Epiphytic and endophytic microbiome of the seagrass Zostera marina: Do they contribute to pathogen reduction in seawater?

Microbial dynamics in seagrass restoration: Unveiling hidden indicators of ecological success

Seagrass transplantation significantly alters sediment microbial communities, shaping their composition and metabolic functions. One year after Zostera marina transplantation, the microbial community structure and functions at the recipient site began shifting toward those of the donor site. Key microbial taxa associated with seagrass meadow sediment, such as Firmicutes (Hungateiclostridiaceae, Defluviitaleaceae) and Campylobacterota (Sulfurovum), increased in abundance, correlating with sediment organic matter content and carbon availability. Four functional groups were identified, each with distinct metabolic roles: (1) Opportunistic Anaerobic Degraders, (2) Seagrass-Driven Carbon Recyclers, (3) Anaerobic Fermenters and Hydrocarbon Recyclers and (4) Oxygen-Linked Carbon and Sulfur Cyclers. The sediments of transplanted Z. marina meadows exhibited increased cellulolysis and aerobic chemoheterotrophy, along with a reduction in nitrogen metabolism one year post transplant. Despite these microbial shifts, sediment isotopic signatures remained indicative of algal biomass, suggesting an incomplete transition toward a mature seagrass environment. Multivariate analysis further confirmed that the microbial community at the recipient site had not yet fully converged with that of the donor meadow, indicating that complete sediment maturation may require longer timescales. These findings demonstrate that microbial community composition and functional annotations serve as early indicators of seagrass restoration success. Long-term monitoring is essential to track ecosystem recovery and assess the stabilization of sediment conditions.

The temperate seagrass species Cymodocea nodosa and the associated bacteria co-response to sunscreen pollution

Sunscreens are included among the contaminants of emerging concern (CECs) as their production and use have spread over years while damaging aquatic biota. Sunscreens can damage the photosynthetic systems and change the microbiome of seagrasses, triggering alterations in carbon metabolism -including primary production and dissolved organic carbon (DOC) fluxes- and ecological functions. Here, we conducted a 31-day mesocosm experiment exposing Cymodocea nodosa plants to a mixture of commercial sunscreens. Sunscreens produced a significant reduction on net production rates, switching the system from autotrophic to heterotrophic, which was ascribable to an increase in heterotrophic bacteria families (some known to degrade complex substrates) and, more importantly, to a significant reduction of photosynthetic pigments in plants. Moreover, a significant release of DOC at night attributed to exudation from disrupted roots was recorded, which accounted for the observed increase in bacteria abundance and family richness recorded in the phyllosphere. A higher accumulation of starch in rhizomes suggests a certain degree of resistance of this species. However, we observed a trend to reduce some protective bacteria taxa, whereas promoted the growth of other pathogenic ones for seagrasses, along with other taxa related with the consumption of plant-derived polysaccharides and lignin compounds. Therefore, our results indicated that this CEC may reduce the contribution of seagrasses to the blue carbon pool, among others ecosystem services, and suggest a possible prompt of seagrass diseases if stressing conditions are maintained over time.

Metabolomic signatures of pathogen suppression effect of Baltic eelgrass meadows in surrounding seawater

Organic molecules exuded into water column by marine organisms represent a significant portion of marine dissolved organic matter (DOM) that modulates biochemical interactions. Secreted allelochemicals have been suggested to be involved in regulation of pathogen abundance in seagrass meadows, however, seagrass exometabolome has remained unstudied. We aimed to identify seagrass exometabolites, within and outside meadows, and explore their potential involvement in pathogen suppression under varying environmental conditions. We collected seawater (SW) samples from eelgrass (Zostera marina)-vegetated (V) and non-vegetated (NV) areas across 5 locations spanning 270 km of coastline along the German Baltic Sea. Comparative LC-MS/MS-based untargeted computational metabolomics combined with statistical analyses and machine learning tools were employed to pinpoint (exo)metabolomic signatures of eelgrass leaves. Simultaneously, we measured abiotic parameters and the abundance of three common pathogenic taxa in seawater, and investigated spatiotemporal variations. Here we show the correlation of pathogen biomass and eelgrass pathogen reduction effect with increasing seawater temperature, eutrophication and anthropogenic influences. Exometabolomics studies revealed that eelgrass exudates contributed significantly to overall seawater DOM at molecular level, while SW overlying eelgrass meadows contained many chemical features unique to the eelgrass leaf metabolome. We identified four flavone aglycones as key biomarkers distinguishing SW-V and SW-NV samples. Their drastically increased concentrations correlated with the lowest pathogen biomass, suggesting their role in pathogen regulation. These combined analytical and microbiological approaches indicate that flavones are defensive allelochemicals released into eelgrass meadows upon environmental stress and serve as potential bioindicators of eelgrass' sanitation effect.

The silent signals of climate change

Climate-driven changes to the chemical landscape of reefs affect the recovery of kelp forests.

Characterization of surface microbial communities on four seaweed species from the East China Sea

Sargassum thunbergii (ST), Sargassum horneri (SH), Grateloupia livida (GI), and Ulva pertusa (UP) are common seaweed in the East China Sea. This study investigated the epiphytic microbial communities associated with these four seaweeds in the intertidal zone of Gouqi Island, using natural seawater (SW) and sediment (S) as controls. High-throughput sequencing of 16S rRNA gene amplicons was employed to compare the community structures. The results indicated that ST exhibited lower microbial diversity and species richness compared to the other three seaweeds. Non-metric multidimensional scaling (NMDS) and principal coordinate analysis (PCoA) revealed significant differences in the community structures of epiphytic microbiota on the four seaweeds compared to those in seawater (P < 0.05). The microbial community of ST was significantly distinct from those of the other three seaweeds (P < 0.05), while no significant differences were observed among UP, GI, and SH. LEfSe analysis identified 24 biomarkers, distributed as follows: 18 for SW, 4 for ST, 1 for UP, 3 for S, 1 for GI, and 3 for SH. The dominant bacterial phyla in the epiphytic microbial communities of the four seaweeds were Proteobacteria, Firmicutes, and Bacteroidota, with relative abundances ranging from 84.35 % to 94.98 %. At the genus level, the top 10 taxa in relative abundance exhibited distinct compositional differences among the four seaweeds, demonstrating host specificity. Ecological functional predictions using FAPROTAX indicated that the epiphytic microbial communities were associated with metabolic processes such as nitrogen respiration, nitrate respiration, sulfur compound respiration, and oil bioremediation. By comparing the diversity and structural characteristics of epiphytic microbial communities on the four seaweeds, this study provides a theoretical basis for further understanding the ecological roles of seaweed.

Revealing the potential of biochar for heavy metal polluted seagrass remediation from microbial perspective

Seagrass meadows are under threat due to climate change and human activities, including heavy metal contamination, which can accumulate in seagrass tissues and harm their health and productivity. Despite extensive research, effective remediation strategies are lacking. This study investigated biochar's potential as a remediation agent for seagrass meadows affected by heavy metal pollution. Heavy metal pollution was simulated by adding copper (Cu) and chromium (Cd) to seagrass Thalassia hemprichii, and the remediation effects of biochar were evaluated by monitoring seagrass physiology, root-associated microbial communities, and heavy metal concentrations. Seagrasses can accumulate heavy metals, which adversely affect their health and alter microbial communities. Seagrasses may resist heavy metal stress by releasing dissolved organic carbon (DOC) and recruiting beneficial bacteria. Biochar reduced heavy metal bioavailability and restored seagrass ecosystem health, as evidenced by restored microbial community dynamics. This study highlights biochar's promising role in seagrass meadow restoration impacted by heavy metal pollution.

Low impact of Zostera marina meadows on sediment and water microbiota under brackish conditions

BACKGROUND

Zostera marina is an important ecosystem engineer influencing shallow water environments and possibly shaping the microbiota in surrounding sediments and water. Z. marina is typically found in marine systems, but it can also proliferate under brackish conditions. Changes in salinity generally have a strong impact on the biota, especially at the salty divide between salinity 6 and 9. To better understand the impact of the salty divide on the interaction between Z. marina and the surrounding sediment and water microbiota, we investigated the effects of Z. marina meadows on the surrounding microbiota across a salinity range of 6-15 in the Baltic Sea during the summer using 16S and 18S rRNA gene amplicon sequencing.

RESULTS

Salinity was the most important factor for structuring the microbiota within both water and sediment. The presence of Z. marina affected the composition of the bacterial and eukaryotic community and bacterial alpha diversity in the sediment. However, this effect was confined to alpha-mesohaline conditions (salinity 9-15). The impact of Z. marina below salinity 9 on water and sediment microbiota was insignificant.

CONCLUSIONS

Increasing salinity was associated with a longer leaf length of Z. marina, causing an increased canopy height, which affects the sediment microbiota through reduced water velocity. Hence, we propose that the canopy effect may be the major predictor explaining Z. marina's interactions with the surrounding microbiota at salinity 9-15. These findings emphasize the importance of the physical effects of Z. marina meadow ecosystem services and have important implications for Z. marina management under brackish conditions in a changing climate.

Marine heatwaves and disease alter community metabolism and DOC fluxes on a widespread habitat-forming seagrass species (Zostera marina)

Climate change and disease are two major threats to maintaining healthy seagrass habitats. Seagrasses, and the ecosystems they support, play a critical ecological role in global carbon (C) cycles, providing key ecosystem services, such as blue carbon storage. Zostera marina (eelgrass), the most widespread seagrass species globally, is increasingly affected by warming and is also regularly infected by the endophytic pathogen Labyrinthula zosterae. Both stressors negatively impact plant physiology and population distributions, yet the effects of these stressors on C cycling, and particularly on C metabolism and dissolved organic carbon (DOC) fluxes in eelgrass, remain largely unexplored. Through a mesocosm experiment simulating a marine heatwave (MHW) followed by pathogen challenge with L. zosterae, it was observed that the simulated MHW initially decreased daily community DOC fluxes and Net Production Rates (NPR), while not changing Respiration Rates. DOC released into the water column at the end of the MHW also was less bioavailable than DOC from the control treatment. Importantly, community NPR recovered to control levels after the simulated MHW was over, demonstrating the community's resilience to warming. On the other hand, plants challenged with L. zosterae, which caused a significant decrease in aboveground biomass, exhibited significant decreases in DOC and NPR up to 20 days after the infection. These results have important implications in blue carbon processes, given that both stressors significantly impact the quantity and quality of DOC produced by Z. marina communities. These findings also highlight the differing levels of resilience of C cycling in this system by showing that the impacts of the simulated heat wave may be more transient when compared to the effects of disease.

Beach wracks microbiome and its putative function in plastic polluted Mediterranean marine ecosystem

The coasts of the world's oceans and seas accumulate various types of floating debris, commonly known as beach wracks, including organic seaweeds, seagrass, and ubiquitous anthropogenic waste, mainly plastic. Beach wrack microbiome (MB), surviving in the form of a biofilm, ensures decomposition and remineralization of wracks, but can also serve as a vector of potential pathogens in the environment. Through the interdisciplinary approach and comprehensive sampling design that includes geological analysis of the sediment, plastic debris composition analysis (ATR-FTIR) and application of 16S rRNA gene metabarcoding of beach wrack MBs, this study aims to describe MB in relation to beach exposure, sediment type and plastic pollution. Major contributors in beach wrack MB were Proteobacteria, Bacteroidetes, Actinobacteria, Planctomycetes, Verrucomicrobia and Firmicutes and there was significant dissimilarity between sample groups with Vibrio, Cobetia and Planococcus shaping the Exposed beach sample group and Cyclobacteriaceae and Flavobacterium shaping the Sheltered beach sample group. Our results suggest plastisphere MB is mostly shaped by beach exposure, type of seagrass, sediment type and probably beach naturalness with heavy influence of seawater MB and shows no significant dissimilarity between MBs from a variety of microplastics (MP). Putative functional analysis of MB detected plastic degradation and potential human pathogen bacteria in both beach wrack and seawater MB. The research provides the next crucial step in beach wrack MP accumulation research, MB composition and functional investigation with focus on beach exposure as an important variable.

Potentially Pathogenic Vibrio spp. in Algal Wrack Accumulations on Baltic Sea Sandy Beaches

The Vibrio bacteria known to cause infections to humans and wildlife have been largely overlooked in coastal environments affected by beach wrack accumulations from seaweed or seagrasses. This study presents findings on the presence and distribution of potentially pathogenic Vibrio species on coastal beaches that are used for recreation and are affected by red-algae-dominated wrack. Using species-specific primers and 16S rRNA gene amplicon sequencing, we identified V. vulnificus, V. cholerae (non-toxigenic), and V. alginolyticus, along with 14 operational taxonomic units (OTUs) belonging to the Vibrio genus in such an environment. V. vulnificus and V. cholerae were most frequently found in water at wrack accumulation sites and within the wrack itself compared to sites without wrack. Several OTUs were exclusive to wrack accumulation sites. For the abundance and presence of V. vulnificus and the presence of V. cholerae, the most important factors in the water were the proportion of V. fucoides in the wrack, chl-a, and CDOM. Specific Vibrio OTUs correlated with salinity, water temperature, cryptophyte, and blue-green algae concentrations. To better understand the role of wrack accumulations in Vibrio abundance and community composition, future research should include different degradation stages of wrack, evaluate the link with nutrient release, and investigate microbial food-web interactions within such ecosystems, focusing on potentially pathogenic Vibrio species that could be harmful both for humans and wildlife.

Seagrass Meadows: Prospective Candidates for Bioactive Molecules

Seagrass meadows consist of angiosperms that thrive fully submerged in marine environments and form distinct ecosystems. They provide essential support for many organisms, acting as nursery grounds for species of economic importance. Beyond their ecological roles, seagrasses and their associated microbiomes are rich sources of bioactive compounds with the potential to address numerous human healthcare challenges. Seagrasses produce bioactive molecules responding to physical, chemical, and biological environmental changes. These activities can treat microbe-borne diseases, skin diseases, diabetes, muscle pain, helminthic diseases, and wounds. Seagrasses also offer potential secondary metabolites that can be used for societal benefits. Despite numerous results on their presence and bioactive derivatives, only a few studies have explored the functional and therapeutic properties of secondary metabolites from seagrass. With the increasing spread of epidemics and pandemics worldwide, the demand for alternative drug sources and drug discovery has become an indispensable area of research. Seagrasses present a reliable natural source, making this an opportune moment for further exploration of their pharmacological activities with minimal side effects. This review provides a comprehensive overview of the biochemical, phytochemical, and biomedical applications of seagrasses globally over the last two decades, highlighting the prospective areas of future research for identifying biomedical applications.

Exploring Fungal Diversity in Seagrass Ecosystems for Pharmaceutical and Ecological Insights

Marine ecosystems are important in discovering novel fungi with interesting metabolites that have shown great potential in pharmaceutical and biotechnological industries. Seagrasses, the sole submerged marine angiosperm, host diverse fungal taxa with mostly unknown metabolic capabilities. They are considered to be one of the least studied marine fungal habitats in the world. This review gathers and analyzes data from studies related to seagrasses-associated fungi, including taxonomy and biogeography, and highlights existing research gaps. The significance of the seagrass-fungal associations remains largely unknown, and current understanding of fungal diversity is limited to specific geographical regions such as the Tropical Atlantic, Mediterranean, and Indo-Pacific. Our survey yielded 29 culture-dependent studies on seagrass-associated endophytic and epiphytic fungi, and 13 miscellaneous studies, as well as 11 meta-studies, with no pathogenic true fungi described. There is a significant opportunity to expand existing studies and conduct multidisciplinary research into novel species and their potential applications, especially from understudied geographical locations. Future research should prioritize high-throughput sequencing and mycobiome studies, utilizing both culture-dependent and -independent approaches to effectively identify novel seagrass-associated fungal taxa.

Mass spectrometry-based metabolomics for the investigation of antibiotic-bacterial interactions

With the development of analytical technologies especially mass spectrometry, metabolomics is becoming increasingly hot in the field of studying antibiotic-bacterial interactions. On the one hand, metabolomics can reveal metabolic perturbations in bacteria in the presence of antibiotics and expose metabolic mechanisms. On the other hand, through in-depth analysis of bacterial metabolic profiles, biomarkers and bioactive secondary metabolites with great potential as drug precursors can be discovered. This review focuses on the experimental workflow of bacterial metabolomics and its application to study the interaction between bacteria and antibiotics. Metabolomics improves the understanding of antibiotic lethality, reveals metabolic perturbations in antibiotic-resistant bacteria, guides the diagnosis and antibiotic treatment of infectious diseases, and aids in the exploration of antibacterial metabolites in nature. Furthermore, current limitations and directions for future developments in this area are discussed.

Halophilomyces hongkongensis, a Novel Species and Genus in the Lulworthiaceae with Antibacterial Potential, Colonizing the Roots and Rhizomes of the Seagrass Halophila ovalis

Seagrass serves as a quintessential reservoir for obligate marine Lulworthiaceae fungi. Our current knowledge of the mycological diversity associated with seagrass in Hong Kong remains poor. We analyzed the diversity of fungi associated with the most widely distributed seagrass species in Hong Kong Halophila ovalis (Hydrocharitaceae), using a combination of culture-based methods and high-throughput amplicon sequencing. Halophilomyces hongkongensis, a novel fungal species in a newly proposed genus within the Lulworthiaceae family, was isolated from H. ovalis roots and rhizomes. The novel fungus showed distinct morphological characteristics, while both combined 18S-28S and internal transcribed spacer (ITS) phylogenetic trees based on maximum likelihood and Bayesian methods supported its discrimination from other existing Lulworthiaceae members. The ITS2 region in the Illumina sequencing results of multiple H. ovalis compartments, water, and adjacent non-seagrass sediments revealed continuous recruitment of H. hongkongensis by H. ovalis throughout the year despite dramatically fluctuating environmental conditions, with remarkably high proportions of this taxon found in root and rhizome internal tissues, possibly indicating a strong and specialized relationship established between the Lulworthiaceae fungal partner and its seagrass host. The inhibitory abilities exhibited by H. hongkongensis against Staphylococcus aureus SA29213 and ATCC 43300 (methicillin-resistant) may imply its capacity in producing (novel) antibacterial compounds. The discovery of H. hongkongensis as the first novel Lulworthiaceae taxon in Hong Kong, along with its distributional pattern in the seagrass meadow, provides valuable insights into the systematics and ecology of this strictly marine fungal family.

Antibacterial insights into alternariol and its derivative alternariol monomethyl ether produced by a marine fungus

Alternaria alternata FB1 is a marine fungus identified as a candidate for plastic degradation in our previous study. This fungus has been recently shown to produce secondary metabolites with significant antimicrobial activity against various pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and the notorious aquaculture pathogen Vibrio anguillarum. The antibacterial compounds were purified and identified as alternariol (AOH) and its derivative, alternariol monomethyl ether (AME). We found that AOH and AME primarily inhibited pathogenic bacteria (MRSA or V. anguillarum) by disordering cell division and some other key physiological and biochemical processes. We further demonstrated that AOH could effectively inhibit the unwinding activity of MRSA topoisomerases, which are closely related to cell division and are the potential action target of AOH. The antibacterial activities of AOH and AME were verified by using zebrafish as the in vivo model. Notably, AOH and AME did not significantly affect the viability of normal human liver cells at concentrations that effectively inhibited MRSA or V. anguillarum. Finally, we developed the genetic operation system of A. alternata FB1 and blocked the biosynthesis of AME by knocking out omtI (encoding an O-methyl transferase), which facilitated A. alternata FB1 to only produce AOH. The development of this system in the marine fungus will accelerate the discovery of novel natural products and further bioactivity study.IMPORTANCEMore and more scientific reports indicate that alternariol (AOH) and its derivative alternariol monomethyl ether (AME) exhibit antibacterial activities. However, limited exploration of their detailed antibacterial mechanisms has been performed. In the present study, the antibacterial mechanisms of AOH and AME produced by the marine fungus Alternaria alternata FB1 were disclosed in vitro and in vivo. Given their low toxicity on the normal human liver cell line under the concentrations exhibiting significant antibacterial activity against different pathogens, AOH and AME are proposed to be good candidates for developing promising antibiotics against methicillin-resistant Staphylococcus aureus and Vibrio anguillarum. We also succeeded in blocking the biosynthesis of AME, which facilitated us to easily obtain pure AOH. Moreover, based on our previous results, A. alternata FB1 was shown to enable polyethylene degradation.