Sponge Holobiont Research Articles

The unique and enigmatic spirochete symbiont of latrunculid sponges.

Bacterial symbionts are critical members of many marine sponge holobionts. Some sponge-associated bacterial lineages, such as Poribacteria, sponge-associated unclassified lineage (SAUL), and Tethybacterales, appear to have broad-host ranges and associate with a diversity of sponge species, while others are more species-specific, having adapted to the niche environment of their host. Host-associated spirochete symbionts that are numerically dominant have been documented in several invertebrates including termites, starfish, and corals. However, dominant spirochete populations are rare in marine sponges, having thus far been observed only in Clathrina clathrus and various species within the Latrunculiidae family, where they are co-dominant alongside Tethybacterales symbionts. This study aimed to characterize these spirochetes and their potential role in the host sponge. Analysis of metagenome-assembled genomes from eight latrunculid sponges revealed that these unusual spirochetes are relatively recent symbionts and are phylogenetically distinct from other sponge-associated spirochetes. Functional comparative analysis suggests that the host sponge may have selected for these spirochetes due to their ability to produce terpenoids and/or possible structural contributions.IMPORTANCESouth African latrunculid sponges are host to co-dominant Tethybacterales and Spirochete symbionts. While the Tethybacterales are broad-host range symbionts, the spirochetes have not been reported as abundant in any other marine sponge except Clathrina clathrus. However, spirochetes are regularly the most dominant populations in marine corals and terrestrial invertebrates where they are predicted to serve as beneficial symbionts. Here, we interrogated eight metagenome-assembled genomes of the latrunculid-associated spirochetes and found that these symbionts are phylogenetically distinct from all invertebrate-associated spirochetes. The symbiosis between the spirochetes and their sponge host appears to have been established relatively recently.

Stable dominance of parasitic dinoflagellates in Antarctic sponges.

Marine sponges are dominant components of Antarctic benthos and representative of the high endemism that characterizes this environment. All microbial groups are part of the Antarctic sponge holobionts, but microbial eukaryotes have been studied less, and their symbiotic role still needs to be better understood. Here, we characterize the dynamics of microbial eukaryotes associated with Antarctic sponges, focusing on dinoflagellates over three summer periods to better understand the members, interannual variations, and trophic and lifestyle strategies. The analysis revealed that dinoflagellates dominate microeukaryotic communities in Antarctic sponges. The results also showed significant differences in the diversity and composition of dinoflagellate communities associated with sponges compared to those in seawater. Antarctic sponges were dominated by a single dinoflagellate family, Syndiniales Dino-Group-I-Clade 1, which was present in high abundance in Antarctic sponges compared to seawater communities. Despite minor differences, the top microeukaryotic amplicon sequence variants (ASVs) showed no significant interannual abundance changes, indicating general temporal stability within the studied sponge species. Our findings highlight the abundance and importance of parasitic groups, particularly the classes Coccidiomorphea, Gregarinomorphea, and Ichthyosporea, with the exclusive dominance of Syndiniales Dino-Group-I-Clade 1 within sponges. The present study comprehensively characterizes the microbial eukaryotes associated with Antarctic sponges, showing a remarkable stability of parasitic dinoflagellates in Antarctic sponges. These findings underscore the significant role of parasites in these marine hosts, with implications for population dynamics of the microeukaryome and the holobiont response to a changing ocean.

Dynamic microbiome diversity shaping the adaptation of sponge holobionts in coastal waters.

During long-term evolution, sponge holobionts, among the oldest symbiotic relationships between microbes and metazoans, developed two distinct phenotypes with high- and low-microbial abundance (HMA and LMA). Despite sporadic studies indicating that the characteristic microbial assemblages present in HMA and LMA sponges, the adaptation strategies of symbionts responding to environments are still unclear. This deficiency limits our understanding of the selection of symbionts and the ecological functions during the evolutionary history and the adaptative assessment of HMA and LMA sponges in variable environments. Here, we explored symbiotic communities with two distinct phenotypes in a 1-year dynamic environment and combined with the meta-analysis of 13 sponges. The different strategies of symbionts in adapting to the environment were basically drawn: microbes with LMA were more acclimated to environmental changes, forming relatively loose-connected communities, while HMA developed relatively tight-connected and more similar communities beyond the divergence of species and geographical location.

Decoupling of strain- and intrastrain-level interactions of microbiomes in a sponge holobiont

Holobionts are highly organized assemblages of eukaryotic hosts, cellular microbial symbionts, and viruses, whose interactions and evolution involve complex biological processes. It is largely unknown which specific determinants drive similarity or individuality in genetic diversity between holobionts. Here, we combine short- and long-read sequencing and DNA-proximity-linkage technologies to investigate intraspecific diversity of the microbiomes, including host-resolved viruses, in individuals of a model marine sponge. We find strong impacts of the sponge host and the cellular hosts of viruses on strain-level organization of the holobiont, whereas substantial overlap in nucleotide diversity between holobionts suggests frequent exchanges of microbial cells and viruses at intrastrain level in the local sponge population. Immune-evasive arms races likely restricted virus-host co-evolution at the intrastrain level, generated holobiont-specific genome variations, and linked virus-host genetics through recombination. Our work shows that a decoupling of strain- and intrastrain-level interactions is a key factor in the genetic diversification of holobionts.

Actinobacteria diversity associated with sponge Amorphinopsis exacavans from Rancabuaya Beach, Garut, Indonesia

Abstract Sponges are complex marine organisms known for their symbiotic relationships with various bacterial communities, which play structural and functional roles within the sponge holobiont. Actinobacteria have received much attention because of their potential pharmaceutical applications. This study examines Amorphinopsis excavans profile, distinguishing between endosymbionts and ectosymbionts. The microbial grouping was characterized using Next Generation Sequencing (NGS) anchored to the 16S rRNA marker. The results showed that Proteobacteria and Firmicutes dominated both the endosymbiont and ectosymbiont communities at the phylum level. The actinobacterial diversity index at the genus level in both communities was 2.77 and 3.11, indicating moderate to high diversity without a dominant genus. For the genus level, the evenness scores of 0.81 and 0.89 indicated a high population type within the bacterial community. Relative abundance revealed the three most prevalent genera in the endosymbiont community were Rothia, Thermoplyspora, and Thermomonospora. In contrast, Kocuria, Mycobacterium, and Bifidobacterium were the three most common genera of ectosymbionts. Leuconostoc citreum was the first species known to be associated with a sponge. This study is a developing metagenomic assessment of actinobacterial diversity in A. excavans, providing novel insights into the sponge’s complex microbial ecosystem.

Co-occurring nitrifying symbiont lineages are vertically inherited and widespread in marine sponges.

Ammonia-oxidizing archaea and nitrite-oxidizing bacteria are common members of marine sponge microbiomes. They derive energy for carbon fixation and growth from nitrification-the aerobic oxidation of ammonia to nitrite and further to nitrate-and are proposed to play essential roles in the carbon and nitrogen cycling of sponge holobionts. In this study, we characterize two novel nitrifying symbiont lineages, Candidatus Nitrosokoinonia and Candidatus Nitrosymbion in the marine sponge Coscinoderma matthewsi using a combination of molecular tools, in situ visualization, and physiological rate measurements. Both represent a new genus in the ammonia-oxidizing archaeal class Nitrososphaeria and the nitrite-oxidizing bacterial order Nitrospirales, respectively. Furthermore, we show that larvae of this viviparous sponge are densely colonized by representatives of Ca. Nitrosokoinonia and Ca. Nitrosymbion indicating vertical transmission. In adults, the representatives of both symbiont genera are located extracellularly in the mesohyl. Comparative metagenome analyses and physiological data suggest that ammonia-oxidizing archaeal symbionts of the genus Ca. Nitrosokoinonia strongly rely on endogenously produced nitrogenous compounds (i.e. ammonium, urea, nitriles/cyanides, and creatinine) rather than on exogenous ammonium sources taken up by the sponge. Additionally, the nitrite-oxidizing bacterial symbionts of the genus Ca. Nitrosymbion may reciprocally support the ammonia-oxidizers with ammonia via the utilization of sponge-derived urea and cyanate. Comparative analyses of published environmental 16S rRNA gene amplicon data revealed that Ca. Nitrosokoinonia and Ca. Nitrosymbion are widely distributed and predominantly associated with marine sponges and corals, suggesting a broad relevance of our findings.

Transcriptomic responses of sponge holobionts to in situ, seasonal anoxia and hypoxia

Transcriptomic responses of sponge holobionts to in situ, seasonal anoxia and hypoxia

Niche-dependent sponge hologenome expression profiles and the host-microbes interplay: a case of the hawaiian demosponge Mycale Grandis.

Most researches on sponge holobionts focus primarily on symbiotic microbes, yet data at the level of the sponge hologenome are still relatively scarce. Understanding of the sponge host and its microbial gene expression profiles and the host-microbes interplay in different niches represents a key aspect of sponge hologenome. Using the Hawaiian demosponge Mycale grandis in different niches as a model, i.e. on rocks, on the surface of coral Porites compressa, under alga Gracilaria salicornia, we compared the bacterial and fungal community structure, functional gene diversity, expression pattern and the host transcriptome by integrating open-format (deep sequencing) and closed-format (GeoChip microarray) high-throughput techniques. Little inter-niche variation in bacterial and fungal phylogenetic diversity was detected for M. grandis in different niches, but a clear niche-dependent variability in the functional gene diversity and expression pattern of M. grandis host and its symbiotic microbiota was uncovered by GeoChip microarray and transcriptome analyses. Particularly, sponge host genes related to innate immunity and microbial recognition showed a strong correlation with the microbial symbionts' functional gene diversity and transcriptional richness in different niches. The cross-niche variability with respect to the symbiont functional gene diversity and the transcriptional richness of M. grandis holobiont putatively reflects the interplay of niche-specific selective pressure and the symbiont functional diversity. Niche-dependent gene expression profiles of M. grandis hologenome and the host-microbes interplay were suggested though little inter-niche variation in bacterial and fungal diversity was detected, particularly the sponge innate immunity was found to be closely related to the symbiotic microbes. Altogether, these findings provide novel insights into the black box of one sponge holobiont in different niches at the hologenome level.

Phylosymbiosis and metabolomics resolve phenotypically plastic and cryptic sponge species in the genus Agelas across the Caribbean basin.

Fundamental to holobiont biology is recognising how variation in microbial composition and function relates to host phenotypic variation. Sponges often exhibit considerable phenotypic plasticity and also harbour dense microbial communities that function to protect and nourish hosts. One of the most prominent sponge genera on Caribbean coral reefs is Agelas. Using a comprehensive set of morphological (growth form, spicule), chemical and molecular data on 13 recognised species of Agelas in the Caribbean basin, we were able to define only five species (=clades) and found that many morphospecies designations were incongruent with phylogenomic and population genetic analyses. Microbial communities were also strongly differentiated between phylogenetic species, showing little evidence of cryptic divergence and relatively low correlation with morphospecies assignment. Metagenomic analyses also showed strong correspondence to phylogenetic species, and to a lesser extent, geographical and morphological characters. Surprisingly, the variation in secondary metabolites produced by sponge holobionts was explained by geography and morphospecies assignment, in addition to phylogenetic species, and covaried significantly with a subset of microbial symbionts. Spicule characteristics were highly plastic, under greater impact from geographical location than phylogeny. Our results suggest that while phenotypic plasticity is rampant in Agelas, morphological differences within phylogenetic species affect functionally important ecological traits, including the composition of the symbiotic microbial communities and metabolomic profiles.

Unveiling microbial guilds and symbiotic relationships in Antarctic sponge microbiomes

Marine sponges host diverse microbial communities. Although we know many of its ecological patterns, a deeper understanding of the polar sponge holobiont is still needed. We combine high-throughput sequencing of ribosomal genes, including the largest taxonomic repertoire of Antarctic sponge species analyzed to date, functional metagenomics, and metagenome-assembled genomes (MAGs). Our findings show that sponges harbor more exclusive bacterial and archaeal communities than seawater, while microbial eukaryotes are mostly shared. Furthermore, bacteria in Antarctic sponge holobionts establish more cooperative interactions than in sponge holobionts from other environments. The bacterial classes that established more positive relations were Bacteroidia, Gamma- and Alphaproteobacteria. Antarctic sponge microbiomes contain microbial guilds that encompass ammonia-oxidizing archaea, ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, and sulfur-oxidizing bacteria. The retrieved MAGs showed a high level of novelty and streamlining signals and belong to the most abundant members of the main microbial guilds in the Antarctic sponge holobiont. Moreover, the genomes of these symbiotic bacteria contain highly abundant functions related to their adaptation to the cold environment, vitamin production, and symbiotic lifestyle, helping the holobiont survive in this extreme environment.

Sponges on shifting reefs: holobionts show similar molecular and physiological responses to coral versus macroalgal food

IntroductionMany coral reefs witness an ongoing coral-to-algae phase shift. Corals and algae release large quantities of (in)organic nutrients daily, of which a large part is utilized by sponges. In turn, sponges are important cyclers of precious resources to other inhabitants on reefs residing in oligotrophic waters. Here, we investigated whether sponge holobionts (i.e., host and prokaryotic symbionts) adapt their physiology to food released by coral- versus macroalgae.MethodsThereto, two sponge species, Plakortis angulospiculatus and Halisarca caerulea (high and low microbial abundance, respectively), were continuously exposed for 12 days to coral and macroalgal exudates in running seawater aquaria. Transcript expression of host and prokaryotic symbionts, changes in prokaryotic community composition, and holobiont physiological responses (i.e., respiratory demand, fluxes of carbon and nitrogen) were investigated after coral- versus macroalgae dominated treatments and compared to a seawater only control treatment.ResultsIn both sponge holobionts differential transcript expression between the coral and macroalgae treatments was very low (<0.01% of total transcripts). Differential expression was found in genes targeting cellular signaling pathways, e.g., cell proliferation (upregulated in coral treatment), and immune response (upregulated in macroalgal treatment). The sponge-associated prokaryotic community composition and sponge physiological responses were similar in all treatments, yet differed significantly between the two species.DiscussionAfter 12 days of exposure sponges appear to opportunistically feed on different food sources without having to adjust their metabolic pathways or associated prokaryotic communities. This suggests that sponges could be well-adapted to predicted changes in food source availability due to coral-to-algal phase shifts on many coral reefs.

Divergent morphological and microbiome strategies of two neighbor sponges to cope with low pH in Mediterranean CO2 vents

Ocean Acidification (OA) profoundly impacts marine biochemistry, resulting in a net loss of biodiversity. Porifera are often forecasted as winner taxa, yet the strategies to cope with OA can vary and may generate diverse fitness status. In this study, microbial shifts based on the V3–V4 16S rRNA gene marker were compared across neighboring Chondrosia reniformis sponges with high microbial abundance (HMA), and Spirastrella cunctatrix with low microbial abundance (LMA) microbiomes. Sponge holobionts co-occurred in a CO2 vent system with low pH (pHT ~ 7.65), and a control site with Ambient pH (pHT ~ 8.05) off Ischia Island, representing natural analogues to study future OA, and species' responses in the face of global environmental change. Microbial diversity and composition varied in both species across sites, yet at different levels. Increased numbers of core taxa were detected in S. cunctatrix, and a more diverse and flexible core microbiome was reported in C. reniformis under OA. Vent S. cunctatrix showed morphological impairment, along with signs of putative stress-induced dysbiosis, manifested by: 1) increases in alpha diversity, 2) shifts from sponge related microbes towards seawater microbes, and 3) high dysbiosis scores. Chondrosia reniformis in lieu, showed no morphological variation, low dysbiosis scores, and experienced a reduction in alpha diversity and less number of core taxa in vent specimens. Therefore, C. reniformis is hypothesized to maintain an state of normobiosis and acclimatize to OA, thanks to a more diverse, and likely metabolically versatile microbiome. A consortium of differentially abundant microbes was identified associated to either vent or control sponges, and chiefly related to carbon, nitrogen and sulfur-metabolisms for nutrient cycling and vitamin production, as well as probiotic symbionts in C. reniformis. Diversified symbiont associates supporting functional convergence could be the key behind resilience towards OA, yet specific acclimatization traits should be further investigated.

Simple Porifera holobiont reveals complex interactions between the host, an archaeon, a bacterium, and a phage.

The basal metazoan phylum Porifera (sponges) is increasingly used as a model to investigate ecological and evolutionary features of microbe-animal symbioses. However, sponges often host complex microbiomes, which has hampered our understanding of their interactions with their microbial symbionts. Here, we describe the discovery and characterization of the simplest sponge holobiont reported to date, consisting of the deep-sea glass sponge Aphrocallistes beatrix and two newly-described microbial symbionts: an autotrophic ammonia-oxidizing archaeon and a bacterial heterotroph. Omics analyses and metabolic modeling revealed the dependency of the ammonia-oxidizing archaea on sponge-derived ammonia to drive primary production, which in turn supports the bacterium's growth by providing the dicarboxylate fumarate. Furthermore, virus-mediated archaeal lysis appears crucial to overcome the bacterium's vitamin B12 auxotrophy. These findings reveal that the exchanges of vitamin B12 and dicarboxylate may be evolutionarily conserved features of symbiosis as they can also be found in interactions between free-living marine bacteria, and between microbes and plants or diatoms.

Future oceanic conditions could leave sponge holobionts breathless – but they won’t let that stop them

Future oceanic conditions could leave sponge holobionts breathless – but they won’t let that stop them

The interactive effects of ocean acidification and warming on bioeroding sponge Spheciospongia vesparium microbiome indicated by metatranscriptomics

Global climate change will cause coral reefs decline and is expected to increase the reef erosion potential of bioeroding sponges. Microbial symbionts are essential for the overall fitness and survival of sponge holobionts in changing ocean environments. However, we rarely know about the impacts of ocean warming and acidification on bioeroding sponge microbiome. Here, the structural and functional changes of the bioeroding sponge Spheciospongia vesparium microbiome, as well as its recovery potential, were investigated at the RNA level in a laboratory system simulating 32 °C and pH 7.7. Based on metatranscriptome analysis, acidification showed no significant impact, while warming or simultaneous warming and acidification disrupted the sponge microbiome. Warming caused microbial dysbiosis and recruited potentially opportunistic and pathogenic members of Nesiotobacter, Oceanospirillaceae, Deltaproteobacteria, Epsilonproteobacteria, Bacteroidetes and Firmicutes. Moreover, warming disrupted nutrient exchange and molecular interactions in the sponge holobiont, accompanied by stimulation of virulence activity and anaerobic metabolism including denitrification and dissimilatory reduction of nitrate and sulfate to promote sponge necrosis. Particularly, the interaction between acidification and warming alleviated the negative effects of warming and enhanced the Rhodobacteraceae-driven ethylmalonyl-CoA pathway and sulfur-oxidizing multienzyme system. The microbiome could not recover during the experiment period after warming or combined stress was removed. This study suggests that warming or combined warming and acidification will irreversibly destabilize the S. vesparium microbial community structure and function, and provides insight into the molecular mechanisms of the interactive effects of acidification and warming on the sponge microbiome.

Host-specific bacterial communities associated with six cold-seep sponge species in the South China Sea

The cold-seep sponge holobionts are attracting growing attention in recent years. In this study, we utilized 16S rRNA amplicons to characterize the bacterial communities of six deep-sea sponge species found in sponge grounds at the Formosa Ridge cold seep in the South China Sea. Bacterial communities in these geographically proximal sponge species are dominated by Proteobacteria (mainly Gammaproteobacteria and Alphaproteobacteria) but exhibit distinct diversity and compositions among communities. Further analysis revealed that the SUP05 clade (Thioglobaceae) dominated most of the sponge samples. Meanwhile, phylogenetic analysis showed that the six sponge species harbored diverse SUP05 OTU phylotypes, indicating significant divergence within this clade. Additionally, operational taxonomic units (OTUs) of the family Methylomonadaceae, another abundant group in these sponges, displayed a significant genetic distance both from each other and from known species. Our findings support the hypothesis of the host-species specificity of sponge-associated bacterial communities, a widely accepted concept in shallow-water and other deep-sea sponges. The presence of dominant functional microbes, such as sulfur- and methanol-oxidizing bacteria, suggests their crucial role as chemosynthetic symbionts in facilitating the niche adaption of sponge hosts to the cold seep ecosystem. In conclusion, our study reveals the diverse and novel bacterial communities in deep-sea sponges from cold seep environments, contributing new knowledge to the host-species specificity of bacterial communities within sponges and highlighting the potential significance of functional microbes in cold seep ecosystems with dynamic energy supplies.

Genome-resolved metagenomic analysis of Great Amazon Reef System sponge-associated Latescibacterota bacteria and their potential contributions to the host sponge and reef

The Great Amazon Reef System (GARS) is an extensive biogenic reef influenced by a plume layer of sediments. This creates an extreme environment where light is reduced, thus affecting physicochemical properties as well as living organisms such as sponges and their microbiomes. The sponge’s microbiome has numerous ecological roles, like participation in biogeochemical cycles and host nutrition, helping the sponge thrive and contributing to the ecosystem. Also, sponges and sponge-associated microorganisms are rich sources of bioactive compounds, and their products are applied in different areas, including textile, pharmaceutical, and food industries. In this context, metagenome-assembled genomes (MAG), obtained from GARS sponges microbiota, were analyzed to predict their ecological function and were prospected for biotechnological features. Thus, in this work, tissues of GARS sponges were collected, their metagenomes were sequenced and assembled, and 1,054 MAGs were recovered. Ten of those MAGs were selected based on their taxonomic classification in the candidate phylum Latescibacterota and this group’s abundance in GARS sponges. The workflow consisted of MAG’s quality definition, taxonomic classification, metabolic reconstruction, and search for bioactive compounds. Metabolic reconstruction from medium to high-quality MAGs revealed genes related to degradation and synthesis pathways, indicating functions that may be performed by GARS sponge-associated Latescibacterota. Heterotrophy, a recurring attribute in Latescibacterota that might be crucial for GARS sponge holobiont nutrition, was verified by the presence of genes related to respiration and fermentation. Also, the analyzed bacteria may contribute to the host’s survival in multiple ways, including host protection via defense systems; aid in nutrient consumption by breaking complex substrates and producing essential nutrients like vitamins and certain amino acids; and detoxification of mercury, arsenic, ammonia, and hydrogen sulfide. Additionally, genes linked to persistent organic pollutant degradation, including glyphosate, and biogeochemical cycles reactions, such as ammonification, sulfate reduction, thiosulfate disproportionation, phosphorus remineralization, and complex organic matter degradation, were identified, suggesting the participation of these Latescibacterota in bioremediation and nutrient cycling. Finally, the investigated MAGs contain genes for numerous bioactive compounds, including industrial enzymes, secondary metabolites, and biologically active peptides, which may have biotechnological value.

Candidatus Nemesobacterales is a sponge-specific clade of the candidate phylum Desulfobacterota adapted to a symbiotic lifestyle.

Members of the candidate phylum Dadabacteria, recently reassigned to the phylum Candidatus Desulfobacterota, are cosmopolitan in the marine environment found both free-living and associated with hosts that are mainly marine sponges. Yet, these microorganisms are poorly characterized, with no cultured representatives and an ambiguous phylogenetic position in the tree of life. Here, we performed genome-centric metagenomics to elucidate their phylogenomic placement and predict the metabolism of the sponge-associated members of this lineage. Rank-based phylogenomics revealed several new species and a novel family (Candidatus Spongomicrobiaceae) within a sponge-specific order, named here Candidatus Nemesobacterales. Metabolic reconstruction suggests that Ca. Nemesobacterales are aerobic heterotrophs, capable of synthesizing most amino acids, vitamins and cofactors and degrading complex carbohydrates. We also report functional divergence between sponge- and seawater-associated metagenome-assembled genomes. Niche-specific adaptations to the sponge holobiont were evident from significantly enriched genes involved in defense mechanisms against foreign DNA and environmental stressors, host-symbiont interactions and secondary metabolite production. Fluorescence in situ hybridization gave a first glimpse of the morphology and lifestyle of a member of Ca. Desulfobacterota. Candidatus Nemesobacterales spp. were found both inside sponge cells centred around sponge nuclei and in the mesohyl of the sponge Geodia barretti. This study sheds light on the enigmatic group Ca. Nemesobacterales and their functional characteristics that reflect a symbiotic lifestyle.

The RNA Viruses in Samples of Endemic Lake Baikal Sponges

Sponges are unusual representatives of the animal kingdom; their viromes, as part of the associated community, began to be studied quite recently, and, accordingly, these studies are gaining momentum. The diversity of viruses in sponges is high, and they most likely play a significant role in the composition of the sponge holobiont, especially under stress conditions. The objects of our metagenomic study were RNA viruses of two common endemic species of Baikal sponges, Lubomirskia baikalensis and Baikalospongia bacillifera. As a result of viral RNA sequencing, we were able to identify fragments of viral genomes related to those from the RefSeq NCBI complete viral genome database. Most of the similar genomes belonged to viruses isolated from various invertebrates; some of the scaffolds were related to known plant viruses, and one of them was related to a vertebrate virus. The similarity of the putative proteins of viral scaffolds from the Baikal sponges with proteins of known viruses turned out to be low (20.7–67.3%), indicating the detection of novel viruses. The samples of diseased and visually healthy sponges were clustered separately, suggesting a shift in sponge virome composition during the course of the disease. In a comparative analysis, the viromes of the Baikal and marine sponges differed significantly, demonstrating the influence of the host species, habitat, and geographical location on virome composition in the sponge holobiont.

Trophic ecology of Angolan cold-water coral reefs (SE Atlantic) based on stable isotope analyses

Cold-water coral (CWC) reefs of the Angolan margin (SE Atlantic) are dominated by Desmophyllum pertusum and support a diverse community of associated fauna, despite hypoxic conditions. In this study, we use carbon and nitrogen stable isotope analyses (δ13C and δ15N) to decipher the trophic network of this relatively unknown CWC province. Although fresh phytodetritus is available to the reef, δ15N signatures indicate that CWCs (12.90 ± 1.00 ‰) sit two trophic levels above Suspended Particulate Organic Matter (SPOM) (4.23 ± 1.64 ‰) suggesting that CWCs are highly reliant on an intermediate food source, which may be zooplankton. Echinoderms and the polychaete Eunice norvegica occupy the same trophic guild, with high δ13C signatures (-14.00 ± 1.08 ‰) pointing to a predatory feeding behavior on CWCs and sponges, although detrital feeding on 13C enriched particles might also be important for this group. Sponges presented the highest δ15N values (20.20 ± 1.87 ‰), which could be due to the role of the sponge holobiont and bacterial food in driving intense nitrogen cycling processes in sponges’ tissue, helping to cope with the hypoxic conditions of the reef. Our study provides first insights to understand trophic interactions of CWC reefs under low-oxygen conditions.