Coastal Vibrio harbor a high diversity of underexplored antimicrobial resistance genes (ARGs). Their selective maintenance in the absence of antibiotic selection is puzzling. While their mobilization is rare, it may just be a question of time when the ecological context turns the coastal environment into a source for ARG spillover.

Coastal environments are increasingly recognised as reservoirs of known antibiotic resistance genes (ARGs), but are less frequently identified as sources of novel ARGs. Here, we investigated class A β-lactamases circulating in European coastal environments used for oyster farming. We examined their diversity, function, and the ecological factors associated with their geographic distribution and environmental dynamics. A high diversity of carbenicillinases was detected in the culturable microbiome of European oysters. The Harveyi and Splendidus clades were key Vibrio lineages structuring the geography of carbenicillinase diversity. The Harveyi clade was primarily associated with the circulation of known carbenicillinases in Mediterranean samples, whereas the Splendidus clade contributed previously uncharacterized carbenicillinase sequences across all Europe. A one-year seasonal monitoring revealed that Vibrio alginolyticus drives the circulation of blaCARB-42 in the Mediterranean Thau lagoon, with dynamics strongly associated with seawater temperature. blaCARB-42 conferred intrinsic resistance to both carboxypenicillins and aminopenicillins in V. alginolyticus, which was found in most other species of the Harveyi clade with additional resistances to aztreonam, third-generation cephalosporins and aminoglycosides. Since the Harveyi clade includes major human pathogens, these findings have direct implication for environmental and One Health surveillance, as rising seawater temperatures may increase coastal exposure to antibiotic-resistant Harveyi clade Vibrio.

Antimicrobial resistance is a major global health threat, and the environment is increasingly recognised as a key reservoir of antibiotic resistance genes (ARGs). In marine ecosystems, filter-feeding organisms such as oysters are sentinel species that reflect the microbiological diversity of their environment. Here, we investigated the ARG content of the Pacific oyster Magallana gigas across four European coastal sites, contrasting anthropogenically impacted aquaculture sites (Bay of Brest, Ebro Delta, Thau lagoon) with natural oyster reefs (Sylt island). ARGs sequencing was enriched using a targeted probe-and-capture metagenomic approach employing 37,826 RNA baits covering over 2000 ARG sequences. Moreover, a novel bioinformatic pipeline ARGifier was specifically designed for sensitive, high-resolution detection and classification of ARGs in metagenomic samples dominated by host DNA. This strategy was able to overcome limitations of cultivation-based methods and whole metagenomic sequencing, enabling the detection of more than 33,000 ARG sequences in the oyster resistome spanning over 450 genes distributed over 23 resistance classes. Reads assigned to ARGs represented 0.23

An increase in temperature due to climate change may affect the geographic ranges of invasive parasites and alter their impact on native hosts. Our goal was to determine if the effects of infection by two species of invasive endoparasitic copepods on native blue mussel hosts (Mytilus edulis) change with increasing temperatures. We investigated this with a laboratory experiment using temperatures that represent annual mean and mean summer water temperatures of past observations and future predictions for the study area, the European Wadden Sea (10–26 °C). Over a period of 8–20 weeks, infection with Mytilicola intestinalis lowered mussel condition and infection with Mytilicola orientalis decreased mussel shell growth. High temperatures decreased mussel growth and condition in general, but only at low temperatures (10–14 °C) the parasite-induced loss of condition was evident compared to uninfected mussels. Mussel mortality and reproductive activity were not affected by parasite infection, although both were impacted by temperature: the highest temperature (26 °C) increased mussel mortality, and gamete ripening only occurred at lower temperatures (10–18 °C). Taken together, these results suggest that both infection and high temperatures have independent negative effects. However, an increase in temperature does not worsen the effect of infection on individual mussel hosts, and neither does infection decrease host tolerance for long-term exposure to high temperatures. These findings add to our understanding of the interplay between increasing temperature and the interaction between invasive parasites and native hosts, and help predicting host and parasite dynamics in systems affected by species invasions and climate change.

It is in the very nature of coastal ecosystems to respond rapidly to external driving factors from land, sea, and man. Human-induced environmental change from local to global scale, and introduced species in particular, has accelerated ecological transformation at many coasts worldwide. A series of major events has fundamentally changed the macrobenthos in a tidal bay of the Wadden Sea (Königshafen, south-eastern North Sea, Atlantic Europe) since 1924. (1) Overexploitation extirpated native oysters. (2) The once dominating seagrasses declined dramatically in the 1930s due to an introduced pandemic disease. This was followed by a loss of fine mud, and an expansion of bioturbating lugworms on sandy flats. (3) In the 1980s–2000s, seagrasses declined further and massive green algal mats spread due to eutrophication. (4) Mussel beds gradually expanded in spite of recurrent resets caused by severe winters. (5) Since the 2000s, introduced Pacific oysters dominated mussel beds. With the spread of the mixed reefs of native mussels and Pacific oysters, the extent of mud flats increased again. (6) Since the last three decades, many more non-native species, that are well adapted to a warmer climate, have increased benthic diversity. The ecological developments in this small bay may anticipate change on larger scales and could serve as a model site for learning about the role of contingent history for the functioning of a novel benthic ecosystem.

Marine protected area (MPA) networks are important for supporting biodiversity, enhancing ecosystem resilience, and facilitating species recovery. For the effectiveness of conservation and restoration, functional connectivity plays a vital role. The dispersal, movement, and successful establishment of organisms between suitable habitats and MPAs ensure long-term sustainability of the populations. Despite its importance, functional connectivity is rarely integrated into restoration planning, which limits the effectiveness of species reintroductions, habitat connectivity, and adaptation to environmental changes. In this study, we applied an integrative approach combining molecular detections (environmental DNA [eDNA] and meroplankton metabarcoding) with biophysical modeling to explore the functional connectivity between two Natura 2000 MPAs in the North Sea: Borkum Reef Ground (BRG) and Sylt Outer Reef (SOR). We focused on the European flat oyster (Ostrea edulis), a reef-building species that once provided vast reef habitats but is now functionally extinct in the German Bight and is therefore the subject of recent restoration measures at BRG. Our results showed partial but informative correspondence between molecular detections of oyster genetic traces and the modeled larval pathways during the June-July 2022 sampling period. We further explored larval dispersal across entire spawning seasons in 2022 and 2023. Connectivity between BRG and SOR was highly dependent on larval drift depth. Surface-drifting larvae showed strong interannual variability, with 3% reaching SOR in 2022 when northwesterly winds dominated, increasing to 22% in 2023 under westerly and southwesterly winds. Larvae drifting at depth, however, exhibited near-zero connectivity, leading to high self-recruitment rates, with over 25% settling near the original restoration sites. Our results demonstrate that wind-driven currents are a key driver of interannual variability in larval retention and dispersal. Additionally, they highlight the role of biological traits, such as vertical positioning and pelagic larval duration, in shaping connectivity between MPAs and oyster restoration sites. These findings emphasize the need to integrate connectivity assessments into MPA management and the restoration planning of reef-building benthic species. The interdisciplinary approach presented here provides a quantitative framework for assessing connectivity under species- and site-specific conditions, offering a transferable tool to evaluate the restoration potential of other species and enhance the functional network between MPAs.

Climate change is likely to affect infectious diseases that are facilitated by biological invasions, with repercussions for wildlife conservation and zoonotic risks. Current invasion management and policy are underprepared for the future risks associated with such invasion-related wildlife diseases. By considering evidence from bioclimatology, invasion biology, and disease research, we illustrate how climate change is anticipated to affect disease agents (parasites and pathogens), hosts, and vectors across the different stages of invasions. We highlight the opportunity to integrate these disciplines to identify the effects of climate change on invasion-related wildlife diseases. In addition, shifting to a proactive stance in implementing management and policy, such as by incorporating climate-change effects either into preventative and mitigation measures for biosecurity or with rapid response protocols to limit disease spread and impacts, could help to combat future ecological, economic, and human health risks stemming from invasion-related wildlife diseases.

The role of microbiota in health and disease is most often expressed by structural shifts of the taxonomic composition of prokaryote communities in infected and healthy individuals. In cultured aquatic animals with open circulatory systems, such as mollusks, microbiota also harbor a wide range of protists, which are unicellular eukaryotes that could also play an important role during infections. To evaluate the effectiveness of eukaryotic vs. prokaryotic microbiota in characterizing infection states, we examined both microbial compartments under natural conditions in two commercially important oyster species, the flat oyster Ostrea edulis and the Pacific oyster Magallana (Crassostrea) gigas. With O. edulis being infected by two protist parasites, Marteilia refringens and Bonamia ostreae, and M. gigas being infected by the ostreid herpes virus OsHV-1, we chose iconic diseases responsible for substantial mortalities and economic damage within the two species. We analyzed and compared the structural and compositional differences between healthy and infected oysters and used random forest machine learning to classify infection states and identify indicator taxa that distinguish healthy from infected individuals. Both at the structural and compositional levels, bacterial microbiota proved to be better predictors of infection states. By eliminating noisy taxa through variable selection in the random forest models, we enhanced the compositional differences between infection states. In all host-pathogen combinations, only a few taxa (<31) were required to achieve optimal separation. While the identity of indicator taxa will partly reflect the specific environmental conditions at the time of sampling, we recovered several previously described indicator taxa, such as Mycoplasma, Vibrio, Photobacterium, and Arcobacter. Next to these we also discovered new taxa like Motiliproteus that exhibited the potential to differentiate between infection states of the investigated O. edulis specimen. The simultaneous characterization of prokaryotic and eukaryotic microbiota suggests that only few prokaryotic indicator species might be needed to reliably differentiate between infected from healthy individuals and monitor infection risks.

Climate change can impact the persistence of native and invasive parasites and their effects on hosts. Given the complexity of interactions in natural systems, models based on parasite-host systems can be helpful to explore long-term impacts. We investigate how two intestinal parasitic copepods impact host populations, and how the predicted temperature increase by year [Formula: see text] may affect the persistence and impacts of the parasites. We study Mytilicola intestinalis (a specialist established in blue mussels, Mytilus edulis) and Mytilicola orientalis (a recent invader infecting mussels and Pacific oysters, Magallana gigas) in the Wadden Sea. The parasites are non-lethal but can influence host maturation and fecundity. Using a mathematical model parametrized with empirical, field and literature data, we explore how temperature increase affects parasite basic reproduction numbers and the long-term population trends of parasites and mussels. Temperature increase reduces mussel populations below the critical community size for M. intestinalis persistence, while allowing M. orientalis to persist without oysters. M. orientalis does not have a negative effect on the host population in additional to that of M. intestinalis when both are present. We show that environmental change can have qualitatively different effects on related parasites by changing the role of the shared host as a maintenance population.

Colistin is a widespread last resort antibiotic for treatment of multidrug-resistant bacteria. The recent worldwide emergence of colistin resistance (Col-R) conferred by mcr-1 in human pathogens has raised concern, but the putative sources and reservoirs of novel mcr genes in the marine environment remain underexplored. We observed a high prevalence of Col-R, particularly in Vibrio isolated from European coastal waters by using the same cohorts of oysters as bioaccumulators in three sites across Europe. The high sequence diversity found in the mcr/eptA gene family was geographically structured, particularly for three novel eptA gene variants, which were restricted to the Mediterranean (France, Spain) and occurred as a dgkA-eptA operon. The RstA/RstB two component system was shown to control both the dgkA-eptA operon and the Col-R phenotype. The analysis of 29 427 Vibrionaceae genomes revealed that this mechanism of intrinsic resistance is prevalent and specific to the Harveyi clade, which includes the human pathogens Vibrio parahaemolyticus and Vibrio alginolyticus. The operon conferred colistin-resistance when transferred to sensitive non-Vibrio strains. In general, eptA gene variants are widespread and evolved with the Vibrio lineage. They occur in clade-specific genomic environments, suggesting that eptA expression responds to distinct environmental signals across the Vibrio phylogeny. However, we also identified mobile eptA paralogues that have been recently transferred between and within Vibrio clades. This highlights Vibrio as a potential source of Col-R mechanisms, emphasizing the need for enhanced surveillance to prevent colistin-resistant infections in coastal areas.

This study investigated the transport of European flat oyster (Ostrea edulis) larvae between two Marine Protected Areas in the North Sea: Borkum Reef Ground (BRG), where oysters were recently reintroduced, and Sylt Outer Reef (SOR). Additionally, we determined the source of oyster genetic material collected during cruises in 2022 between BRG and SOR. To achieve these goals, numerical simulations focusing on surface and depth-averaged water mass transport were conducted using the FESOM-C coastal ocean model with a forward/backward Lagrangian module. Surface drifter data were also analysed to examine surface transport and validate the model output. Our results indicate that typical summer wind conditions, along with tidal residual currents, support the transport of water masses and passive tracers from BRG to SOR. Surface water masses from BRG generally approach SOR from the south and west. However, BRG and SOR are usually connected over periods exceeding two weeks, even considering the fastest surface currents. Strong and persistent south-westerly winds, which are uncommon in summer, can accelerate this connection to under two weeks. Conversely, strong and persistent easterly or south-easterly winds, also rare in summer, can prevent some passive tracers originating from BRG from ever reaching SOR or the eastern North Sea. In the case of depth-averaged transport, significantly more time is required, with a minimum duration of eleven weeks to connect the domains. This connection could be facilitated by an intermediary habitat - as a stepping stone in the transition zone, if that provides suitable habitat for settlement and subsequent larval production.

Exploitation of host resources by parasites can have profound impacts on infected hosts. When prevalence is high parasite infection can even feed back on host population dynamics. Especially when parasites are invasive species, their new native hosts can suffer from exploitation due to a lack of co-evolutionary history. Nevertheless, energetic costs are often subtle at the level of the individual and hard to link to population level effects. Sublethal energetic costs accumulate over time and can in sum be traded off against fitness over longer time scales. To study temporal accumulation of infection costs, we used a series of controlled infection experiments to assess the physiological effects exerted by the invasive parasitic copepod Mytilicola intestinalis on its newly acquired native host, the blue mussel Mytilus edulis in the North Sea, where the parasite can reach prevalences > 70 %. To link short-term physiological responses to long term components of fitness, we combined several methodological approaches measuring changes of physiological traits that act on different time scales. Stable isotope analyses over different seasons and environments revealed a direct consumption of host tissue by M. intestinalis. Tissue repair increased energy demand that could directly be observed in accelerated heartbeat rates, and changes of amino acid metabolism measured by 1H NMR spectroscopy. Although these effects were comparatively small in size, the resulting altered energy budget of the mussel host led to a lower body condition index under controlled laboratory conditions on medium time scales (months) and slower growth in the field over longer time scales (1 year). In combination, our experimental results show that small short-term physiological changes can translate to fitness relevant negative effects on life history traits when integrated across temporal scales.

Climate change effects on coastal ecosystems vary on large spatial scales, but can also be highly site dependent at the regional level. The Wadden Sea in the south-eastern North Sea is warming faster than many other temperate coastal areas, with surface seawater temperature increasing by almost 2 °C over the last 60 years, nearly double the global ocean mean increase. Climate warming is accompanied by rising sea levels, which have increased by approximately 2 mm yr−1 over the last 120 years. For this sedimentary coast, the predicted acceleration of sea-level rise will have profound effects on tidal dynamics and bathymetry in the area. This paper synthesises studies of the effects of ocean warming and sea level rise in the northern Wadden Sea, largely based on research conducted at the Wadden Sea Station Sylt of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. An increasing rate of sea level rise above a critical threshold will lead to coastal erosion and changes in sediment composition, and may cause the transition from a tidal to lagoon-like environment as tidal flats submerge. This involves changes to coastal morphology, and the decline of important habitats such as muddy tidal flats, salt marshes and seagrass meadows, as well as their ecological services (e.g. carbon sequestration). Ocean warming affects plankton dynamics and phenology, as well as benthic community structure by hampering cold-adapted but facilitating warm-adapted species. The latter consist mostly of introduced non-native species originating from warmer coasts, with some epibenthic species acting as ecosystem engineers that create novel habitats on the tidal flats. Warming also changes interactions between species by decoupling existing predator–prey dynamics, as well as forming new interactions in which mass mortalities caused by parasites and pathogens can play an understudied but essential role. However, Wadden Sea organisms can adapt to changing abiotic and biotic parameters via genetic adaptation and phenotypic plasticity, which can also be inherited across generations (transgenerational plasticity), enabling faster plastic responses to future conditions. Important research advances have been made using next-generation molecular tools (-omics), mesocosm experiments simulating future climate scenarios, modelling approaches (ecological network analysis), and internet-based technologies for data collection and archiving. By synthesising these climate change impacts on multiple levels of physical and biological organisation in the northern Wadden Sea, we reveal knowledge gaps that need to be addressed by future investigations and comparative studies in other regions in order to implement management, mitigation and restoration strategies to preserve the uniqueness of this ecosystem of global importance.

Infections with pathogenic Vibrio strains are associated with high summer mortalities of Pacific oysters Magalana (Crassostrea) gigas, affecting production worldwide. This raises the question of how M. gigas cultures can be protected against deadly Vibro infection. There is increasing experimental evidence of immune priming in invertebrates, where previous exposure to a low pathogen load boosts the immune response upon secondary exposure. Priming responses, however, appear to vary in their specificity across host and parasite taxa. To test priming specificity in the Vibrio - M. gigas system, we used two closely related Vibrio splendidus strains with differing degrees of virulence towards M. gigas. These V. splendidus strains were either isolated in the same location as the oysters (sympatric, opening up the potential for co-evolution) or in a different location (allopatric). We extracted cell-free haemolymph plasma from infected and control oysters to test the influence of humoral immune effectors on bacterial growth in vitro. While addition of haemolypmph plasma in general promoted growth of both strains, priming by an exposure to a sublethal dose of bacterial cells lead to inhibitory effects against a subsequent challenge with a potentially lethal dose in vitro. Inhibitory effects and immune priming was strongest when oysters had been primed with the sympatric Vibrio strain, but inhibitory effects were seen both when challenged with the sympatric as well as against allopatric V. splendidus, suggesting some degree of cross protection. The stronger immune priming against the sympatric strain suggests that priming could be more efficient against matching local strains potentially adding a component of local adaptation or co-evolution to immune priming in oysters. These in vitro results, however, were not reflected in the in vivo infection data, where we saw increased bacterial loads following an initial challenge. This discrepancy might suggests that that it is the humoral part of the oyster immune system that produces the priming effects seen in our in vitro experiments.

In an era of accelerating biological globalization and climatic warming, it is vital to understand how introduced species integrate. Pacific Manila clams Ruditapes philippinarum were introduced for aquaculture to Europe in the 1970s, spread and became harvested from the Mediterranean Sea to British coasts. From the Dutch Rhine Delta, Manila clams immigrated stepwise into the southern Wadden Sea, and by long distance jump dispersal they arrived in the northern Wadden Sea in the 2010s. Encounters remained few until 2021, when live clams were often found in intertidal seagrass beds and around mixed beds of Pacific oysters with native mussels. Shell lengths reached up to 74 mm with a longevity of 7-8 years. Adult abundances remained low (<10 clams m(-2)), although larval numbers substantially increased in 2022 and 2023. Strong predation on spat may constitute ecological resistance to the immigrant. Haplotype diversity at mtDNA COI gene fragments is high and its composition is mixed from dominant haplotypes of other European sites, suggesting multiple introductions or an unknown source with an already mixed population. Currently, this is the most northern (55 degrees N) population of R. philippinarum in continental Europe, but with the rapid expansion of this genetically variable population, further northward spread can be expected.

Climate change may exacerbate the impact of invasive parasites from warmer climates through pre-existing temperature adaptations. We investigated temperature impacts on two closely related marine parasitic copepod species that share the blue mussel (Mytilus edulis) as host: Mytilicola orientalis has invaded the system from a warmer climate <20 years ago, whereas its established congener Mytilicola intestinalis has had >90 years to adapt. In laboratory experiments with temperatures 10-26 degrees C, covering current and future temperatures as well as heat waves, the development of both life cycle stages of both species accelerated with increasing temperature. In the parasitic stages, the growth of the established invader increased evenly from 10 degrees C to 22 degrees C, whereas the recent invader barely grew at all at 10 degrees C and grew faster already at 18 degrees C. In contrast, temperature had little effect on the transition success between life cycle stages. However, the highest temperature (26 degrees C) limited the egg development success of the established invader and the host entry success of both species, whereas the infection success of the established invader increased at 18 degrees C and 22 degrees C. In general, our experiments indicate that the main effect of temperature on both species is through development speed and not life cycle stage transition success. Based on regional long-term temperature data and predictions, the numbers of completed life cycles per year will increase for both parasites. The established invader seems better adapted for low current temperatures (around 10 degrees C), whereas the more recent invader barely develops at these temperatures but can cope in high temperatures (around 26 degrees C). Hence, pre-existing temperature adaptations of the recent invader may allow the species to better cope with heat waves.

Many gene families are shared across the tree of life between distantly related species because of horizontal gene transfers (HGTs). However, the frequency of HGTs varies strongly between gene families and biotic realms suggesting differential selection pressures and functional bias. One gene family with a wide distribution are FIC-domain containing enzymes (FicDs). FicDs catalyze AMPylation, a post-translational protein modification consisting in the addition of adenosine monophosphate to accessible residues of target proteins. Beside the well-known conservation of FicDs in deuterostomes, we report the presence of a conserved FicD gene ortholog in a large number of protostomes and microbial eukaryotes. We also reported additional FicD gene copies in the genomes of some rotifers, parasitic worms and bivalves. A few dsDNA viruses of these invertebrates, including White spot syndrome virus, Cherax quadricarinatus iridovirus, Ostreid herpesvirus-1 and the beetle nudivirus, carry copies of FicDs, with phylogenetic analysis suggesting a common origin of these FicD copies and the duplicated FicDs of their invertebrate hosts. HGTs and gene duplications possibly mediated by endogenous viruses or genetic mobile elements seem to have contributed to the transfer of AMPylation ability from bacteria and eukaryotes to pathogenic viruses, where this pathway could have been hijacked to promote viral infection.

Phages depend on their bacterial hosts to replicate. The habitat, density and genetic diversity of host populations are therefore key factors in phage ecology, but our ability to explore their biology depends on the isolation of a diverse and representative collection of phages from different sources. Here, we compared two populations of marine bacterial hosts and their phages collected during a time series sampling program in an oyster farm. The population of Vibrio crassostreae, a species associated specifically to oysters, was genetically structured into clades of near clonal strains, leading to the isolation of closely related phages forming large modules in phage-bacterial infection networks. For Vibrio chagasii, which blooms in the water column, a lower number of closely related hosts and a higher diversity of isolated phages resulted in small modules in the phage-bacterial infection network. Over time, phage load was correlated with V. chagasii abundance, indicating a role of host blooms in driving phage abundance. Genetic experiments further demonstrated that these phage blooms can generate epigenetic and genetic variability that can counteract host defence systems. These results highlight the importance of considering both the environmental dynamics and the genetic structure of the host when interpreting phage-bacteria networks.

For about a century, biodiversity in the tidal Wadden Sea (North Sea, European Atlantic) has increased by more than one hundred introduced species from overseas. Most originate from warmer waters and could facilitate the transformation of this coastal ecosystem to comply with climate warming. Some introduced species promote sediment stabilization and mud accretion. This could help tidal flats to keep up with sea level rise. Although some introduced species also entail negative effects, introductions have diversified lower food web levels, and may benefit foraging birds. So far, no resident populations have gone extinct because an introduced species had established. Rather than degrading the ecosystem, the establishment of introduced species seems to have raised the capacity to follow environmental change. We support increasing efforts against introductions to avoid risk. However, once species are integrated, the common condemnation attitude against "non-natives " or "aliens " ought to be reconsidered for tidal ecosystems of low biodiversity.

Life on tidal coasts presents physiological major challenges for sessile species. Fluctuations in oxygen and temperature can affect bioenergetics and modulate metabolism and redox balance, but their combined effects are not well understood. We investigated the effects of intermittent hypoxia (12h/12h) in combination with different temperature regimes (normal (15 °C), elevated (30 °C) and fluctuating (15 °C water/30 °C air)) on the Pacific oyster Crassostrea (Magallana) gigas. Fluctuating temperature led to energetic costly metabolic rearrangements and accumulation of proteins in oyster tissues. Elevated temperature led to high (60%) mortality and oxidative damage in survivors. Normal temperature had no major negative effects but caused metabolic shifts. Our study shows high plasticity of oyster metabolism in response to oxygen and temperature fluctuations and indicates that metabolic adjustments to oxygen deficiency are strongly modulated by the ambient temperature. Co-exposure to constant elevated temperature and intermittent hypoxia demonstrates the limits of this adaptive metabolic plasticity.