The epiphytic bacteria in aquatic ecosystems, inhabiting a unique ecological niche with significant ecological function, have long been the subject of attention. Habitat characteristics and plant species are believed to be important in controlling the assembly of epiphytic bacteria. However, the underlying principle governing the assembly of the epiphytic bacterial community on macrophytes is far from clear. In this study, we systematically compared the diversity and community composition of epiphytic bacteria both in different habitats and on different species of macrophytes where they were attached. Results suggested that neither the plant species nor the habitat had a significant effect on the diversity and community of epiphytic bacteria independently, indicating that the epiphytic bacterial community composition was correlated to both geographical distance and individual species of macrophytes. Furthermore, almost all of the abundant taxa were shared between different lake regions or macrophyte species, and the most abundant bacteria belonged to Proteobacteria and Firmicutes. Our results demonstrated that the competitive lottery model may explain the pattern of epiphytic bacterial colonization of submerged macrophyte surfaces. This research could provide a new perspective for exploring plant-microbe interaction in aquatic systems and new evidence for the lottery model as the mechanism best explaining the assembly of epiphytic bacteria.

There is an enormous diversity of life forms present in the extremely intricate marine environment. The growth and development of seaweeds in this particular environment are controlled by the bacteria that settle on their surfaces and generate a diverse range of inorganic and organic chemicals. The purpose of this work was to identify epiphytic and endophytic bacterial populations associated with ten common marine macroalgae from various areas along the Mediterranean Sea coast in Alexandria. This was done to target their distribution and possible functional aspects. Examine the effects of the algal habitat on the counting and phenotypic characterization of bacteria, which involves grouping bacteria based on characteristics such as shape, colour, mucoid nature, type of Gram stain, and their ability to generate spores. Furthermore, studying the physiological traits of the isolates under exploration provides insight into the optimum environmental circumstances for bacteria associated with the formation of algae. The majority of the bacterial isolates exhibited a wide range of enzyme activities, with cellulase, alginase, and caseinase being the most prevalent, according to the data. Nevertheless, 26% of the isolates displayed amylolytic activity, while certain isolates from Miami, Eastern Harbor, and Montaza lacked catalase activity. Geographical variations with the addition of algal extract may impact on the enumeration of the bacterial population, and this might have a relationship with host phylogeny. The most significant observation was that endophytic bacteria associated with green algae increased in all sites, while those associated with red algae increased in Abu Qir and Miami sites and decreased in Eastern Harbor. At the species level, the addition of algal extract led to a ninefold increase in the estimated number of epiphytic bacteria for Cladophora pellucida in Montaza. Notably, after adding algal extract, the number of presented endophytic bacteria associated with Codium sp. increased in Abu Qir while decreasing with the same species in Montaza. In addition to having the most different varieties of algae, Abu Qir has the most different bacterial isolates.

OBJECTIVE: Macroalgae harbor a rich epiphytic microbiota that plays a crucial role in algal morphogenesis and defense mechanisms. This study aims to isolate epiphytic cultivable microbiota from Ulva sp. surfaces. Various culture media were employed to evaluate a wide range of cultivable microbiota. Our objective was to assess the antibacterial and biofilm-modulating activities of supernatants from isolated bacteria.
RESULTS: Sixty-nine bacterial isolates from Ulva sp. were identified based on 16S rRNA gene sequencing. Their antibacterial activity and biofilm modulation potential were screened against three target marine bacteria: 45%, mostly affiliated with Gammaproteobacteria and mainly grown on diluted R2A medium (R2Ad), showed strong antibacterial activity, while 18% had a significant impact on biofilm modulation. Molecular network analysis was carried out on four bioactive bacterial supernatants, revealing new molecules potentially responsible for their activities.
CONCLUSIONS: R2Ad offered the greatest diversity and proportion of active isolates. The molecular network approach holds promise for both identifying bacterial isolates based on their molecular production and characterizing antibacterial and biofilm-modulating activities.

Biofilms attached to submerged macrophytes play an important role in improving the water quality of the water environment supplemented with reclaimed water. In order to explore the effects of reclaimed water quality and submerged macrophyte species on the characteristics of an epiphytic bacterial community, different types of submerged macrophytes were selected as research objects in this study. 16S rRNA high-throughput sequencing technology was used on the epiphytic bacteria and the surrounding environmental samples to analyze the bacterial community structure and functional genes. The results showed that approximately 20%-35% of the nitrogen and phosphorus nutrients were absorbed and utilized in the water environment supplemented with reclaimed water. However, the COD, turbidity, and chroma of the downstream water were significantly increased. The bacterial community of the biofilms attached to submerged macrophytes was significantly different from that in the surrounding environment (soil, sediment, and water body) and in the activated sludge that was treated by reclaimed water. In terms of bacterial community diversity, the richness and diversity were significantly lower than those of soil and sediment but higher than those of plankton bacteria in water. In terms of bacterial community composition, dominant genera and corresponding abundances were also different from those of other samples. The main dominant bacterial genera were Sphingomonas, Aeromonas, Pseudomonas, and Acinetobacter, accounting for 7%-40%, respectively. Both macrophyte species and the quality of reclaimed water (BOD5, TN, NH4+-N, and TP) could affect the bacterial community. However, the effect of water quality of the bacterial community was greater than that of macrophytes species. Additionally, the quality of reclaimed water also affected the abundance of functional genes in the bacterial community, and the relative abundance of nitrogen and phosphorus cycling functional genes was higher in areas with higher nitrogen and phosphorus concentrations.

Marine macroalgae are the habitat of epiphytic bacteria and provide several conditions for a beneficial biological interaction to thrive. Although Bacillus is one of the most abundant epiphytic genera, genomic information on marine macroalgae-associated Bacillus species remains scarce. In this study, we further investigated our previously published genome of the epiphytic strain Bacillus altitudinis 19_A to find features that could be translated to potential metabolites produced by this microorganism, as well as genes that play a role in its interaction with its macroalgal host. To achieve this goal, we performed a pan-genome analysis of Bacillus sp. and a codon bias assessment, including the genome of the strain Bacillus altitudinis 19_A and 29 complete genome sequences of closely related Bacillus strains isolated from soil, marine environments, plants, extreme environments, air, and food. This genomic analysis revealed that Bacillus altitudinis 19_A possessed unique genes encoding proteins involved in horizontal gene transfer, DNA repair, transcriptional regulation, and bacteriocin biosynthesis. In this comparative analysis, codon bias was not associated with the habitat of the strains studied. Some accessory genes were identified in the Bacillus altitudinis 19_A genome that could be related to its epiphytic lifestyle, as well as gene clusters for the biosynthesis of a sporulation-killing factor and a bacteriocin, showing their potential as a source of antimicrobial peptides. Our results provide a comprehensive view of the Bacillus altitudinis 19_A genome to understand its adaptation to the marine environment and its potential as a producer of bioactive compounds.

Epiphytic bacteria constitute a vital component of aquatic ecosystems, pivotal in regulating elemental cycling. Despite their significance, the diversity and functions of epiphytic bacterial communities adhering to various submerged macrophytes remain largely unexplored. In this study, we employed a metagenomic approach to investigate the diversity and function of epiphytic bacterial communities associated with six submerged macrophytes: Ceratophyllum demersum, Hydrilla verticillata, Myriophyllum verticillatum, Potamogeton lucens, Stuckenia pectinata, and Najas marina. The results revealed that the predominant epiphytic bacterial species for each plant type included Pseudomonas spp., Microbacterium spp., and Stenotrophomonas rhizophila. Multiple comparisons and linear discriminant analysis effect size indicated a significant divergence in the community composition of epiphytic bacteria among the six submerged macrophytes, with 0.3-1% of species uniquely identified. Epiphytic bacterial richness associated with S. pectinata significantly differed from that of both C. demersum and H. verticillata, although no significant differences were observed in diversity and evenness. Functionally, notable variations were observed in the relative abundances of genes associated with carbon, nitrogen, and phosphorus cycling within epiphytic bacterial communities on the submerged macrophyte hosts. Among these communities, H. verticillata exhibited enrichment in genes related to the 3-hydroxypropionate bicycle and nitrogen assimilation, translocation, and denitrification. Conversely, M. verticillatum showcased enrichment in genes linked to the reductive citric acid cycle (Arnon-Buchanan cycle), reductive pentose phosphate cycle (Calvin cycle), polyphosphate degradation, and organic nitrogen metabolism. In summary, our findings offer valuable insights into the diversity and function of epiphytic bacteria on submerged macrophyte leaves, shedding light on their roles in lake ecosystems.

Marine bacteria play important roles in the degradation and cycling of algal polysaccharides. However, the dynamics of epiphytic bacterial communities and their roles in algal polysaccharide degradation during kelp decay are still unclear. Here, we performed metagenomic analyses to investigate the identities and predicted metabolic abilities of epiphytic bacterial communities during the early and late decay stages of the kelp Saccharina japonica. During kelp decay, the dominant epiphytic bacterial communities shifted from Gammaproteobacteria to Verrucomicrobia and Bacteroidetes. In the early decay stage of S. japonica, epiphytic bacteria primarily targeted kelp-derived labile alginate for degradation, among which the gammaproteobacterial Vibrionaceae (particularly Vibrio) and Psychromonadaceae (particularly Psychromonas), abundant in alginate lyases belonging to the polysaccharide lyase (PL) families PL6, PL7, and PL17, were key alginate degraders. More complex fucoidan was preferred to be degraded in the late decay stage of S. japonica by epiphytic bacteria, predominantly from Verrucomicrobia (particularly Lentimonas), Pirellulaceae of Planctomycetes (particularly Rhodopirellula), Pontiellaceae of Kiritimatiellota, and Flavobacteriaceae of Bacteroidetes, which depended on using glycoside hydrolases (GHs) from the GH29, GH95, and GH141 families and sulfatases from the S1_15, S1_16, S1_17, and S1_25 families to depolymerize fucoidan. The pathways for algal polysaccharide degradation in dominant epiphytic bacterial groups were reconstructed based on analyses of metagenome-assembled genomes. This study sheds light on the roles of different epiphytic bacteria in the degradation of brown algal polysaccharides.IMPORTANCEKelps are important primary producers in coastal marine ecosystems. Polysaccharides, as major components of brown algal biomass, constitute a large fraction of organic carbon in the ocean. However, knowledge of the identities and pathways of epiphytic bacteria involved in the degradation process of brown algal polysaccharides during kelp decay is still elusive. Here, based on metagenomic analyses, the succession of epiphytic bacterial communities and their metabolic potential were investigated during the early and late decay stages of Saccharina japonica. Our study revealed a transition in algal polysaccharide-degrading bacteria during kelp decay, shifting from alginate-degrading Gammaproteobacteria to fucoidan-degrading Verrucomicrobia, Planctomycetes, Kiritimatiellota, and Bacteroidetes. A model for the dynamic degradation of algal cell wall polysaccharides, a complex organic carbon, by epiphytic microbiota during kelp decay was proposed. This study deepens our understanding of the role of epiphytic bacteria in marine algal carbon cycling as well as pathogen control in algal culture.

Halymenia durvillei is a red alga that is commonly utilized in the Philippines as food and as a source of high-value natural products for industrial applications. However, there are no studies regarding the microbial community associated with H. durvillei and its potential applications. This study aimed to isolate and identify the epiphytic bacteria of H. durvillei and determine their antimicrobial and quorum sensing inhibitory (QSI) effects. The thalli of H. durvillei were collected at the shores of Santa Fe, Bantayan, Cebu, Philippines. Bacterial isolates were identified using 16S rRNA, and their ethyl acetate (EtOAc) extracts were subjected to antimicrobial susceptibility tests against representative species of yeast and Gram-negative and Gram-positive bacteria. Their QSI activity against Chromobacterium violaceum was also determined. Fourteen distinct bacterial colonies belonging to four genera, namely Alteromonas (3), Bacillus (5), Oceanobacillus (1) and Vibrio (5), were successfully isolated and identified. All 14 bacterial isolates exhibited antibacterial effects. EPB9, identified as Bacillus safensis , consistently showed the strongest inhibition against Escherichia coli , Staphylococcus aureus and Staphylococcus epidermidis , with minimum inhibitory concentrations (MICs) ranging from 0.0625 to 1.0 mg ml-1. In contrast, all 14 isolates showed weak antifungal effects. Both B. safensis (EPB9) and Bacillus australimaris (EPB15) exhibited QSI effects at 100 mg ml-1, showing opaque zones of 3.1±0.9 and 3.8±0.4 mm, respectively. This study is the first to isolate and identify the distinct microbial epiphytic bacterial community of H. durvillei and its potential as an abundant resource for new antibacterial and QSI bioactives.

Profound growth differences such as seedling length and biomass are often observed during the cultivation of Sargassum fusiforme despite the absence of detectable variance in abiotic factors that could have affected this process. This highlights the importance of biotic factors such as epiphytic microbiota in controlling seedling growth. Yet, how, and to what extent microbial activities can affect host growth in the presence of seawater flow and continuous erosion remains debatable. Particularly, the contribution of microbial network interactions to the growth of macroalgae remains poorly understood. This study aimed to compare the physicochemical properties of S. fusiforme seedlings via 16S rRNA gene Illumina sequencing-based profiling of the epiphytic microbial communities of seedlings with different lengths. Significantly different epiphytic bacterial communities were observed among S. fusiforme seedlings of different lengths. The result showed that community from longer seedlings maintained higher bacterial diversity with the taxa Gammaproteobacteria, Burkholderiales, Alteromonadales, Vibrionaceae, Ralstonia, Colwelliaceae, and Thalassotalea being selectively enriched. More importantly, microbial interspecific interactions, which were predominantly positive, were enhanced consistently in communities of the longer seedlings, indicative of reinforced prevalent and mutually cooperative relationships among the microorganisms associated with S. fusiforme seedlings of greater length. Furthermore, longer seedlings also displayed up-regulation of microbial functional potentials involved in N fixation and mineralization, P mineralization and transportation, and ion transportation compared with shorter ones. Lastly, stochastic processes dominated the community assembly of the epiphytic microorganisms. These findings could provide new insights into the relationship between microbial communities and growth in S. fusiforme seedlings and enable us to predict the community diversity and assembly of macroalgae-associated microbial communities. This could have important implications for linking microbial community diversity and network interactions to their host productivity.

With the rapid development of industry and agriculture, excessive nitrogen and phosphorus released into natural surface water have caused eutrophication. Applying submerged plants to manage eutrophic water has attracted widespread attention. However, there are limited studies on the effects of different nitrogen and phosphorus in the water environment on submerged plants and their epiphytic biofilm. Therefore, this paper investigated the effects of eutrophic water with ammonium chloride (IN), urea (ON), potassium dihydrogen phosphate (IP), and sodium-β-glycerophosphate (OP) on Myriophyllum verticillatum and epiphytic biofilms. The results showed that Myriophyllum verticillatum exhibited a good purification effect on the eutrophic water with inorganic phosphorus, the removal rates of IP were 68.0%, and the plants grew best in this condition. The fresh weight of the IN group and ON group increased by 12.24% and 7.12%, and the shoot length of the IN group and the ON group increased by 17.71% and 8.33%; the fresh weight of the IP group and OP group increased by 19.19% and 10.83%, the shoot length of the IP group and the OP group increased by 21.09% and 18.23%. In addition, the enzyme activities of superoxide dismutase, catalase, nitrate reductase, and acid phosphatase in plant leaves were significantly changed in eutrophic water with different forms of nitrogen and phosphorus. Finally, the analysis of the epiphytic bacteria showed that different forms of nitrogen and phosphorus nutrients could significantly alter the abundance and structure of microorganisms and microbial metabolism also had significant changes. This study provides a new theoretical basis for evaluating the removal of different forms of nitrogen and phosphorus by Myriophyllum verticillatum, and it also provides new insights for the subsequent engineering of epiphytic microorganisms to improve the capability of submerged plants to treat eutrophic water.