Marine sponges host diverse microbial communities that play essential roles in nutrient cycling and host health. During a study of the microbiota of marine sponges from Taiwan, two Gram-stain-negative, motile and facultatively anaerobic strains, XeTr1T and StTr2, were isolated from Xestospongia sp. and Stylissa carteri, respectively. Cells were rod-shaped and showed catalase and oxidase activities. Growth was observed at 15-37 °C, pH 6.0-9.0 and 1.0-3.0% (w/v) NaCl. Strains XeTr1T and StTr2 contained ubiquinone-8 and ubiquinone-9 as major quinones and summed feature 3 (C16:1  ω7c), summed features 8 (C18:1  ω7c) and C16:0 as major fatty acids. The major polar lipids identified in strains XeTr1T and StTr2 were phosphatidylethanolamine and phosphatidylglycerol. Phylogenetic analysis based on the 16S rRNA gene sequence placed strains XeTr1T and StTr2 within the family Endozoicomonadaceae, forming a distinct lineage that shared 94% or less sequence homology with its closest relatives, including Endozoicomonas lisbonensis NE40T, Endozoicomonas arenosclerae CBAS 572T and Endozoicomonas montiporae CL-33T. The average amino acid identity values were below the cutoff range for species of the same genus, supporting the placement of strains XeTr1T and StTr2 as representing a novel genus within the family Endozoicomonadaceae. Distinct phenotypic and chemotaxonomic characteristics, combined with low sequence similarity and a clear phylogenetic separation from known genera, support the proposal of a novel genus and species in the family Endozoicomonadeae, for which the name Spongorhabdus nitratireducens gen. nov., sp. nov. is proposed. The type strain is XeTr1T (=JCM 37577T=UCCCB 249T).

In an experimental marine culture system, humic substances (HS), Eisenia bicyclis (EB) extract, and their combination (MIX) were used to modulate biofilter, water, and fish skin mucus bacteriomes. In addition to this, we assessed their impact on water quality and seabass (Dicentrarchus labrax) health. Our results revealed that the addition of EB significantly increased total ammonia nitrogen (TAN) levels. Nevertheless, there were no adverse effects of the natural organic substances tested on fish health. The bacteriome analyses showed that these substances induced significant shifts in structural diversity across various biotopes in the culture system. For example, HS selectively enriched bacterial taxa including Bacteroidota in water and skin mucus, and significantly enriched nitrifying bacteria belonging to Nitrosomonadaceae in biofilters. Conversely, EB adversely affected putative nitrifying bacteria but enriched potentially beneficial Rhodobacterales populations in skin mucus. The MIX treatment enhanced microbial evenness in fish skin mucus and enriched Verrucomicrobiales populations, particularly Rubritalea sp., a genus linked to fish health and bioactive compound production. Our findings further showed that all treatments modulated putative functions relevant to host health and pathogen antagonism within the culture system. This study highlights the potential of natural organic substances to drive system-wide shifts in microbial communities across biofilter, water, and fish skin biotopes, offering a promising strategy for targeted microbiome modulation in the aquaculture sector.

A Gram-stain-negative, aerobic, motile-by-gliding bacterium, designated strain RHTr2T, was isolated from the marine sponge Rhabdastrella globostellata collected in waters surrounding the Penghu Islands, Taiwan. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain RHTr2T displayed the highest 16S rRNA gene sequence similarities to Aquimarina versatilis CBA3207T (94.2%), Aquimarina acroporae D1M17T (93.7%) and Aquimarina intermedia LMG 23204T (93.2%). Strain RHTr2T grew with 1-5% (w/v) NaCl (optimum, 2-3%), at 17-37 °C (optimum, 30 °C) and at pH 6.0-8.0 (optimum, pH 6.0-7.0). Strain RHTr2T was able to degrade starch, Tweens (20, 40 and 80) and gelatin; flexirubin-type pigments were produced. Strain RHTr2T contained menaquinone 6 as the major respiratory quinone, and iso-C15 : 0 and iso-C17 : 0 3-OH as major fatty acids (> 10%). The polar lipids of strain RHTr2T were an unidentified aminophospholipid, three aminolipids, a phospholipid and an unidentified polar lipid. The DNA G+C content was 31.4 mol% based on genome sequence analysis. The average nucleotide identity and digital DNA-DNA hybridization values between strain RHTr2T and the most closely related members of the Aquimarina genus were lower than the recommended thresholds for species delineation. Based on its phenotypic and genotypic characteristics and on the phylogenetic and phylogenomic evidence presented, strain RHTr2T represents a novel species of the genus Aquimarina for which the name Aquimarina rhabdastrellae sp. nov. is proposed. The type strain is RHTr2T (=JCM 37598T=UCCCB 217T).

AIMS:In the present study, we tested whether terrestrially derived humic substances (HS) could mitigate the adverse effects of elevated temperature and ultraviolet B (UVB) radiation on the bacterial communities of two hard corals (Montipora digitata and M. capricornis), one soft coral (Sarcophyton glaucum), sediment and water. We also examined the impact of temperature, UVB radiation, and HS supplementation on coral photosynthetic activity, a proxy for coral bleaching. METHODS AND RESULTS:We performed a multifactorial experiment using a randomized-controlled microcosm setup. Coral photosynthetic efficiency was measured in vivo using a pulse amplitude modulation fluorometer. Bacterial communities were analyzed using 16S rRNA gene sequencing. Corals in HS-supplemented microcosms had significantly higher photosynthetic activities than those in microcosms subjected to elevated temperature and UVB radiation. Additionally, HS supplementation significantly influenced the composition of sediment, water, and host-associated bacterial communities. Reef organisms in HS supplemented microcosms contained distinct bacterial communities enriched with groups of potentially beneficial bacteria. In the hard coral M. digitata, we observed an interactive effect of HS supplementation, UVB radiation, and temperature. CONCLUSION:Our findings indicate that HS significantly modulates coral reef bacterial communities and support the hypothesis that these substances contribute to improved reef resistance to the adverse effects of elevated temperature and UVB radiation.

This study compared the efficacy of three individual lytic phages, PSA_LMAPSA-2T (PSA-2T), PSA_LMAPSA-6F (PSA-6F) and PSA_LMAPSA-7F (PSA-7F) and four phage cocktails (dual and triple combinations) in inactivating Pseudomonas syringae pv. actinidiae. Phages were isolated from kiwifruit leaves and soil samples contaminated with P. syringae pv. actinidiae and characterized by host spectrum, growth parameters, adsorption rate, genomic analysis, inactivation efficiency and viability under variable environmental conditions in orchard environments (temperature, pH and solar radiation). Phage PSA-2T showed the highest in vitro efficacy, achieving a 3.2 log CFU/mL maximum reduction after 18 h, outperforming PSA-6F and PSA-7F (0.6 and 1.5 log reductions, respectively). Phage cocktails achieved reductions of 1.0–2.2 log CFU/mL, but none exceeded the performance of PSA-2T alone. Phage viability was most affected by high temperature and acidic pH, with PSA-7F showing the greatest sensitivity. Nonetheless, all phages remained stable under typical orchard conditions. Phage PSA-2T significantly reduced P. syringae pv. actinidiae levels (1.5-log CFU/mL) on artificially contaminated kiwifruit leaves after a single treatment. These results demonstrate the potential of PSA-2T and phage cocktails as sustainable alternatives to copper and antibiotics, warranting further study of repeated treatments and broad-host-range phage formulations for field use.

AIMS:The main goal of this study was to investigate the diversity, genetic composition, and potential ecological relevance of circular putative plasmids within the microbiomes of the sponge Stylissa carteri across thousands of kilometers (~6000 km) in the Indo-Pacific region. METHODS AND RESULTS:To achieve this, we applied a selective multiply-primed rolling circle amplification method combined with high-throughput sequencing to enrich and characterize circular putative plasmids from sponge samples. Our results revealed highly diverse and site-specific plasmid assemblages, primarily consisting of small cryptic plasmids (<5 kbp) of unknown function. We also observed the widespread distribution of two putatively cryptic plasmids across all sampling locations, and 22 additional plasmids present in at least three locations. We, furthermore, detected a striking prevalence of putative genes encoding for immunoglobulin-like (Ig-like) and eukaryotic-like protein (ELP) domains, such as Calx-beta, fibronectin type III (fn3), ankyrin (ANK), and scavenger receptor cysteine-rich, as well as RVT-CRISPR-related reverse transcriptases (RVTs) and unclassified RVTs. The widespread presence of genes encoding ELPs in putative plasmids, which may play key roles in host colonization and immune evasion, underscores the need for a deeper understanding of how these plasmid-carried genes influence microbe-sponge ecological interactions. CONCLUSIONS:Our findings shed new light on the potential role of plasmids as a reservoir of genetic diversity in the context of sponge microbe symbioses.

The early developmental stages of fish larvae are critical for establishing their microbial communities. However, significant gaps remain in our understanding of microbial colonisation and succession during these stages, as well as their role in optimising larviculture practices to enhance larval survival and health. This study investigated the ecology of bacterial communities associated with turbot (Scophthalmus maximus) larvae during their first month of development, generating fundamental knowledge that can guide targeted strategies to actively shape the fish bacteriome towards a healthier state. Our results revealed rapid bacterial colonisation and displacement in turbot larvae, marked by significant structural differences between water and larval communities during the initial developmental stages. During this period, larval bacterial communities largely followed neutral model predictions, indicating that stochastic processes governed early bacterial colonisation. As the larvae matured, we observed a convergence in the bacterial community structures of both water and fish, along with increasing deviations from neutral model predictions. This suggests increased niche specialisation and/or competitive exclusion of the bacteriome as the host fish aged. Moreover, this study identified specific bacterial taxa of interest that deviated from neutral model predictions, indicating potential positive or negative host selection. We conclude by discussing the importance of our findings for the development of larviculture microbiome interventions.

In the present study, five new bacteriophages (or phages) were characterized, and their efficacy in controlling pathogenic bacteria-Escherichia coli, Salmonella enterica serovar Typhimurium, Salmonella enterica serovar Enteritidis, Aeromonas hydrophila, and Vibrio parahaemolyticus-associated with bivalve consumption was evaluated. The isolated phages include both siphovirus [vB_EcoS_UALMA_PCEc3 (PCEc3), vB_SeTS_UALMA_PCST1 (PCST1), and vB_VpaS_UALMA_PCVp3 (PCVp3)] and myovirus [vB_SeEM_UALMA_PCSE1 (PCSE1) and vB_AhyM_UALMA_PCAh2 (PCAh2)] morphotypes. Four phages are safe for bacterial control, with only one (PCAh2) showing potential lysogenic characteristics. All phages exhibited a narrow host range, capable of infecting up to six additional bacterial strains besides their original host, and four could infect the host bacteria of other phages. Adsorption rates ranged from 24% and 98% within 1 h. One-step growth assays revealed different latent periods, ranging from 10 to 120 min, and low to average burst sizes, ranging from 7.60 to 83.97 PFU/mL. Generally, increasing the multiplicity of infection (MOI) enhanced phage efficiency significantly. All phages effectively reduced the bacterial load of their respective hosts, achieving maximum reductions between 3.73 and 5.57 log CFU/mL within 10 h of treatment. These results suggest that phage biocontrol can be an effective alternative to combat pathogenic bacteria associated with bivalve consumption.

Marine karst ecosystems exist at the land-sea interface and are characterised by underwater formations sculpted over time by the action of seawater. Submerged caves and crevices of these ecosystems host a rich array of marine life of which sponges are among the most abundant and diverse components. In the present study, we describe elements of the sponge fauna sampled from a unique karst ecosystem at a remote island, Orchid Island, off the southeastern coast of Taiwan. The present study includes several understudied sponge taxa, including sclerosponges ( Acanthochaetetes wellsi , and Astrosclera willeyana ) and several lithistid species from dark, shallow-water caves. Prokaryotic communities were obtained from a total of 22 demosponge species, of which 11 are potentially new to science. The tetracladinid, lithistids harboured prokaryotic communities, which clustered separately from all other sponge species, contrasting with the non-tetracladinid, lithistid Vetulina incrustans . The tetracladinid, lithistids, furthermore, formed two distinct clusters with species of the Spirophorina suborder clustering apart from those of the Astrophorina suborder. The sclerosponge A. wellsi also harboured a distinct prokaryotic community in terms of composition including five unique, abundant OTUs with relatively low sequence similarities to organisms in GenBank. All cave sponges were enriched with SAR202 members, a group of bacteria known for their role in the degradation of recalcitrant compounds. The highest relative abundance of SAR202 was found in A. wellsi . We propose that the cave sponges of Orchid Island may play an as-yet uncharted role in nutrient dynamics at the land-sea interface.

Urinary tract infections (UTIs) caused by Klebsiella pneumoniae can lead to severe clinical complications and even death. An alternative treatment option for infected patients is using bacteriophages (phages). In the present study, we isolated phage KP-1 from sewage water using K. pneumoniae as a host. Whole genome analysis indicated that the genome was a double-stranded linear 176,096-bp long DNA molecule with 41.8% GC content and did not contain virulence or antibiotic resistance genes. The inactivation potential of phage KP-1 was assessed in broth at an MOI of 1 and 10, and a maximum inactivation of 4.9 and 5.4 log CFU/mL was observed after 9 h, respectively. The MOI of 10 efficacy was also assessed in urine to evaluate the phage’s performance in an acidic environment. A maximum inactivation of 3.8 log CFU/mL was observed after 9 h. The results indicate that phage KP-1 could be used to control UTIs caused by K. pneumoniae; however, further studies using phage cocktails to broader the phage’s spectrum of activity and phages in micro- or nanocarriers to avoid the negative effect of the low pH of the urine on phage viability, are necessary.

Pseudomonas aeruginosa is a common cause of hospital-acquired infections and exhibits a strong resistance to antibiotics. An alternative treatment option for bacterial infections is the use of bacteriophages (or phages). In this study, two distinct phages, VB_PaD_phPA-G (phPA-G) and VB_PaN_phPA-Intesti (phPA-Intesti), were used as single suspensions or in a phage cocktail to inactivate the planktonic cells and biofilms of P. aeruginosa. Preliminary experiments in culture medium showed that phage phPA-Intesti (reductions of 4.5–4.9 log CFU/mL) outperformed phPA-G (reductions of 0.6–2.6 log CFU/mL) and the phage cocktail (reduction of 4.2 log CFU/mL). Phage phPA-Intesti caused a maximum reduction of 5.5 log CFU/cm2 in the P. aeruginosa biofilm in urine after 4 h of incubation. The combination of phage phPA-Intesti and ciprofloxacin did not improve the efficacy of bacterial inactivation nor reduce the development of resistant mutants. However, the development of resistant bacteria was lower in the combined treatment with the phage and the antibiotic compared to treatment with the antibiotic alone. This phage lacks known toxins, virulence, antibiotic resistance, and integrase genes. Overall, the results suggest that the use of phage phPA-Intesti could be a potential approach to control urinary tract infections (UTIs), namely those caused by biofilm-producing and multidrug-resistant strains of P. aeruginosa.

Sponges are one of the oldest lineages of animals on Earth and play key roles in shaping marine ecosystems. They are diverse, with more than 9600 species known to science, and come in a wide range of shapes, sizes, and colours. Sponges are, furthermore, known to host diverse communities of microbial symbionts, which play important roles in their physiology and ecology. In the present study, we sampled prokaryotic communities from 24 sponge species inhabiting coral reef flat and slope habitats off the coast of SW Celebes (Indonesia) in addition to sediment and seawater. The prokaryotic profiles of several sponge species were characterised for the first time. In line with previous studies, we revealed pronounced variation in diversity and composition among species with high microbial abundance (HMA) or low microbial abundance (LMA) status playing an important role in structuring prokaryotic communities across host sponge species. In addition to this, reef habitat (flat versus slope) also played a significant role in structuring prokaryotic communities. Most species in the reef slope habitat housed prokaryotic communities with a consistent profile of several cyanobacterial and prokaryotic OTUs, whereas these OTUs were largely absent from sponges inhabiting the reef flat habitat. Instead, they tended to house highly abundant bacterial populations related to the Synechococcus spongiarum group. We propose that specific strains of S. spongiarum may play a key role in enabling their host sponges to survive in an, otherwise, inhospitable environment (e.g., high irradiance and temperature) and, thus, help to explain differences in sponge composition between coral reef flat and slope habitats.

Previous studies on disease in coral reef organisms have neglected the natural distribution of potential pathogens and the genetic factors that underlie disease incidence. This study explores the intricate associations between hosts, microbial communities, putative pathogens, antibiotic resistance genes (ARGs) and virulence factors (VFs) across diverse coral reef biotopes. We observed a substantial compositional overlap of putative bacterial pathogens, VFs and ARGs across biotopes, consistent with the 'everything is everywhere, but the environment selects' hypothesis. However, flatworms and soft corals deviated from this pattern, harbouring the least diverse microbial communities and the lowest diversity of putative pathogens and ARGs. Notably, our study revealed a significant congruence between the distribution of putative pathogens, ARGs and microbial assemblages across different biotopes, suggesting an association between pathogen and ARG occurrence. This study sheds light on the existence of this latent pathobiome, the disturbance of which may contribute to disease onset in coral reef organisms.

Over the past decade, an increasing number of studies have emphasized the importance of the host microbiome in influencing organismal health and development. Aligned with this understanding, our study aimed to investigate the potential association between the turbot (Scophthalmus maximus) phenotypic traits and the post-larval bacteriome. Turbot post-larvae were sampled from twenty randomly selected production cycles thirty days after hatching (DAH) across multiple post-larval production batches over a three-month period (April to June). Fish were selectively sampled based on five phenotypic traits, namely, normal, large, small, malformed, and depigmented. Our results showed that small-sized post-larvae had significantly higher bacterial phylogenetic diversity in their bacterial communities than all other phenotypes. A more in-depth compositional analysis also revealed specific associations between certain bacterial taxa and fish phenotypes. For example, the genera Aliivibrio and Sulfitobacter were enriched in small-sized post-larvae, while the family Micrococcaceae were predominantly found in larger post-larvae. Furthermore, genus Exiguobacterium was linked to depigmented larvae, and genus Pantoea was more prevalent in normal post-larvae. These observations underscore the importance of further research to understand the roles of these bacterial taxa in larval growth and phenotypic differentiation. Such insights could contribute to developing microbiome modulation strategies, which may enhance turbot post-larval health and quality and improve larviculture production.

Aims This study aimed to evaluate the efficiency of two phages [VB_VaC_TDDLMA (phage TDD) and VB_VaC_SRILMA (phage SRI)] alone and in a cocktail to control Vibrio alginolyticus in brine shrimp before their administration in larviculture.Methods and results Phages were isolated from seawater samples and characterized by host spectrum, growth parameters, adsorption rate, genomic analysis, and inactivation efficiency. Both phages belong to the Caudoviricetes class and lack known virulence or antibiotic-resistance genes. They exhibit specificity, infecting only their host, V. alginolyticus CECT 521. Preliminary experiments in a culture medium showed that phage TDD (reduction of 5.8 log CFU ml-1 after 10 h) outperformed phage SRI (reduction of 4.6 log CFU ml-1 after 6 h) and the cocktail TDD/SRI (reduction of 5.2 log CFU ml-1 after 8 h). In artificial marine water experiments with Artemia franciscana, both single phage suspensions and the phage cocktail, effectively inactivated V. alginolyticus in culture water (reduction of 4.3, 2.1, and 1.9 log CFU ml-1 for phages TDD, SRI, and the phage cocktail, respectively, after 12 h) and in A. franciscana (reduction of 51.6%, 87.3%, and 85.3% for phages TDD, SRI, and the phage cocktail, respectively, after 24 h). The two phages and the phage cocktail did not affect A. franciscana natural microbiota or other Vibrio species in the brine shrimp.Conclusions The results suggest that phages can safely and effectively control V. alginolyticus in A. franciscana prior to its administration in larviculture.

In the present study, we assessed the sponge fauna, sponge-associated, and planktonic prokaryotic communities residing in Burgers' Zoo Ocean aquarium, Arnhem, the Netherlands. The Ocean aquarium consisted of separate displays and life support systems, and included fish-only systems in addition to a large, 750,000 L tank containing a living, tropical coral reef ecosystem. Sponges were observed throughout the aquarium system and were identified as belonging to the genera Chalinula, Chondrilla, Chondrosia, Cinachyrella, Stylissa, Suberites and Tethya. There was a highly significant difference in composition between sponge- associated and planktonic prokaryotic communities. The tanks in which the sponges were sampled appeared to have a secondary structural effect on prokaryotic composition with sponges and water from the same tanks sharing several microorganisms. Both sponge-associated and planktonic prokaryotic communities housed prokaryotic taxa, which were highly similar to microorganisms previously recorded in sponges or coral reef environments, including taxa potentially involved in nitrification, denitrification, sulphur oxidation, and antibiotic biosynthesis. Several abundant microorganisms were only recorded in sponges and these may play a role in maintaining water quality in the aquarium system. Potential pathogens, e.g. related to Photobacterium damselae, and beneficial organisms, e.g. related to Pseudovibrio denitrificans, were also detected. The present study showed that Burgers' Zoo Ocean aquarium housed diverse free-living and host-associated prokaryotic communities. Future research should focus on identifying conditions and microbial communities conducive to a healthy aquarium environment.

Urinary tract infections (UTIs) caused by resistant Klebsiella pneumoniae can lead to severe clinical complications and even death. An alternative treatment option for infected patients is using bacteriophages. In the present study, we isolated phage VB_KPM_KP1LMA (KP1LMA) from sewage water using a K. pneumoniae strain as a host. Whole-genome analysis indicated that the genome was a double-stranded linear 176,096-bp long DNA molecule with 41.8% GC content and did not contain virulence or antibiotic resistance genes. The inactivation potential of phage KP1LMA was assessed in broth at an MOI of 1 and 10, and a maximum inactivation of 4.9 and 5.4 log CFU/mL, respectively, was observed after 9 h. The efficacy at an MOI of 10 was also assessed in urine to evaluate the phage's performance in an acidic environment. A maximum inactivation of 3.8 log CFU/mL was observed after 9 h. The results suggest that phage KP1LMA could potentially control a UTI caused by this strain of K. pneumoniae, indicating that the same procedure can be used to control UTIs caused by other strains if new specific phages are isolated. Although phage KP1LMA has a narrow host range, in the future, efforts can be made to expand its spectrum of activity and also to combine this phage with others, potentially enabling its use against other K. pneumoniae strains involved in UTIs.

Ecosystem functioning depends on complex interactions between microorganisms, hosts, and the environment. Changes in environmental conditions (e.g., ocean acidification) in combination with anthropogenic pollution have been shown to affect the composition and function of free-living microbial communities, but little is known about the effects these stressors on host-associated communities. This study aims to characterize the response of host-associated bacterial communities of the bottom-dwelling polychaete Hediste diversicolor and the epibenthic gastropod Peringia ulvae to oil contamination and reduced seawater pH. The independent and interactive effects of both stressors were simulated under controlled conditions. The response of host-associated bacterial communities was assessed using the high-throughput sequencing of the 16S rRNA gene and several biochemical markers related to host metabolic pathways, e.g., neurotransmission, anaerobic metabolism, biotransformation, oxidative stress, and energy consumption. In H. diversicolor, reduced seawater pH was associated with a high relative abundance of Cyanobacteria, while in P. ulvae oil contamination was associated with a reduction in the relative abundance of Chitinophagales. In P. ulvae, enrichment with oil hydrocarbon-degrading bacteria suggests a possible role of these organisms in the dispersion of oil hydrocarbon degraders. Furthermore, oil supplementation shifted some specific biochemical markers of gastropods related to oxidative stress and energy consumption, which suggests host stress. In general, the bacterial communities and biochemical markers of the gastropod were more affected by stressors than those of the polychaete. Overall, this study contributes to a better understanding of the response of host-associated bacterial communities of benthic macrofauna to anthropogenic contamination and environmental change.

Mesophotic reefs, located in the "Twilight Zone" of the ocean, are coral reefs that exist in relatively deep waters ranging from approximately 30 to 150 m below the surface. These reefs are situated just beyond the reach of conventional SCUBA diving and are typically explored using advanced diving techniques or with the aid of submersibles. In the present study, we used a state-of-the-art submersible to sample 26 sponge specimens belonging to 11 species. High (HMA) or low (LMA) microbial abundance status was assigned to species based on TEM imagery. Prokaryotic communities associated with these sponges were, furthermore, assessed using high-throughput sequencing. Proteobacteria, Chloroflexi, Actinobacteriota, and Acidobacteriota were the most abundant phyla overall. HMA/LMA status proved to be a highly significant predictor of prokaryotic composition. HMA sponges also tended to be more diverse in terms of richness and evenness than LMA sponges. 14 predictor-classes were identified using an exploratory technique based on machine learning including classes within the phyla Chloroflexi (e.g., Dehalococcoidia and JG30-KF-CM66) and Acidobacteriota (Thermoanaerobaculia and Subgroups 11 and 21). Previous studies have demonstrated the prevalence of the HMA/LMA dichotomy in shallow waters and, recently, the deep sea. Our results demonstrate its prevalence in the mesophotic realm.

In the present study, we developed and validated an experimental life support system (ELSS) designed to investigate coral reef associated bacterial communities. The microcosms in the ELSS consisted of coral reef sediment, synthetic seawater, and specimens of five benthic reef species. These included two hard corals Montipora digitata and Montipora capricornis, a soft coral Sarcophyton glaucum, a zoanthid Zoanthus sp., and a sponge Chondrilla sp.. Physicochemical parameters and bacterial communities in the ELSS were similar to those observed at shallow coral reef sites. Sediment bacterial evenness and higher taxonomic composition were more similar to natural-type communities at days 29 and 34 than at day 8 after transfer to the microcosms, suggesting microbial stabilization after an initial recovery period. Biotopes were compositionally distinct but shared a number of ASVs. At day 34, sediment specific ASVs were found in hosts and visa versa. Transplantation significantly altered the bacterial community composition of M. digitata and Chondrilla sp., suggesting microbial adaptation to altered environmental conditions. Altogether, our results support the suitability of the ELSS developed in this study as a model system to investigate coral reef associated bacterial communities using multi-factorial experiments.