Salinity affects compositional traits of epibacterial communities on the brown macroalga Fucus vesiculosus

Community dynamics and metagenomic analyses reveal Bacteroidota's role in widespread enzymatic Fucus vesiculosus cell wall degradation

Enzymatic degradation of algae cell wall carbohydrates by microorganisms is under increasing investigation as marine organic matter gains more value as a sustainable resource. The fate of carbon in the marine ecosystem is in part driven by these degradation processes. In this study, we observe the microbiome dynamics of the macroalga Fucus vesiculosus in 25-day-enrichment cultures resulting in partial degradation of the brown algae. Microbial community analyses revealed the phylum Pseudomonadota as the main bacterial fraction dominated by the genera Marinomonas and Vibrio. More importantly, a metagenome-based Hidden Markov model for specific glycosyl hydrolyses and sulphatases identified Bacteroidota as the phylum with the highest potential for cell wall degradation, contrary to their low abundance. For experimental verification, we cloned, expressed, and biochemically characterised two α-L-fucosidases, FUJM18 and FUJM20. While protein structure predictions suggest the highest similarity to a Bacillota origin, protein-protein blasts solely showed weak similarities to defined Bacteroidota proteins. Both enzymes were remarkably active at elevated temperatures and are the basis for a potential synthetic enzyme cocktail for large-scale algal destruction.

Salinity and host drive Ulva-associated bacterial communities across the Atlantic-Baltic Sea gradient

The green seaweed Ulva is a model system to study seaweed-bacteria interactions, but the impact of environmental drivers on the dynamics of these interactions is little understood. In this study, we investigated the stability and variability of the seaweed-associated bacteria across the Atlantic-Baltic Sea salinity gradient. We characterized the bacterial communities of 15 Ulva sensu lato species along 2,000 km of coastline in a total of 481 samples. Our results demonstrate that the Ulva-associated bacterial composition was strongly structured by both salinity and host species (together explaining between 34% and 91% of the variation in the abundance of the different bacterial genera). The largest shift in the bacterial consortia coincided with the horohalinicum (5-8 PSU, known as the transition zone from freshwater to marine conditions). Low-salinity communities especially contained high relative abundances of Luteolibacter, Cyanobium, Pirellula, Lacihabitans and an uncultured Spirosomaceae, whereas high-salinity communities were predominantly enriched in Litorimonas, Leucothrix, Sulfurovum, Algibacter and Dokdonia. We identified a small taxonomic core community (consisting of Paracoccus, Sulfitobacter and an uncultured Rhodobacteraceae), which together contributed to 14% of the reads per sample, on average. Additional core taxa followed a gradient model, as more core taxa were shared between neighbouring salinity ranges than between ranges at opposite ends of the Atlantic-Baltic Sea gradient. Our results contradict earlier statements that Ulva-associated bacterial communities are taxonomically highly variable across individuals and largely stochastically defined. Characteristic bacterial communities associated with distinct salinity regions may therefore facilitate the host's adaptation across the environmental gradient.

Applications of Ulva Biomass and Strategies to Improve Its Yield and Composition: A Perspective for Ulva Aquaculture

Sea lettuce (Ulva spp.), with its worldwide distribution and remarkable ability to grow rapidly under various conditions, represents an important natural resource that is still under-exploited. Its biomass can be used for a wide range of applications in the food/feed, pharmaceutical, nutraceutical, biofuel, and bioremediation industries. However, knowledge of the factors affecting Ulva biomass yield and composition is far from complete. Indeed, the respective contributions of the microbiome, natural genetic variation in Ulva species, environmental conditions and importantly, the interactions between these three factors on the Ulva biomass, have been only partially elucidated. Further investigation is important for the implementation of large-scale Ulva aquaculture, which requires stable and controlled biomass composition and yields. In this review, we document Ulva biomass composition, describe the uses of Ulva biomass and we propose different strategies for developing a sustainable and profitable Ulva aquaculture industry.

Hyunsoonleella sp. HU1-3 Increased the Biomass of Ulva fasciata

Green algae are photosynthetic organisms and play an important role in coastal environment. The microbial community on the surface of green algae has an effect on the health and nutrition of the host. However, few species of epiphytic microbiota have been reported to play a role in promoting the growth of algae. In this study, 16S rDNA sequencing was used to study the changes of microbial composition on the surface of Ulva fasciata at different growth stages. Some growth promoting bacteria were identified. The possible growth-promoting behavior of the strains were verified by co-culture of pure bacteria obtained from the surface of U. fasciata with its sterile host. Among the identified species, a new bacterial species, Hyunsoonleella sp. HU1-3 (belonging to the family Flavobacteriaceae) significantly promoted the growth of U. fasciata. The results also showed that there were many genes involved in the synthesis of growth hormone and cytokinin in the genome of Hyunsoonleella sp. HU1-3. This study identified the bacterium Hyunsoonleella sp. HU1-3 for the first time, in which this bacterium has strong growth-promoting effects on U. fasciata. Our findings not only provide insights on the establishment of the surface microbiota of U. fasciata, but also indicate that Hyunsoonleella sp. HU1-3 is one of the important species to promote the growth of U. fasciata.

A mesocosm study on bacteria-kelp interactions: Importance of nitrogen availability and kelp genetics

Macroalgal holobiont studies involve understanding interactions between the host, its microbiota, and the environment. We analyzed the effect of bacteria-kelp interactions on phenotypic responses of two genetically distinct populations of giant kelp, Macrocystis pyrifera (north and south), exposed to different nitrogen (N) concentrations. In co-culture experiments with different N concentration treatments, we evaluated kelp growth responses and changes in three specific molecular markers associated with the N cycle, both in epiphytic bacteria (relative abundance of nrfA-gene: cytochrome c nitrite reductase) and macroalgae (expression of NR-gene: nitrate reductase; GluSyn-gene: glutamate synthase). Both kelp populations responded differently to N limitation, with M. pyrifera-south sporophytes having a lower specific growth rate (SGR) under N-limiting conditions than the northern population; M. pyrifera-north sporophytes showed no significant differences in SGR when exposed to low-N and high-N concentrations. This corresponded to a higher GluSyn-gene expression in the M. pyrifera-north sporophytes and the co-occurrence of specific nrfA bacterial taxa. These bacteria may increase ammonium availability under low-N concentrations, allowing M. pyrifera-north to optimize nutrient assimilation by increasing the expression of GluSyn. We conclude that bacteria-kelp interactions are important in enhancing kelp growth rates under low N availability, although this effect may be regulated by the genetic background of kelp populations.

Integration of spatio-temporal variations of surface metabolomes and epibacterial communities highlights the importance of copper stress as a major factor shaping host-microbiota interactions within a Mediterranean seaweed holobiont

BACKGROUND

Although considered as holobionts, macroalgae and their surface microbiota share intimate interactions that are still poorly understood. Little is known on the effect of environmental parameters on the close relationships between the host and its surface-associated microbiota, and even more in a context of coastal pollutions. Therefore, the main objective of this study was to decipher the impact of local environmental parameters, especially trace metal concentrations, on an algal holobiont dynamics using the Phaeophyta Taonia atomaria as a model. Through a multidisciplinary multi-omics approach combining metabarcoding and untargeted LC-MS-based metabolomics, the epibacterial communities and the surface metabolome of T. atomaria were monitored along a spatio-temporal gradient in the bay of Toulon (Northwestern Mediterranean coast) and its surrounding. Indeed, this geographical area displays a well-described trace metal gradient particularly relevant to investigate the effect of such pollutants on marine organisms.

RESULTS

Epibacterial communities of T. atomaria exhibited a high specificity whatever the five environmentally contrasted collecting sites investigated on the NW Mediterranean coast. By integrating metabarcoding and metabolomics analyses, the holobiont dynamics varied as a whole. During the occurrence period of T. atomaria, epibacterial densities and α-diversity increased while the relative proportion of core communities decreased. Pioneer bacterial colonizers constituted a large part of the specific and core taxa, and their decrease might be linked to biofilm maturation through time. Then, the temporal increase of the Roseobacter was proposed to result from the higher temperature conditions, but also the increased production of dimethylsulfoniopropionate (DMSP) at the algal surface which could constitute of the source of carbon and sulfur for the catabolism pathways of these taxa. Finally, as a major result of this study, copper concentration constituted a key factor shaping the holobiont system. Thus, the higher expression of carotenoids suggested an oxidative stress which might result from an adaptation of the algal surface metabolome to high copper levels. In turn, this change in the surface metabolome composition could result in the selection of particular epibacterial taxa.

CONCLUSION

We showed that associated epibacterial communities were highly specific to the algal host and that the holobiont dynamics varied as a whole. While temperature increase was confirmed to be one of the main parameters associated to Taonia dynamics, the originality of this study was highlighting copper-stress as a major driver of seaweed-epibacterial interactions. In a context of global change, this study brought new insights on the dynamics of a Mediterranean algal holobiont submitted to heavy anthropic pressures. Video abstract.

The Brown Seaweeds of Scotland, Their Importance and Applications

More than 50% of the UK coastline is situated in Scotland under legislative jurisdiction; therefore, there is a great opportunity for regionally focused economic development by the rational use of sustainable marine bio-sources. We review the importance of seaweeds in general, and more specifically, wrack brown seaweeds which are washed from the sea and accumulated in the wrack zone and their economic impact. Rules and regulations governing the harvesting of seaweed, potential sites for harvesting, along with the status of industrial application are discussed. We describe extraction and separation methods of natural products from these seaweeds along with their phytochemical profiles. Many potential applications for these derivatives exist in agriculture, energy, nutrition, biomaterials, waste treatment (composting), pharmaceuticals, cosmetics and other applications. The chemical diversity of the natural compounds present in these seaweeds is an opportunity to further investigate a range of chemical scaffolds, evaluate their biological activities, and develop them for better pharmaceutical or biotechnological applications. The key message is the significant opportunity for the development of high value products from a seaweed processing industry in Scotland, based on a sustainable resource, and locally regulated.

Environmental factors shape the epiphytic bacterial communities of Gracilariopsis lemaneiformis

Macroalgae host various symbionts on their surface, which play a critical role in their growth and development processes. However, there is still incomplete understanding of this epiphytic bacteria-host algae interactions. This study comprehensively analysed variation of the epiphytic bacterial communities (EBC) composition of red macroalga Gracilariopsis lemaneiformis at different geographic locations and environmental factors (i.e., nitrogen and phosphorus), which shape the EBC composition of G. lemaneiformis. The composition and structure of EBC were characterized using high throughput sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. The results revealed that epiphytic bacteria varied significantly among three different geographic locations in China, i.e., Nan'ao Island (NA), Lianjiang County (LJ), and Nanri Island (NR). Redundancy analysis (RDA) showed that the relative abundance of Bacteroidetes, Firmicutes, Verrucomicrobia, and Epsilonbacteraeota at NR were strongly positively correlated with total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO₃-N), and dissolved inorganic nitrogen (DIN), but negatively correlated with nitrite nitrogen (NO₂-N). The relative abundance of Cyanobacteria at NA and LJ were strongly positively correlated with NO₂-N, but negatively correlated with TN, TP, NO₃-N, and DIN. Besides, the Mantel test results indicated that the EBC composition was significantly correlated with these environmental factors, which was also confirmed by Spearman correlation analysis. Thus, environmental factors such as NO₃-N and DIN play a key role in the community composition of epiphytic bacteria on G. lemaneiformis. This study provides important baseline knowledge on the community composition of epiphytic bacteria on G. lemaneiformis and shows correlation between different epiphytic bacteria and their surrounding environmental factors.

Chemically Mediated Microbial "Gardening" Capacity of a Seaweed Holobiont Is Dynamic

Terrestrial plants are known to “garden” the microbiota of their rhizosphere via released metabolites (that can attract beneficial microbes and deter pathogenic microbes). Such a “gardening” capacity is also known to be dynamic in plants. Although microbial “gardening” has been recently demonstrated for seaweeds, we do not know whether this capacity is a dynamic property in any aquatic flora like in terrestrial plants. Here, we tested the dynamic microbial “gardening” capacity of seaweeds using the model invasive red seaweed Agarophyton vermiculophyllum. Following an initial extraction of surface-associated metabolites (immediately after field collection), we conducted a long-term mesocosm experiment for 5 months to test the effect of two different salinities (low = 8.5 and medium = 16.5) on the microbial “gardening” capacity of the alga over time. We tested “gardening” capacity of A. vermiculophyllum originating from two different salinity levels (after 5 months treatments) in settlement assays against three disease causing pathogenic bacteria and seven protective bacteria. We also compared the capacity of the alga with field-collected samples. Abiotic factors like low salinity significantly increased the capacity of the alga to deter colonization by pathogenic bacteria while medium salinity significantly decreased the capacity of the alga over time when compared to field-collected samples. However, capacity to attract beneficial bacteria significantly decreased at both tested salinity levels when compared to field-collected samples. Dynamic microbial “gardening” capacity of a seaweed to attract beneficial bacteria and deter pathogenic bacteria is demonstrated for the first time. Such a dynamic capacity as found in the current study could also be applicable to other aquatic host–microbe interactions. Our results may provide an attractive direction of research towards manipulation of salinity and other abiotic factors leading to better defended A. vermiculophyllum towards pathogenic bacteria thereby enhancing sustained production of healthy A. vermiculophyllum in farms.

Bacterial Communities Show Algal Host (Fucus spp.)/Zone Differentiation Across the Stress Gradient of the Intertidal Zone

The intertidal zone often has varying levels of environmental stresses (desiccation, temperature, light) that result in highly stress-tolerant macrobiota occupying the upper zone while less tolerant species occupy the lower zone, but little comparative information is available for intertidal bacteria. Here we describe natural (unmanipulated) bacterial communities of three Fucus congeners (F. spiralis, high zone; F. vesiculosus, mid zone; F. distichus, low zone) as well as those of F. vesiculosus transplanted to the high zone (Dry and Watered treatments) and to the mid zone (Procedural Control) during summer in Maine (United States). We predicted that bacterial communities would be different among the differently zoned natural congeners, and that higher levels of desiccation stress in the high zone would cause bacterial communities of Dry transplants to become similar to F. spiralis, whereas relieving desiccation stress on Watered transplants would maintain the mid-zone F. vesiculosus bacterial community. Bacteria were identified as amplicon sequence variants (ASVs) after sequencing the V4 hypervariable region of the 16S rRNA gene. Microbiome composition and structure were significantly different between the differently zoned congeners at each tissue type (holdfasts, receptacles, vegetative tips). ASVs significantly associated with the mid-zone congener were frequently also present on the high-zone or low-zone congener, whereas overlap in ASVs between the high-zone and low-zone congeners was rare. Only 7 of 6,320 total ASVs were shared among tissues over all congeners and transplant treatments. Holdfast bacterial community composition of Dry transplants was not significantly different from that of F. spiralis, but Watered holdfast communities were significantly different from those of F. spiralis and not significantly different from those of procedural controls. Additional stressor(s) appeared important, because bacterial communities of Dry and Watered transplants were only marginally different from each other (p = 0.059). The relative abundance of Rhodobacteraceae associated with holdfasts generally correlated with environmental stress with highest abundance associated with F. spiralis and the two high-zone transplant treatments. These findings suggest that the abiotic stressors that shape distributional patterns of host species also affect their bacterial communities.

Structural Variability and Functional Prediction in the Epiphytic Bacteria Assemblies of Myriophyllum spicatum

The underlying principles influencing bacteria community assembly have long been of interest in the field of microbial ecology. Environmental heterogeneity is believed to be important in controlling the uniqueness and variability of communities. However, little is known about the influence of the host macrophytes on epiphytic bacteria assembly. In this study, we used two contrasting artificial water environments (eutrophic and oligotrophic) for reciprocal transplant experiment of Myriophyllum spicatum to examine the colonization of epiphytic bacteria accompanied with plants growth. Comparative analysis addressed a higher species diversity in epiphytic bacteria than in bacterioplankton, and the highest microbiome richness in sediment. Our data revealed that the organization of epiphytic bacterial community was affected by both plant status (i.e. branch number, net photosynthesis rate etc.) and water bodies (i.e. total phosphate, total nitrogen, pH etc.). Moreover, plant status effected the assembly in priority to water. 16S rRNA gene sequencing further indicated that the epiphytic assemblies were motivated by functionalization and interplay with hosts as a whole. The results complemented new evidences for the 'lottery process' in the epiphytic bacteria assembly traits and shed insights into the assembly patterns referring to functional adaptation across epiphytic bacteria and macrophytes.

Warming, but Not Acidification, Restructures Epibacterial Communities of the Baltic Macroalga Fucus vesiculosus With Seasonal Variability

Due to ocean acidification and global warming, surface seawater of the western Baltic Sea is expected to reach an average of ∼1100 μatm pCO2 and an increase of ∼5°C by the year 2100. In four consecutive experiments (spanning 10-11 weeks each) in all seasons within 1 year, the abiotic factors temperature (+5°C above in situ) and pCO2 (adjusted to ∼1100 μatm) were tested for their single and combined effects on epibacterial communities of the brown macroalga Fucus vesiculosus and on bacteria present in the surrounding seawater. The experiments were set up in three biological replicates using the Kiel Outdoor Benthocosm facility (Kiel, Germany). Phylogenetic analyses of the respective microbiota were performed by bacterial 16S (V1-V2) rDNA Illumina MiSeq amplicon sequencing after 0, 4, 8, and 10/11 weeks per season. The results demonstrate (I) that the bacterial community composition varied in time and (II) that relationships between operational taxonomic units (OTUs) within an OTU association network were mainly governed by the habitat. (III) Neither single pCO2 nor pCO2:Temperature interaction effects were statistically significant. However, significant impact of ocean warming was detected varying among seasons. (IV) An indicator OTU (iOTU) analysis identified several iOTUs that were strongly influenced by temperature in spring, summer, and winter. In the warming treatments of these three seasons, we observed decreasing numbers of bacteria that are commonly associated with a healthy marine microbial community and-particularly during spring and summer-an increase in potentially pathogenic and bacteria related to intensified microfouling. This might lead to severe consequences for the F. vesiculosus holobiont finally affecting the marine ecosystem.

A Multi-Omics Analysis Suggests Links Between the Differentiated Surface Metabolome and Epiphytic Microbiota Along the Thallus of a Mediterranean Seaweed Holobiont

Marine macroalgae constitute an important living resource in marine ecosystems and complex ecological interactions occur at their surfaces with microbial communities. In this context, the present study aimed to investigate how the surface metabolome of the algal holobiont Taonia atomaria could drive epiphytic microbiota variations at the thallus scale. First, a clear discrimination was observed between algal surface, planktonic and rocky prokaryotic communities. These data strengthened the hypothesis of an active role of the algal host in the selection of epiphytic communities. Moreover, significant higher epibacterial density and α-diversity were found at the basal algal parts compared to the apical ones, suggesting a maturation gradient of the community along the thallus. In parallel, a multiplatform mass spectrometry-based metabolomics study, using molecular networking to annotate relevant metabolites, highlighted a clear chemical differentiation at the algal surface along the thallus with similar clustering as for microbial communities. In that respect, higher amounts of sesquiterpenes, phosphatidylcholines (PCs), and diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanines (DGTAs) were observed at the apical regions while dimethylsulfoniopropionate (DMSP) and carotenoids were predominantly found at the basal parts of the thalli. A weighted UniFrac distance-based redundancy analysis linking the metabolomics and metabarcoding datasets indicated that these surface compounds, presumably of algal origin, may drive the zonal variability of the epibacterial communities. As only few studies were focused on microbiota and metabolome variation along a single algal thallus, these results improved our understanding about seaweed holobionts. Through this multi-omics approach at the thallus scale, we suggested a plausible scenario where the chemical production at the surface of T. atomaria, mainly induced by the algal physiology, could explain the specificity and the variations of the surface microbiota along the thallus.

Evaluation of a new primer combination to minimize plastid contamination in 16S rDNA metabarcoding analyses of alga-associated bacterial communities

Plant- and alga-associated bacterial communities are generally described via 16S rDNA metabarcoding using universal primers. As plastid genomes encode 16S rDNA related to cyanobacteria, these data sets frequently contain >90% plastidial sequences, and the bacterial diversity may be under-sampled. To overcome this limitation we evaluated in silico the taxonomic coverage for four primer combinations targeting the 16S rDNA V3-V4 region. They included a forward primer universal to Bacteria (S-D-Bact-0341-b-S-17) and four reverse primers designed to avoid plastid DNA amplification. The best primer combination (NOCHL) was compared to the universal primer set in the wet lab using a synthetic community and samples from three macroalgal species. The proportion of plastid sequences was reduced by 99%-100% with the NOCHL primers compared to the universal primers, irrespective of algal hosts, sample collection and extraction protocols. Additionally, the NOCHL primers yielded a higher richness while maintaining the community structure. As Planctomycetes, Verrucomicrobia and Cyanobacteria were underrepresented (70%-90%) compared to universal primers, combining the NOCHL set with taxon-specific primers may be useful for a complete description of the alga-associated bacterial diversity. The NOCHL primers represent an innovation to study algal holobionts without amplifying host plastid sequences and may further be applied to other photosynthetic hosts.

Salinity and Time Can Alter Epibacterial Communities of an Invasive Seaweed

The establishment of epibacterial communities is fundamental to seaweed health and fitness, in modulating ecological interactions and may also facilitate adaptation to new environments. Abiotic factors like salinity can determine bacterial abundance, growth and community composition. However, influence of salinity as a driver of epibacterial community composition (until species level) has not been investigated for seaweeds and especially under long time scales. We also do not know how abiotic stressors may influence the 'core' bacterial species of seaweeds. Following an initial (immediately after field collection) sampling of epibacterial community of an invasive red seaweed Agarophyton vermicullophylum, we conducted a long term mesocosm experiment for 5 months, to examine the influence of three different salinities (low, medium and high) at two different time points (3 months after start of experiment and 5 months, i.e., at the end of experiment) on the epibacterial community richness and composition of Agarophyton. Metagenomic sequencing showed that epibacterial communities changed significantly according to salinity and time points sampled. Epibacterial richness was significantly different between low and high salinities at both time points. Epibacterial richness also varied significantly between 3 months (after start of experiment) and 5 months (end of experiment) within low, medium and high salinity level. Irrespective of salinity levels and time points sampled 727 taxa consistently appeared in all Agarophyton samples hinting at the presence of core bacterial species on the surface of the alga. Our results indicate that both salinity and time can be major driving forces in structuring epibacterial communities of seaweeds with respect to richness and β-diversity. We highlight the necessity of conducting long term experiments allowing us to detect and understand epibacterial succession over time on seaweeds.

Temporal covariation of epibacterial community and surface metabolome in the Mediterranean seaweed holobiont Taonia atomaria

An integrative multi-omics approach allowed monthly variations for a year of the surface metabolome and the epibacterial community of the Mediterranean Phaeophyceae Taonia atomaria to be investigated. The LC-MS-based metabolomics and 16S rDNA metabarcoding data sets were integrated in a multivariate meta-omics analysis (multi-block PLS-DA from the MixOmic DIABLO analysis) showing a strong seasonal covariation (Mantel test: p < 0.01). A network based on positive and negative correlations between the two data sets revealed two clusters of variables, one relative to the 'spring period' and a second to the 'summer period'. The 'spring period' cluster was mainly characterized by dipeptides positively correlated with a single bacterial taxon of the Alteromonadaceae family (BD1-7 clade). Moreover, 'summer' dominant epibacterial taxa from the second cluster (including Erythrobacteraceae, Rhodospirillaceae, Oceanospirillaceae and Flammeovirgaceae) showed positive correlations with few metabolites known as macroalgal antifouling defences [e.g. dimethylsulphoniopropionate (DMSP) and proline] which exhibited a key role within the correlation network. Despite a core community that represents a significant part of the total epibacteria, changes in the microbiota structure associated with surface metabolome variations suggested that both environment and algal host shape the bacterial surface microbiota.

Mapping the Surface Microbiome and Metabolome of Brown Seaweed Fucus vesiculosus by Amplicon Sequencing, Integrated Metabolomics and Imaging Techniques

The brown alga Fucus vesiculosus is a keystone marine species, which is subject to heavy surface colonisation. This study was designed to analyse the surface epibiome of F. vesiculosus in conjunction with the composition and spatial distribution of its surface metabolome. The amplicon sequencing, SEM and CARD-FISH imaging studies showed Alphaproteobacteria to predominate the epibiotic bacteria. Fungi of the class Eurotiomycetes were visualised for the first time on an algal surface. An untargeted metabolomics approach using molecular networks, in silico prediction and manual dereplication showed the differential metabolome of the surface and the whole tissue extracts. In total, 50 compounds were putatively dereplicated by UPLC-MS/MS, 37 of which were previously reported from both seaweeds and microorganisms. Untargeted spatial metabolomics by DESI-Imaging MS identified the specific localisation and distribution of various primary and secondary metabolites on surface imprints and in algal cross sections. The UPLC-MS, DESI-IMS and NMR analyses failed to confirm the presence of any surface-associated metabolite, except for mannitol, which were previously reported from F. vesiculosus. This is the first study analysing the seaweed surface microbiome in conjunction with untargeted surface metabolomics and spatial metabolomics approaches.

Spatial distribution of the culturable bacterial community associated with the invasive alga Caulerpa cylindracea in the Mediterranean Sea

Understanding the mechanisms underlying the complex seaweed-bacteria associations in nature may provide information on the fitness of an invasive host. This may require the use of different approaches. In this study, we employed, for the first time, the Biolog system-Ecoplates™ to analyze the functional diversity of the culturable fraction of the bacterial assemblages associated with the surface of Caulerpa cylindracea, the invasive seaweed of the Mediterranean Sea. Seaweed samples were collected at five sites across the basin. A high similarity in the bacterial activity, expressed as Average Well Color Development (AWCD), among the study sites was observed. Culturable heterotrophic bacteria at 22 °C showed mean values ranging from 1.4 × 10⁵ CFU g^-1 at Porto Cesareo (Ionian Sea, Italy) to 5.8 × 10⁶ CFU g^-1 at Othonoi, Diapontine Island (Ionian Sea, Greece). The analysis of the DNA sequences on isolated bacteria demonstrated that the genera Shewanella, Marinobacter, Vibrio, Granulosicoccus and the family Rhodobacteraceae are consistently present on C. cylindracea, irrespective of its geographical origin. The present study provided new insights into the complex association between bacteria and this algal species, suggesting a specific composition and function of the associated culturable bacteria across the basin.

Restructuring of Epibacterial Communities on Fucus vesiculosus forma mytili in Response to Elevated pCO2 and Increased Temperature Levels

Marine multicellular organisms in composition with their associated microbiota-representing metaorganisms-are confronted with constantly changing environmental conditions. In 2110, the seawater temperature is predicted to be increased by ~5°C, and the atmospheric carbon dioxide partial pressure (pCO2) is expected to reach approximately 1000 ppm. In order to assess the response of marine metaorganisms to global changes, e.g., by effects on host-microbe interactions, we evaluated the response of epibacterial communities associated with Fucus vesiculosus forma mytili (F. mytili) to future climate conditions. During an 11-week lasting mesocosm experiment on the island of Sylt (Germany) in spring 2014, North Sea F. mytili individuals were exposed to elevated pCO2 (1000 ppm) and increased temperature levels (Δ+5°C). Both abiotic factors were tested for single and combined effects on the epibacterial community composition over time, with three replicates per treatment. The respective community structures of bacterial consortia associated to the surface of F. mytili were analyzed by Illumina MiSeq 16S rDNA amplicon sequencing after 0, 4, 8, and 11 weeks of treatment (in total 96 samples). The results demonstrated that the epibacterial community structure was strongly affected by temperature, but only weakly by elevated pCO2. No interaction effect of both factors was observed in the combined treatment. We identified several indicator operational taxonomic units (iOTUs) that were strongly influenced by the respective experimental factors. An OTU association network analysis revealed that relationships between OTUs were mainly governed by habitat. Overall, this study contributes to a better understanding of how epibacterial communities associated with F. mytili may adapt to future changes in seawater acidity and temperature, ultimately with potential consequences for host-microbe interactions.

The Cultivable Surface Microbiota of the Brown Alga Ascophyllum nodosum is Enriched in Macroalgal-Polysaccharide-Degrading Bacteria

Bacteria degrading algal polysaccharides are key players in the global carbon cycle and in algal biomass recycling. Yet the water column, which has been studied largely by metagenomic approaches, is poor in such bacteria and their algal-polysaccharide-degrading enzymes. Even more surprisingly, the few published studies on seaweed-associated microbiomes have revealed low abundances of such bacteria and their specific enzymes. However, as macroalgal cell-wall polysaccharides do not accumulate in nature, these bacteria and their unique polysaccharidases must not be that uncommon. We, therefore, looked at the polysaccharide-degrading activity of the cultivable bacterial subpopulation associated with Ascophyllum nodosum. From A. nodosum triplicates, 324 bacteria were isolated and taxonomically identified. Out of these isolates, 78 (~25%) were found to act on at least one tested algal polysaccharide (agar, ι- or κ-carrageenan, or alginate). The isolates “active” on algal-polysaccharides belong to 11 genera: Cellulophaga, Maribacter, Algibacter, and Zobellia in the class Flavobacteriia (41) and Pseudoalteromonas, Vibrio, Cobetia, Shewanella, Colwellia, Marinomonas, and Paraglaceciola in the class Gammaproteobacteria (37). A major part represents likely novel species. Different proportions of bacterial phyla and classes were observed between the isolated cultivable subpopulation and the total microbial community previously identified on other brown algae. Here, Bacteroidetes and Gammaproteobacteria were found to be the most abundant and some phyla (as Planctomycetes and Cyanobacteria) frequently encountered on brown algae weren't identified. At a lower taxonomic level, twelve genera, well-known to be associated with algae (with the exception for Colwellia), were consistently found on all three A. nosodum samples. Even more interesting, 9 of the 11 above mentioned genera containing polysaccharolytic isolates were predominant in this common core. The cultivable fraction of the bacterial community associated with A. nodosum is, thus, significantly enriched in macroalgal-polysaccharide-degrading bacteria and these bacteria seem important for the seaweed holobiont even though they are under-represented in alga-associated microbiome studies.

Composition of Bacterial Communities Associated with Aurelia aurita Changes with Compartment, Life Stage, and Population

The scyphozoan Aurelia aurita is recognized as a key player in marine ecosystems and a driver of ecosystem change. It is thus intensely studied to address ecological questions, although its associations with microorganisms remain so far undescribed. In the present study, the microbiota associated with A. aurita was visualized with fluorescence in situ hybridization (FISH) analysis, and community structure was analyzed with respect to different life stages, compartments, and populations of A. aurita by 16S rRNA gene amplicon sequencing. We demonstrate that the composition of the A. aurita microbiota is generally highly distinct from the composition of communities present in ambient water. Comparison of microbial communities from different developmental stages reveals evidence for life stage-specific community patterns. Significant restructuring of the microbiota during strobilation from benthic polyp to planktonic life stages is present, arguing for a restructuring during the course of metamorphosis. Furthermore, the microbiota present in different compartments of the adult medusa (exumbrella mucus and gastric cavity) display significant differences, indicating body part-specific colonization. A novel Mycoplasma strain was identified in both compartment-specific microbiota and is most likely present inside the epithelium as indicated by FISH analysis of polyps, indicating potential endosymbiosis. Finally, comparison of polyps of different populations kept under the same controlled laboratory conditions in the same ambient water showed population-specific community patterns, most likely due the genetic background of the host. In conclusion, the presented data indicate that the associated microbiota of A. aurita may play important functional roles, e.g., during the life cycle.

Spatial variability of microbial assemblages associated with a dominant habitat-forming seaweed

Macroalgal surfaces support abundant and diverse microorganisms within biofilms, which are often involved in fundamental functions relating to the health and defense of their seaweed hosts, including algal development, facilitation of spore release, and chemical antifouling. Given these intimate and important interactions, environmental changes have the potential to negatively impact macroalgae by disrupting seaweed-microbe interactions. We used the disappearance of the dominant canopy-forming fucoid Phyllospora comosa from the metropolitan coast of Sydney, NSW, Australia as a model system to study these interactions. We transplanted Phyllospora individuals from nearby, extant populations back onto reefs in Sydney to test whether bacterial assemblages associated with seaweed surfaces would be influenced by (i) the host itself, independently of where it occurs, (ii) the type of habitat where the host occurs, or (iii) site-specific differences. Analyses of bacterial DNA fingerprints (terminal fragment length polymorphisms) indicated that assemblages of bacteria on Phyllospora were not habitat-specific. Rather, they were primarily influenced by local, site-specific conditions with some evidence for host-specificity in some cases. This could suggest a lottery model of host-surface colonization, by which hosts are colonized by 'suitable' bacteria available in the local species pool, resulting in high variability in assemblage structure across sites, but where some species in the community are specific to the host and possibly influenced by differences in host traits.