish blooms, specifically Aurelia aurita and Mnemiopsis leidyi, can significantly increase the presence of antimicrobial resistance genes and mobile genetic elements in marine microbial communities by up to one order of magnitude. By providing abundant nutrients and surfaces for bacterial colonization, organic matter from these blooms enhances ARG proliferation, including transfer to and mobility in potentially pathogenic bacteria like Vibrio. Understanding this connection highlights the importance of monitoring jellyfish blooms as part of marine health assessments and developing strategies to mitigate the spread of AMR in coastal ecosystems.

The assessment and monitoring of microbial plankton biodiversity are essential to obtain a robust evaluation of the health status of marine environments. The PETRI-MED project addresses this imperative by developing novel strategies to monitor the microbial plankton community composition and function, based on satellite observations. PETRI-MED will focus on the Mediterranean Sea as a global biodiversity hotspot with profound ecological and cultural importance. The primary objectives of PETRI-MED project encompass (i) the development of innovative satellite-based indicators to determine the biodiversity status and trends of microbial plankton community, (ii) the identification of spatio-temporal patterns in microbial plankton distribution and diversity, and (iii) the elucidation of key controls of biodiversity patterns, including ecological connectivity, natural and human-related forcings, by focusing on key indicators of ocean’s health and/or biogeochemical state. To do so, PETRI-MED will largely rely on satellite optical radiometric measurements (i.e, Ocean Colour, OC), exploiting the combined temporal and spatial characteristics of latest OC European datasets (i.e., Copernicus Sentinel-3 and European Space Agency OC-CCI) with state-of-the-art remote sensing observations and biogeochemical models (as provided by Copernicus Marine), marine currents modelling, and genomic techniques. To achieve the ambitious goal of merging remote sensing, biogeochemical/physical modelling, and in situ omics measurements, PETRI-MED will rely on Artificial Intelligence (AI). The overarching goal of PETRI-MED is to empower policymakers and stakeholders with the necessary knowledge to adopt prioritization approaches for ecosystem management based on quantitative, real-time metrics. This includes the design and implementation of protection strategies and policies to safeguard biodiversity, quantifying the impact of implemented actions at various levels, and enabling systematic, fact-supported management of Marine Protected Areas (MPAs), Key Biodiversity Areas, and Ecologically or Biologically Significant Marine Areas. Furthermore, PETRI-MED seeks to evaluate the viability of MPA management in response to climate change, ensuring adaptive strategies for the conservation of marine ecosystems in the face of environmental challenges. In summary, PETRI-MED represents a comprehensive and innovative approach to advancing our understanding of microbial plankton biodiversity in the Mediterranean Sea. Through the integration of satellite technology, omics techniques and AI, the project contributes valuable insights and tools for effective marine ecosystem management and conservation strategies.

Many coastal ecosystems worldwide are impacted by wastewater discharges, which introduce nutrients, pollutants, and allochthonous microbes that can alter microbiome composition and function. Although the severity and distribution of these impacts vary across regions, their potential consequences for key ecological processes remain a concern. The resilience and functional adaptability of native coastal microbiomes are still poorly understood. To study the immediate ecological impact of wastewater discharge on a coastal seawater microbiome, we conducted short-term microcosm experiments, exposing a coastal microbiome to two types of treated wastewater: (i) unfiltered wastewater containing nutrients, pollutants, and allochthonous microbes; and (ii) filtered wastewater containing only nutrients and pollutants. By integrating multi-omics and metabolic assays, we show that wastewater-derived organic matter and nutrients (mostly ammonia and phosphate) did not alter the taxonomic composition of the coastal microbiota, but triggered reorganization of metabolic pathways in them. We observed enhanced metabolism of proteins, amino acids, lipids, and carbohydrates, particularly of the lineages Alteromonadales, Rhodobacterales, and Flavobacteriales. Glaciecola (Alteromonadales), a copiotroph with antagonistic traits, significantly contributed to these shifts. Conversely, allochthonous taxa like Legionellales and Pseudomonadales had minimal impact. Elevated phosphorus concentrations resulting from wastewater input reduced the synthesis of proteins linked to scavenging phosphorus from organic phosphorus compounds, including alkaline phosphatase activity in native Rhodobacterales and Flavobacteriales, with important ecological implications for phosphorus-depleted coastal ecosystems. Furthermore, the presence of wastewater caused a decline in relative abundance and metabolic activity of Synechococcus, potentially affecting carbon cycling. Yet, the coastal microbiome rapidly respired wastewater-derived dissolved organic carbon, resulting in bacterial growth efficiencies consistent with global coastal averages. Our findings highlight the capacity of coastal microbiomes to withstand wastewater discharge, with critical implications for assessment of anthropogenic perturbations in coastal ecosystems. However, wastewater-driven changes in metabolic functions and niche utilization within the autochthonous microbial community, impacting phosphorus cycling and potentially affecting carbon cycling, may have long-term consequences for ecosystem functioning.

The investigation of the microbial community change in the biofilm, growing on the walls of a containment tank of TRIGA nuclear reactor revealed a thriving community in an oligotrophic and heavy-metal-laden environment, periodically exposed to high pulses of ionizing radiation (IR). We observed a vertical IR resistance/tolerance stratification of microbial genera, with higher resistance and less diversity closer to the reactor core. One of the isolated Bacillus strains survived 15 kGy of combined gamma and proton radiation, which was surprising. It appears that there is a succession of genera that colonizes or re-colonizes new or IR-sterilized surfaces, led by Bacilli and/or Actinobacteria, upon which a photoautotrophic and diazotrophic community is established within a fortnight. The temporal progression of the biofilm community was evaluated also as a proxy for microbial response to radiological contamination events. This indicated there is a need for better dose-response models that could describe microbial response to contamination events. Overall, TRIGA nuclear reactor offers a unique insight into IR microbiology and provides useful means to study relevant microbial dose-thresholds during and after radiological contamination.

High abundances of gelatinous zooplankton (GZ) can significantly impact marine ecosystem by acting as both sink and source of organic matter (OM) and nutrients. The decay of GZ bloom can introduce significant amount of OM to the ocean interior, with its variability influenced by GZ life traits and environmental factors, impacting microbial communities vital to marine biogeochemical cycles. The invasive ctenophores Mnemiopsis leidyi has formed massive blooms in the northern Adriatic Sea since 2016. However, the variability in the chemical composition and egg production of blooming populations, as well as the role of environmental factors in governing this variability, remains largely unknown. Our analysis of biometry, chemical composition, and fecundity of M. leidyi sampled in the Gulf of Trieste in 2021 revealed stable carbon and nitrogen content throughout bloom development, with no significant correlation with seawater temperature, salinity, oxygen, and chlorophyll a concentration. Although the studied population exhibited homogeneity in terms of biometry and chemical composition, the number of produced eggs varied substantially, showing no clear correlation with environmental variables and being somewhat lower than previously reported for the study area and other Mediterranean areas. We observed a positive correlation between the wet weight of individuals and the percentage of hatched eggs, as well as a significant positive correlation between the percentage of hatched eggs and ambient seawater temperature. Additionally, we noted that the speed of hatching decreased with decreasing seawater temperature in autumn, corresponding to the end of M. leidyi bloom.

Blooms of gelatinous zooplankton, an important source of protein-rich biomass in coastal waters, often collapse rapidly, releasing large amounts of labile detrital organic matter (OM) into the surrounding water. Although these blooms have the potential to cause major perturbations in the marine ecosystem, their effects on the microbial community and hence on the biogeochemical cycles have yet to be elucidated. We conducted microcosm experiments simulating the scenario experienced by coastal bacterial communities after the decay of a ctenophore (Mnemiopsis leidyi) bloom in the northern Adriatic Sea. Within 24 h, a rapid response of bacterial communities to the M. leidyi OM was observed, characterized by elevated bacterial biomass production and respiration rates. However, compared to our previous microcosm study of jellyfish (Aurelia aurita s.l.), M. leidyi OM degradation was characterized by significantly lower bacterial growth efficiency, meaning that the carbon stored in the OM was mostly respired. Combined metagenomic and metaproteomic analysis indicated that the degradation activity was mainly performed by Pseudoalteromonas, producing a large amount of proteolytic extracellular enzymes and exhibiting high metabolic activity. Interestingly, the reconstructed metagenome-assembled genome (MAG) of Pseudoalteromonas phenolica was almost identical (average nucleotide identity >99%) to the MAG previously reconstructed in our A. aurita microcosm study, despite the fundamental genetic and biochemical differences of the two gelatinous zooplankton species. Taken together, our data suggest that blooms of different gelatinous zooplankton are likely triggering a consistent response from natural bacterial communities, with specific bacterial lineages driving the remineralization of the gelatinous OM.IMPORTANCEJellyfish blooms are increasingly becoming a recurring seasonal event in marine ecosystems, characterized by a rapid build-up of gelatinous biomass that collapses rapidly. Although these blooms have the potential to cause major perturbations, their impact on marine microbial communities is largely unknown. We conducted an incubation experiment simulating a bloom of the ctenophore Mnemiopsis leidyi in the Northern Adriatic, where we investigated the bacterial response to the gelatinous biomass. We found that the bacterial communities actively degraded the gelatinous organic matter, and overall showed a striking similarity to the dynamics previously observed after a simulated bloom of the jellyfish Aurelia aurita s.l. In both cases, we found that a single bacterial species, Pseudoalteromonas phenolica, was responsible for most of the degradation activity. This suggests that blooms of different jellyfish are likely to trigger a consistent response from natural bacterial communities, with specific bacterial species driving the remineralization of gelatinous biomass.

Jellyfish, and gelatinous zooplankton (GZ) in general, represent an important component of marine food webs. Certain GZ species are capable of generating massive blooms of severe environmental impact. These blooms are often followed by a sudden collapse of the entire population, introducing considerable amounts of organic matter (GZ-OM) in the ocean’s interior. GZ-OM represents an abundant substrate to promote bacterial growth and copious colonizable surface for microbial interactions. Hence we hypothesized that this GZ-OM serves as a yet overlooked hotspot for transmitting antimicrobial resistance genes (ARGs) in marine environments. For this we experimentally evolved and analyzed marine microbial communities in microcosms in presence and absence of OM from scyphozoan Aurelia aurita s.l. and ctenophore Mnemiopsis leidyi . Communities evolved under GZ-OM exposure displayed an up to 4-fold increase in relative ARG and an up to 10-fold increase in abundance of horizontally transferable mobile genetic elements (MGEs) per 16S rRNA gene copy compared to the controls. This trait was consistent across ARG and MGE classes and independent of the GZ species, suggesting that the underlying mechanism is indeed based on the general influx of nutrients and colonizable surfaces. Potential ARG carriers included known key GZ-OM degraders, but also genera containing potential pathogens hinting towards an increased risk of ARG transfer to pathogenic strains. Here, Vibrio were pinpointed as potential key species directly associated with several significantly elevated ARGs and MGEs. Subsequent whole-genome sequencing of a Vibrio isolate from the microcosm experiment revealed the genetic potential for the mobilization and transfer of ARGs in GZ-OM degrading microbial consortia. With this study, we established the first link between two emerging issues of marine coastal zones, jellyfish blooms and AMR spread, both likely increasing in projected future ocean scenarios. ### Competing Interest Statement The authors have declared no competing interest.

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L−1; median: 8.0 µmol N L−1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

It was once believed that only microbes and viruses inhabited the subseafloor crust beneath hydrothermal vents. Yet, on the seafloor, animals like the giant tubeworm Riftia pachyptila thrive. Their larvae are thought to disperse in the water column, despite never being observed there. We hypothesized that these larvae travel through the subseafloor via vent fluids. In our exploration, lifting lobate lava shelves revealed adult tubeworms and other vent animals in subseafloor cavities. The discovery of vent endemic animals below the visible seafloor shows that the seafloor and subseafloor faunal communities are connected. The presence of adult tubeworms suggests larval dispersal through the recharge zone of the hydrothermal circulation system. Given that many of these animals are host to dense bacterial communities that oxidize reduced chemicals and fix carbon, the extension of animal habitats into the subseafloor has implications for local and regional geochemical flux measurements. These findings underscore the need for protecting vents, as the extent of these habitats has yet to be fully ascertained. Microbes and viruses inhabit the subseafloor crust beneath hydrothermal vents. Here the authors show that vent endemic animals such as giant tubeworms also live in vent subseafloor cavities, implicating subseafloor dispersal of vent larvae and the need to protect seafloor and subseafloor vent habitats.

The diverse microbial community in the ocean, encompassing various metabolic types, interacts with the wide array of compounds in the dissolved organic matter (DOM) pool, thereby influencing the ocean’s biogeochemical state and, consequently, the global climate. Our understanding of the interactions between specific DOM constituents and microbial consortia remains limited, necessitating further refinement to achieve a mechanistic comprehension of the relationship between the DOM field and the microbial metabolic network. Attaining this level of understanding is crucial for accurately predicting the marine ecosystem’s response to natural and anthropogenic perturbations. To address this gap, we developed a bacterial population model based on the von Foerster equation. This model aims to describe the complex microbial-mediated degradation of gelatinous zooplankton (hereinafter ‘jellyfish’) detritus, as an important, but largely overlooked source of DOM in the ocean. By considering microbial growth and decay, as well as DOM uptake, and nutrient release, the model is able to describe the microbial community’s life cycle, and the biochemical transformations of the jellyfish-derived organic matter. We fitted the model to results of laboratory microcosm experiments conducted to simulate scenarios experienced by ambient microbiomes during decay of two different jellyfish species in the northern Adriatic Sea. By interpreting the fitted parameters, we highlight the differences in the microbial response to different jellyfish species, namely how these affect the microbial community composition and the release of nutrients. This model has been specifically designed for integration with ocean circulation models to create a comprehensive physical-biogeochemical ocean model. Such an extended model can be utilized for multi-scale simulations to assess the system’s response to jellyfish and jellyfish-derived organic matter. Given that jellyfish blooms may become more prevalent under future ocean scenarios, this modeling approach is essential for understanding their potential impact on marine ecosystems.