Species Specificity of Bacteria Associated to the Brown Seaweeds Lobophora (Dictyotales, Phaeophyceae) and Their Potential for Induction of Rapid Coral Bleaching in Acropora muricata

High-Throughput Sequencing Analysis Revealed a Preference for Animal-Based Food in Purple Sea Urchins

Sea urchins play an important role in marine ecosystems. Owing to limitations in previous research methods, there has been insufficient understanding of the food sources and ecological functional value of purple sea urchins, leading to considerable controversy regarding their functional positioning. We focused on Daya Bay as the research area, utilizing stable isotope technology and high-throughput sequencing of 16S rDNA and 18S rDNA to analyze sea urchins and their potential food sources in stone and algae areas. The results showed that the δ13C range of purple sea urchins in the stone area is -11.42~-8.17‱, and the δ15N range is 9.15~10.31‱. However, in the algal area, the δ13C range is -13.97~-12.44‱, and the δ15N range is 8.75~10.14‱. There was a significant difference in δ13C between the two areas (p < 0.05), but there was no significant difference in δ15N (p > 0.05). The main food source for purple sea urchins in both areas is sediment. The sequencing results of 18S rDNA revealed that, in the algal area, the highest proportion in the sea urchin gut was Molluska (57.37%). In the stone area, the highest proportion was Arthropoda (76.71%). The sequencing results of 16S rDNA revealed that, in the algal area, Bacteroidetes was the dominant group in the sea urchin gut (28.87%), whereas, in the stone area, Proteobacteria was the dominant group (37.83%). Diversity detection revealed a significant difference in the number of gut microbes and eukaryotes between the stone and algal areas (p < 0.05). The results revealed that the main food source of purple sea urchins in both areas is sediment, but the organic nutritional value is greater in the algal area, and the richness of microbiota and eukaryotes in the gut of purple sea urchins in the stone area is greater. These results indicated that purple sea urchins are likely omnivores and that the area where they occur impacts their growth and development. The results of this study provide a theoretical basis for the restoration of wild purple sea urchin resources and the selection of areas for restocking and release.

Coral thermal stress and bleaching enrich and restructure reef microbial communities via altered organic matter exudation

Coral bleaching is a well-documented and increasingly widespread phenomenon in reefs across the globe, yet there has been relatively little research on the implications for reef water column microbiology and biogeochemistry. A mesocosm heating experiment and bottle incubation compared how unbleached and bleached corals alter dissolved organic matter (DOM) exudation in response to thermal stress and subsequent effects on microbial growth and community structure in the water column. Thermal stress of healthy corals tripled DOM flux relative to ambient corals. DOM exudates from stressed corals (heated and/or previously bleached) were compositionally distinct from healthy corals and significantly increased growth of bacterioplankton, enriching copiotrophs and putative pathogens. Together these results demonstrate how the impacts of both short-term thermal stress and long-term bleaching may extend into the water column, with altered coral DOM exudation driving microbial feedbacks that influence how coral reefs respond to and recover from mass bleaching events.

Bacterial Microbiota of Ostreobium, the Coral-Isolated Chlorophyte Ectosymbiont, at Contrasted Salinities

Microscopic filaments of the siphonous green algae Ostreobium (Ulvophyceae, Bryopsidales) colonize and dissolve the calcium carbonate skeletons of coral colonies in reefs of contrasted salinities. Here, we analyzed their bacterial community's composition and plasticity in response to salinity. Multiple cultures of Pocillopora coral-isolated Ostreobium strains from two distinct rbcL lineages representative of IndoPacific environmental phylotypes were pre-acclimatized (>9 months) to three ecologically relevant reef salinities: 32.9, 35.1, and 40.2 psu. Bacterial phylotypes were visualized for the first time at filament scale by CARD-FISH in algal tissue sections, within siphons, at their surface or in their mucilage. Ostreobium-associated microbiota, characterized by bacterial 16S rDNA metabarcoding of cultured thalli and their corresponding supernatants, were structured by host genotype (Ostreobium strain lineage), with dominant Kiloniellaceae or Rhodospirillaceae (Alphaproteobacteria, Rhodospirillales) depending on Ostreobium lineage, and shifted Rhizobiales' abundances in response to the salinity increase. A small core microbiota composed of seven ASVs (~1.5% of thalli ASVs, 19-36% cumulated proportions) was persistent across three salinities in both genotypes, with putative intracellular Amoebophilaceae and Rickettsiales_AB1, as well as Hyphomonadaceae and Rhodospirillaceae also detected within environmental (Ostreobium-colonized) Pocillopora coral skeletons. This novel knowledge on the taxonomic diversity of Ostreobium bacteria paves the way to functional interaction studies within the coral holobiont.

Exploring Diversity and Polymer Degrading Potential of Epiphytic Bacteria Isolated from Marine Macroalgae

The macroalgae surface allows specific bacterial communities to colonize, resulting in complex biological interactions. In recent years, several researchers have studied the diversity and function of the epiphytic bacteria associated with algal host, but largely these interactions remain underexplored. In the present study we analysed the cultivable diversity and polymer degradation potential of epiphytic bacteria associated with five different marine macroalgae (Sargassum, Ulva, Padina, Dictyota and Pterocladia sp.) sampled from the central west coast of India. Out of the total 360 strains isolated, purified and preserved, about 238 strains were identified through 16S rRNA gene sequence analysis and processed for polymer (cellulose, pectin, xylan and starch) degrading activities. Phylogeny placed the strains within the classes Actinobacteria, Bacilli, Alpha-proteobacteria, and Gamma-proteobacteria and clustered them into 45 genera, wherein Vibrio, Bacillus, Pseudoalteromonas, Alteromonas, Staphylococcus and Kocuria spp. were the most abundant with 20 strains identified as potentially novel taxa within the genera Bacillus, Cellulosimicrobium, Gordonia, Marinomonas, Vibrio, Luteimonas and Pseudoalteromonas. In terms of polymer hydrolysis potential, 61.3% had xylanase activity, while 59.7%, 58.8%, and 52.2% had amylase, cellulase, and pectinase activity, respectively. Overall, 75.6% of the strains degraded more than one polysaccharide, 24% degraded all polymers, while nine strains (3.8%) degraded raw sugarcane bagasse. This study showed great potential for seaweed-associated bacteria in the bio-remediation of agro-waste based raw materials, which can be employed in the form of green technology.

Alternative approaches to identify core bacteria in Fucus distichus microbiome and assess their distribution and host-specificity

BACKGROUND

Identifying meaningful ecological associations between host and components of the microbiome is challenging. This is especially true for hosts such as marine macroalgae where the taxonomic composition of the microbiome is highly diverse and variable in space and time. Identifying core taxa is one way forward but there are many methods and thresholds in use. This study leverages a large dataset of microbial communities associated with the widespread brown macroalga, Fucus distichus, across sites and years on one island in British Columbia, Canada. We compare three different methodological approaches to identify core taxa at the amplicon sequence variant (ASV) level from this dataset: (1) frequency analysis of taxa on F. distichus performed over the whole dataset, (2) indicator species analysis (IndVal) over the whole dataset that identifies frequent taxa that are enriched on F. distichus in comparison to the local environment, and (3) a two-step IndVal method that identifies taxa that are consistently enriched on F. distichus across sites and time points. We then investigated a F. distichus time-series dataset to see if those core taxa are seasonally consistent on another remote island in British Columbia, Canada. We then evaluate host-specificity of the identified F. distichus core ASVs using comparative data from 32 other macroalgal species sampled at one of the sites.

RESULTS

We show that a handful of core ASVs are consistently identified by both frequency analysis and IndVal approaches with alternative definitions, although no ASVs were always present on F. distichus and IndVal identified a diverse array of F. distichus indicator taxa across sites on Calvert Island in multiple years. Frequency analysis captured a broader suit of taxa, while IndVal was better at identifying host-specific microbes. Finally, two-step IndVal identified hundreds of indicator ASVs for particular sites/timepoints but only 12 that were indicators in a majority (> 6 out of 11) of sites/timepoints. Ten of these ASVs were also indicators on Quadra Island, 250 km away. Many F. distichus-core ASVs are generally found on multiple macroalgal species, while a few ASVs are highly specific to F. distichus.

CONCLUSIONS

Different methodological approaches with variable set thresholds influence core identification, but a handful of core taxa are apparently identifiable as they are widespread and temporally associated with F. distichus and enriched in comparison to the environment. Moreover, we show that many of these core ASVs of F. distichus are found on multiple macroalgal hosts, indicating that most occupy a macroalgal generalist niche rather than forming highly specialized associations with F. distichus. Further studies should test whether macroalgal generalists or specialists are more likely to engage in biologically important exchanges with host.

Red, Gold and Green: Microbial Contribution of Rhodophyta and Other Algae to Green Turtle (Chelonia mydas) Gut Microbiome

The fitness of the endangered green sea turtle (Chelonia mydas) may be strongly affected by its gut microbiome, as microbes play important roles in host nutrition and health. This study aimed at establishing environmental microbial baselines that can be used to assess turtle health under altered future conditions. We characterized the microbiome associated with the gastrointestinal tract of green turtles from Guinea Bissau in different life stages and associated with their food items, using 16S rRNA metabarcoding. We found that the most abundant (% relative abundance) bacterial phyla across the gastrointestinal sections were Proteobacteria (68.1 ± 13.9% “amplicon sequence variants”, ASVs), Bacteroidetes (15.1 ± 10.1%) and Firmicutes (14.7 ± 21.7%). Additionally, we found the presence of two red algae bacterial indicator ASVs (the Alphaproteobacteria Brucella pinnipedialis with 75 ± 0% and a Gammaproteobacteria identified as methanotrophic endosymbiont of Bathymodiolus, with <1%) in cloacal compartments, along with six bacterial ASVs shared only between cloacal and local environmental red algae samples. We corroborate previous results demonstrating that green turtles fed on red algae (but, to a lower extent, also seagrass and brown algae), thus, acquiring microbial components that potentially aid them digest these food items. This study is a foundation for better understanding the microbial composition of sea turtle digestive tracts.

Exposure of Goniopora columna to polyethylene microplastics (PE-MPs): Effects of PE-MP concentration on extracellular polymeric substances and microbial community

Although the pollution of coral reefs by microplastics (MPs) is an environmental problem of global significance, the effects of MP concentration on scleractinian corals remain largely underexplored. Herein, we exposed a representative scleractinian coral (Goniopora columna) to different concentrations (5-300 mg L^-1) of polyethylene microplastics (PE-MPs; 40-48 μm) over seven days and evaluated the changes in microbial community and extracellular polymeric substances (EPS) using fluorescence excitation-emission matrix spectroscopy and amplicon sequence variants (ASV). At a PE-MP concentration of 300 mg L^-1, the relative abundance of Bacillus (Firmicutes phylum) and Ruegeria (Proteobacteria phylum) in PE-MP-associated EPS increased and decreased, respectively, while the effects of exposure depended on the particle size of the extracellular polymeric substance (EPS)-based matrix and the humification index. Humic- and fulvic-like substances were identified as critical EPS components produced by microbial activity. The results have shed new insights into short-term responses of G. columna during exposure to different PE-MP concentrations and reveal important coral-MP-microbiome interactions in coral reef ecosystems. Results demonstrated that the coral-MPs interactions should be further evaluated to gain a deeper understanding of the underlying ecotoxicological risks.

Exploring the Interactions between Algae and Bacteria

Humans have used algae for hundreds of years to make various products viz. agar, fertilizer, food, and pigments. Algae are also used in bioremediation to clean up polluted water and as essential laboratory tools in genomics, proteomics, and other research applications such as environmental warnings. Several special features of algae, including the oxygenic photosynthesis, higher yield in biomass, growth on the non-arable lands, their survival in a wide range of water supplies (contaminated or filtered waters), the production of necessary byproducts and biofuels, the enhancement of soil productivity, and the greenhouse gas emissions, etc. altogether rendered them as vital bio-resources in the sustainable development. Algae and bacteria have been assumed to coexist from the early stages of the development of the earth, and a wide variety of interactions were observed between them which have influenced the ecosystems ranging from the oceans to the lichens. Research has shown that bacteria and algae interact synergistically, especially roseobacter-algae interactions being the most common. These interactions are common to all ecosystems and characterize their primary efficiency. The commercialization of algae for industrial purposes, an important field, is also influenced by this interaction which frequently results in bacterial infections among the consumers. However, the recent findings have revealed that the bacteria improve algal growth and support flocculation which are very crucial in algal biotechnology. Some of the most exciting advancements in the area of algal biotic interactions and potential difficulties were reviewed in this article. Information gleaned in this study would provide a firm foundation for launching more contemporaneous research efforts in understanding and utilizing the algal species in biotechnology industries and medical sectors.

Bacterial Composition Associated With Giant Colonies of the Harmful Algal Species Phaeocystis globosa

The cosmopolitan algae Phaeocystis globosa forms harmful algal blooms frequently in a number of tropical and subtropical coastal regions in the past two decades. During the bloom, the giant colony, which is formed by P. globosa, is the dominant morphotype. However, the microenvironment and the microbial composition in the intracolonial fluid are poorly understood. Here, we used high-throughput 16S rRNA amplicon sequencing to examine the bacterial composition and predicted functions in intracolonial fluid. Compared with the bacterial consortia in ambient seawater, intracolonial fluids possessed the lower levels of microbial richness and diversity, implying selectivity of bacteria by the unique intracolonial microenvironment enclosed within the P. globosa polysaccharide envelope. The bacterial consortia in intracolonial fluid were dominated by Balneola (48.6% of total abundance) and Labrezia (28.5%). The bacteria and microbial function enriched in intracolonial fluid were involved in aromatic benzenoid compounds degradation, DMSP and DMS production and consumption, and antibacterial compounds synthesis. We suggest that the P. globosa colonial envelope allows for the formation of a specific microenvironment; thus, the unique microbial consortia inhabiting intracolonial fluid has close interaction with P. globosa cells, which may benefit colony development.

An investigation of antibacterial activity of New Zealand seaweed-associated marine bacteria

Aim: To explore seaweed-associated bacteria as prospective producers of bioactive material with antibacterial properties. Materials & methods: 143 bacterial species were isolated from the surface of 15 New Zealand marine macroalgae. Bacterial extracts obtained using dimethyl sulfoxide and ethyl acetate were screened for antagonistic activities against three antimicrobial susceptibility indicators: Kocuria rhizophila, Staphylococcus epidermidis and Escherichia coli, using well-diffusion method. For selected species, minimum inhibitory concentration was determined, followed by a phylogenetic identification based on 16S rRNA gene sequences. Results: Among all bacteria screened, seven that belonged to the genera Vibrio, Pseudoalteromonas, Psychromonas and Cobetia, showed antagonistic activity against all three indicators. Conclusion: Seaweed-associated bacteria produce bioactive compounds with antimicrobial potential and possible biomedical application in aquatic habitats.

Calcified macroalgae and their bacterial community in relation to larval settlement and metamorphosis of reef-building coral Pocillopora damicornis

Calcified macroalgae play an important role in the settlement and metamorphosis of invertebrate larvae in coral reef ecosystems. However, little is known about the algal-associated bacterial communities and their effects on larval settlement. In this study, the responses of larvae of the coral Pocillopora damicornis to calcified algae (Porolithon onkodes, Halimeda cylindracea, Halimeda opuntia and Amphiroa fragilissima) were evaluated. The results revealed that Por. onkodes and H. cylindracea significantly enhanced the rates of settlement and metamorphosis, whereas fewer larvae settled on Am. fragilissima and H. opuntia. Amplicon pyrosequencing of the V3-V4 region of 16S rDNA was applied to investigate the relationship between algal bacterial community and larval settlement. Principal coordinates analysis demonstrated that the bacterial community composition of H. opuntia was more similar to that of Am. fragilissima, but clearly distinct from those of H. cylindracea and Por. onkodes. Furthermore, the relative abundances of bacteria were highly diverse among different algae. H. opuntia had higher percentages of Thalassobius, Pelagibius and SM1A02, whereas the abundances of Mycoplasma and Suttonella were significantly higher in H. cylindracea than other algae. Our results showed that larval settlement/metamorphosis was strongly correlated with the bacterial community composition and with the relative abundance of a few operational taxonomic units.

Bacterial diversity in six species of fresh edible seaweeds submitted to high pressure processing and long-term refrigerated storage

Seaweeds are highly perishable foods due to their richness in nutrients. High pressure processing (HPP) has been applied for extending the shelf life of fresh seaweeds but there is no information on the effect of HPP on the bacterial diversity of seaweeds. The culturable bacteria of six species of fresh edible seaweeds (green seaweeds Codium fragile and Ulva lactuca, brown seaweeds Himanthalia elongata, Laminaria ochroleuca and Undaria pinnatifida, and red seaweed Chondrus crispus) were investigated and compared to those of HPP-treated (400 and 600 MPa for 5 min) seaweeds, at the start and end of their refrigerated storage period. A total of 523 and 506 bacterial isolates were respectively retrieved from untreated and HPP-treated seaweeds. Isolates from untreated seaweeds belonged to 18 orders, 35 families, 71 genera and 135 species whereas isolates from HPP-treated seaweeds belonged to 13 orders, 23 families, 43 genera and 103 species. HPP treatment significantly reduced the number of isolates belonging to 6 families and greatly increased the number of Bacillaceae isolates. At the end of storage, decreases in bacterial diversity at the genus and species level were observed for untreated as well as for HPP-treated seaweeds.

Niche-based assembly of bacterial consortia on the diatom Thalassiosira rotula is stable and reproducible

With each cell division, phytoplankton create new space for primary colonization by marine bacteria. Although this surface microenvironment is available to all planktonic bacterial colonizers, we show the assembly of bacterial consortia on a cosmopolitan marine diatom to be highly specific and reproducible. While phytoplankton-bacteria interactions play fundamental roles in marine ecosystems, namely primary production and the carbon cycle, the ecological paradigm behind epiphytic microbiome assembly remains poorly understood. In a replicated and repeated primary colonization experiment, we exposed the axenic diatom Thalassiosira rotula to several complex and compositionally different bacterial inocula derived from phytoplankton species of varying degrees of relatedness to the axenic Thalassiosira host or natural seawater. This revealed a convergent assembly of diverse and compositionally different bacterial inocula, containing up to 2071 operational taxonomic units (OTUs), towards a stable and reproducible core community. Four of these OTUs already accounted for a cumulative abundance of 60%. This core community was dominated by Rhodobacteraceae (30.5%), Alteromonadaceae (27.7%), and Oceanospirillales (18.5%) which was qualitatively and quantitatively most similar to its conspecific original. These findings reject a lottery assembly model of bacterial colonization and suggest selective microhabitat filtering. This is likely due to diatom host traits such as surface properties and different levels of specialization resulting in reciprocal stable-state associations.

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.

Distribution, Interaction and Functional Profiles of Epiphytic Bacterial Communities from the Rocky Intertidal Seaweeds, South Africa

Interrelations between epiphytic bacteria and macroalgae are multifaceted and complicated, though little is known about the community structure, interaction and functions of those epiphytic bacteria. This study comprehensively characterized the epiphytic bacterial communities associated with eight different common seaweeds collected from a rocky intertidal zone on the Indian Ocean at Cape Vidal, South Africa. High-throughput sequencing analyses indicated that seaweed-associated bacterial communities were dominated by the phyla Proteobacteria, Bacteroidetes, Firmicutes, Cyanobacteria, Planctomycetes, Actinobacteria and Verrucomicrobia. Energy-dispersive X-ray (EDX) analysis showed the presence of elemental composition in the surface of examined seaweeds, in varying concentrations. Cluster analysis showed that bacterial communities of brown seaweeds (SW2 and SW4) were closely resembled those of green seaweeds (SW1) and red seaweeds (SW7) while those of brown seaweeds formed a separate branch. Predicted functional capabilities of epiphytic bacteria using PICRUSt analysis revealed abundance of genes related to metabolic and biosynthetic activities. Further important identified functional interactions included genes for bacterial chemotaxis, which could be responsible for the observed association and network of elemental-microbes interaction. The study concludes that the diversity of epiphytic bacteria on seaweed surfaces is greatly influenced by algal organic exudates as well as elemental deposits on their surfaces, which triggers chemotaxis responses from epiphytic bacteria with the requisite genes to metabolise those substrates.

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.

Coral-algal interactions at Weizhou Island in the northern South China Sea: variations by taxa and the exacerbating impact of sediments trapped in turf algae

Competitive interactions between corals and benthic algae are increasingly frequent on degrading coral reefs, but the processes and mechanisms surrounding the interactions, as well as the exacerbating effects of sediments trapped in turf algae, are poorly described. We surveyed the frequency, proportion, and outcomes of interactions between benthic algae (turf algae and macroalgae) and 631 corals (genera: Porites, Favites, Favia, Platygyra, and Pavona) on a degenerating reef in the northern South China Sea, with a specific focus on the negative effects of algal contact on corals. Our data indicated that turf algae were the main algal competitors for each surveyed coral genus and the proportion of algal contact along the coral edges varied significantly among the coral genera and the algal types. The proportions of algal wins between corals and turf algae or macroalgae differed significantly among coral genera. Compared to macroalgae, turf algae consistently yielded more algal wins and fewer coral wins on all coral genera. Amongst the coral genera, Porites was the most easily damaged by algal competition. The proportions of turf algal wins on the coral genera increased 1.1–1.9 times in the presence of sediments. Furthermore, the proportions of algal wins on massive and encrusting corals significantly increased with the combination of sediments and turf algae as the algal type. However, the variation in proportions of algal wins between massive and encrusting corals disappeared as sediments became trapped in turf algae. Sediments bound within turf algae further induced damage to corals and reduced the competitive advantage of the different coral growth forms in their competitive interactions with adjacent turf algae.

Positive association between epiphytes and competitiveness of the brown algal genus Lobophora against corals

Observations of coral–algal competition can provide valuable information about the state of coral reef ecosystems. Here, we report contact rates and apparent competition states for six shallow lagoonal reefs in Fiji. A total of 81.4% of examined coral perimeters were found to be in contact with algae, with turf algae (54.7%) and macroalgae of the genus Lobophora (16.8%) representing the most frequently observed contacts. Turf algae competitiveness was low, with 21.8% of coral–turf contacts being won by the algae (i.e. overgrowth or bleaching of coral tissue). In contrast, Lobophora competitiveness against corals was high, with 62.5% of contacts being won by the alga. The presence of epiphytic algae on Lobophora was associated with significantly greater algal competitiveness against corals, with 75.8% and 21.1% of interactions recorded as algal wins in the presence and absence of epiphytes, respectively. Sedimentation rate, herbivorous fish biomass, and coral colony size did not have a significant effect on Lobophora–coral interactions. This research indicates a novel and important role of epiphytes in driving the outcome of coral–algal contacts.

Settlement performance of the Mediterranean reef-builders Dendropoma cristatum (Biondi 1859) in response to natural bacterial films

The gastropod Dendropoma cristatum is a biogenic engineer of the central Mediterranean, forming reefs along the lower rocky intertidal fringe with a remarkable ecological role. To understand whether reef-associated biofilm cultivable bacterial and biofilm ageing may trigger the settlement of the juvenile snails, a combination of laboratory techniques and field experiments was used. Reef-associated biofilm cultivable bacteria were isolated, and a settlement-choice experiment was performed in situ on artificial biofilms composed of i) a mixture of six biofilm-forming selected isolates, ii) all the cultivable bacteria, and iii) 13-, 23-, 32-day old biofilms formed under natural conditions. Overall, settlement rate significantly differed among biofilm treatments (p < 0.0001). A significant positive correlation between biofilm ageing and juvenile D. cristatum settlement was assessed (r = 0.69 (p < 0.001), whereas the biofilm bacterial composition (relatively to the cultivable fraction) did not show any effect on the vermetid's settlement rate.

Acidification increases abundances of Vibrionales and Planctomycetia associated to a seaweed-grazer system: potential consequences for disease and prey digestion efficiency

Ocean acidification significantly affects marine organisms in several ways, with complex interactions. Seaweeds might benefit from rising CO₂ through increased photosynthesis and carbon acquisition, with subsequent higher growth rates. However, changes in seaweed chemistry due to increased CO₂ may change the nutritional quality of tissue for grazers. In addition, organisms live in close association with a diverse microbiota, which can also be influenced by environmental changes, with feedback effects. As gut microbiomes are often linked to diet, changes in seaweed characteristics and associated microbiome can affect the gut microbiome of the grazer, with possible fitness consequences. In this study, we experimentally investigated the effects of acidification on the microbiome of the invasive brown seaweed Sargassum muticum and a native isopod consumer Synisoma nadejda. Both were exposed to ambient CO₂ conditions (380 ppm, pH 8.16) and an acidification treatment (1,000 ppm, pH 7.86) for three weeks. Microbiome diversity and composition were determined using high-throughput sequencing of the variable regions V5-7 of 16S rRNA. We anticipated that as a result of acidification, the seaweed-associated bacterial community would change, leading to further changes in the gut microbiome of grazers. However, no significant effects of elevated CO₂ on the overall bacterial community structure and composition were revealed in the seaweed. In contrast, significant changes were observed in the bacterial community of the grazer gut. Although the bacterial community of S. muticum as whole did not change, Oceanospirillales and Vibrionales (mainly Pseudoalteromonas) significantly increased their abundance in acidified conditions. The former, which uses organic matter compounds as its main source, may have opportunistically taken advantage of the possible increase of the C/N ratio in the seaweed under acidified conditions. Pseudoalteromonas, commonly associated to diseased seaweeds, suggesting that acidification may facilitate opportunistic/pathogenic bacteria. In the gut of S. nadejda, the bacterial genus Planctomycetia increased abundance under elevated CO₂. This shift might be associated to changes in food (S. muticum) quality under acidification. Planctomycetia are slow-acting decomposers of algal polymers that could be providing the isopod with an elevated algal digestion and availability of inorganic compounds to compensate the shifted C/N ratio under acidification in their food.

In conclusion, our results indicate that even after only three weeks of acidified conditions, bacterial communities associated to ungrazed seaweed and to an isopod grazer show specific, differential shifts in associated bacterial community. These have potential consequences for seaweed health (as shown in corals) and isopod food digestion. The observed changes in the gut microbiome of the grazer seem to reflect changes in the seaweed chemistry rather than its microbial composition.

A Functional Perspective Analysis of Macroalgae and Epiphytic Bacterial Community Interaction

Macroalgae are photosynthetic, multicellular, sessile eukaryotic organisms that offer diverse habitats for the colonization of epiphytic bacteria, therefore establishing biological interactions of diverse complexity. This review focusses on the interactions between macroalgae and their Epiphytic Bacterial Community (EBC); the main aims are to ascertain whether (1) the epiphytic bacterial groups differ at the phylum and genus levels of the macroalgae; (2) the methodologies used so far to study these microorganisms are related in any way to eventual variations of the EBCs on macroalgae; and (3) the EBC of macroalgae has a functional means rather a simple taxonomic grouping. Results showed firstly the taxonomic grouping of macroalgae does not explain the composition and structure of the EBCs. Secondly, the methodology used is important for describing EBCs; and thirdly, multiple bacteria can have the same function and thus to describe the functionality of EBCs it is important to recognize host-specific and generalist bacteria. We recommend the incorporation of a complementary approach between the taxonomic composition and the functional composition analyzes of EBCs, as well as the use of methodological tools that allow analysis of interactions between the EBCs and their hosts, based on the "holobiont" concept.

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.

Allelochemicals Produced by Brown Macroalgae of the Lobophora Genus Are Active against Coral Larvae and Associated Bacteria, Supporting Pathogenic Shifts to Vibrio Dominance

Pervasive environmental stressors on coral reefs are attributed with shifting the competitive balance in favor of alternative dominants, such as macroalgae. Previous studies have demonstrated that macroalgae compete with corals via a number of mechanisms, including the production of potent primary and secondary metabolites that can influence coral-associated microbial communities. The present study investigates the effects of the Pacific brown macroalga Lobophora sp. (due to the shifting nature of the Lobophora species complex, it will be referred to here as Lobophora sp.) on coral bacterial isolates, coral larvae, and the microbiome associated with the coral Porites cylindrica. Crude aqueous and organic macroalgal extracts were found to inhibit the growth of coral-associated bacteria. Extracts and fractions were also shown to inhibit coral larval settlement and cause mortality at concentrations lower (<0.3 mg · ml-1) than calculated natural concentrations (4.4 mg · ml-1). Microbial communities associated with coral tissues exposed to aqueous (e.g., hydrophilic) crude extracts demonstrated a significant shift to Vibrio dominance and a loss of sequences related to the putative coral bacterial symbiont, Endozoicomonas sp., based on 16S rRNA amplicon sequencing. This study contributes to growing evidence that macroalgal allelochemicals, dissolved organic material, and native macroalgal microbial assemblages all play a role in shifting the microbial equilibrium of the coral holobiont away from a beneficial state, contributing to a decline in coral fitness and a shift in ecosystem structure.

IMPORTANCE

Diverse microbial communities associate with coral tissues and mucus, providing important protective and nutritional services, but once disturbed, the microbial equilibrium may shift from a beneficial state to one that is detrimental or pathogenic. Macroalgae (e.g., seaweeds) can physically and chemically interact with corals, causing abrasion, bleaching, and overall stress. This study contributes to a growing body of evidence suggesting that macroalgae play a critical role in shifting the coral holobiont equilibrium, which may promote the invasion of opportunistic pathogens and cause coral mortality, facilitating additional macroalgal growth and invasion in the reef. Thus, macroalgae not only contribute to a decline in coral fitness but also influence coral reef ecosystem structure.