Adomaviruses are circular double-stranded DNA viruses that have been mostly described in bony fish species and a few amphibians, but reports of their presence in elasmobranchs have so far been lacking. The discovery of this new viral entity in the broadnose sevengill shark marks a significant extension of the known host range of the family and raises intriguing questions about the evolutionary trajectories of these viruses in aquatic environments. Researchers employed a multidisciplinary approach combining high-throughput metagenomics, histopathology, and electron microscopy to unveil the viral genome and its biological impact on its shark host.

The study began with the identification of unusual proliferative skin lesions observed on several wild-caught broadnose sevengill sharks during routine health assessments. These lesions presented as irregular, raised nodules that contrasted with the normally smooth integument of the sharks. Histological analysis revealed epidermal hyperplasia and inclusion bodies suggestive of viral infection. Given the lack of previously reported viral pathogens capable of causing such lesions in elasmobranchs, the team sought to isolate and sequence the underlying causative agent using advanced nucleic acid screening methods.

By extracting DNA directly from the lesion tissue and applying targeted viral enrichment protocols, the researchers generated high-quality sequencing libraries for metagenomic analysis. Bioinformatic pipelines tailored to viral genome assembly identified a circular DNA virus with hallmark ORFs typical of adomaviruses, including genes encoding a replicative helicase, a major capsid protein, and various transcriptional regulators. Comparative genomics showed that this virus shares approximately 60% genome-wide nucleotide identity with the closest known adomavirus species but possesses distinct structural protein variants consistent with host adaptation.

Intriguingly, phylogenetic reconstruction places the newly discovered adomavirus on a distinct evolutionary branch within the family, suggesting a lineage that diverged early to colonize elasmobranch hosts. This finding enriches the viral tree of life by defining a shark-specific adomavirus clade and supports the hypothesis that adomaviruses have co-evolved with diverse aquatic vertebrates over hundreds of millions of years. Moreover, the research highlights the potential role of these viruses in marine vertebrate health and ecology, emphasizing how viral infections may contribute to skin disease dynamics in sharks.

At the microscopic level, electron microscopy revealed icosahedral virions approximately 60 nanometers in diameter within the infected epidermis. These virus particles were observed budding from keratinocytes, suggesting active replication and horizontal spread within the skin tissue. The presence of viral inclusion bodies and virion assemblages coincided with cellular atypia and proliferative tissue changes, establishing a direct link between viral activity and lesion formation. These pathological observations provide crucial evidence that the adomavirus is not a benign commensal but a potential pathogen exerting measurable effects on shark integument.

Beyond clinical pathology, the study also explored the molecular biology of replication and gene expression using RNA sequencing of lesion tissues. The virus exhibits a temporal pattern of transcript accumulation consistent with other well-studied adomaviruses: early expression of replication-associated genes is followed by late expression of structural proteins critical for virion assembly. This tightly regulated gene expression program underscores the sophisticated viral machinery adapted for persistence in a complex vertebrate host environment. Insights into these mechanisms offer promising avenues for understanding virus-host interactions in marine species.

Regarding transmission dynamics, the researchers hypothesize that the virus spreads through direct contact or environmental exposure in the shark’s coastal habitat, where individuals aggregate for feeding and mating. The detection of viral DNA in lesion-free skin samples from some individuals suggests that subclinical infection or viral latency might occur, facilitating maintenance of the virus in the population. Elucidating transmission pathways is essential for gauging the ecological impact of the virus and developing potential management strategies, particularly given the conservation status of broadnose sevengill sharks in certain regions.

This discovery holds significant implications for marine wildlife virology and shark conservation biology. The skin lesions, while not immediately life-threatening, could impair barrier function and sensory inputs, potentially increasing vulnerability to secondary infections and environmental stressors. Considering that sharks play key roles as apex predators in marine ecosystems, understanding factors that affect their health is critical. The identification of this adomavirus expands the catalog of infectious agents of elasmobranchs, encouraging further surveillance and research into viral diseases of cartilaginous fishes.

From an evolutionary standpoint, the findings support the idea that adomaviruses underwent intricate co-divergence with vertebrate hosts, with host-switching events shaping their current diversity. The unique genomic features detected in the shark adomavirus, including novel protein motifs and regulatory elements, highlight the plasticity of viral genomes in adapting to distinct cellular environments. Such genomic innovations may reflect selective pressures in the marine milieu, where virus-host interactions are influenced by factors like temperature fluctuations, salinity, and host immune defenses.

Furthermore, the technological advances showcased in this work demonstrate the power of integrated omics and microscopy approaches for virus discovery in understudied species. The combination of targeted viral genomic enrichment with high-resolution imaging offers a template for future studies aiming to identify emerging pathogens in wildlife. This is particularly relevant in light of increasing anthropogenic impacts on marine biodiversity, where novel pathogens may have unforeseen consequences on vulnerable populations.

Ongoing investigations will aim to experimentally determine the pathogenicity of the virus through challenge studies and to elucidate the immunological responses elicited in infected sharks. These data will be critical for understanding the balance between viral persistence and host defense, which ultimately shapes disease outcomes. Additionally, characterization of viral protein functions at the biochemical level is expected to yield new molecular targets that could inform antiviral strategies should interventions be warranted.

The broader scientific community now has cause to rethink the ecological significance of viral entities in ocean health. Viruses are the most abundant biological agents in seawater and play essential roles in nutrient cycling, microbial population control, and shaping host genome evolution. Identifying viruses that specifically infect higher vertebrates like sharks adds a vital piece to this complex puzzle, with implications for marine disease ecology, evolutionary biology, and conservation medicine.

In sum, this seminal discovery of a novel adomavirus in the broadnose sevengill shark serves as a testament to the hidden diversity of the ocean’s virome and the intricate interplay between viruses and their vertebrate hosts. As virus discovery accelerates through technological innovation, the marine virology field stands poised for transformative insights that will deepen our understanding of life beneath the waves and the emerging challenges facing marine wildlife in a rapidly changing world.

Subject of Research: Novel adomavirus infecting broadnose sevengill shark (Notorynchus cepedianus) skin lesions

Article Title: A novel adomavirus from proliferative skin lesions of a broadnose sevengill shark (Notorynchus cepedianus)

Article References:
Gordon, L.M., Sevigny, J.L., Buck, C.B. et al. A novel adomavirus from proliferative skin lesions of a broadnose sevengill shark (Notorynchus cepedianus). npj Viruses (2026). https://doi.org/10.1038/s44298-026-00210-8

Image Credits: AI Generated

For decades, deep-sea ecosystems were considered limited in biological productivity due to harsh abiotic conditions. However, recent technological advances, including high-throughput sequencing and metagenomic analyses, have unraveled a startling abundance and diversity of uncultivated viral communities residing in these extreme habitats. Viruses, often overlooked in marine ecology, are now recognized as critical agents of genetic exchange, biological diversity, and metabolic modulation. The study conducted by Wang and colleagues compellingly positions these viral entities as pivotal drivers in the degradation and transformation of complex organic substrates buried within the abyssal sediments and deep ocean waters.

Elucidating viral diversity in deep-sea environments entails overcoming logistical and methodological challenges. The team employed a multifaceted sampling strategy, acquiring viral assemblages from hydrothermal vent plumes, abyssal plains, and organic-rich sediment layers. By applying virion enrichment protocols followed by shotgun metagenomic sequencing, the researchers constructed comprehensive viral gene catalogs. Bioinformatic analyses then revealed an extraordinary spectrum of viral taxa, many of which belong to previously unknown lineages. These novel viral genotypes suggest a vibrant viral ecosystem coevolving alongside the deep microbial community, maintaining ecological equilibrium under extremes of temperature, pressure, and nutrient scarcity.

One of the study’s most salient revelations concerns the auxiliary metabolic genes (AMGs) encoded within viral genomes. These AMGs are viral-encoded genes capable of supplementing or even redirecting host metabolic pathways during infection. Intriguingly, many identified AMGs are directly implicated in the breakdown, assimilation, and remodeling of complex organic molecules such as polysaccharides, proteins, and lipids. This viral strategy enables infected microbes to access and metabolize recalcitrant organic matter more efficiently, essentially hijacking the host’s capabilities and expanding their ecological niches in the deep ocean’s oligotrophic milieu.

The functional implications of viral AMGs extend beyond individual host cells. Infected microbial populations, modulated by viral infection cycles, contribute to enhanced carbon turnover and nutrient regeneration at ecosystem scales. This dynamic suggests viruses act as molecular engineers, accelerating the biogeochemical transformation of sedimentary and dissolved organic carbon reservoirs that would otherwise remain stable over extended periods. Such accelerated organic matter cycling might influence deep-ocean carbon sequestration processes, with potential feedbacks on global climate regulation given the ocean’s integral role in carbon storage.

Wang et al. also drew attention to specific viral-host interactions, highlighting viral infections that enhance the degradation pathways for complex carbohydrates like chitin and cellulose, abundant in marine detritus. These findings demonstrate that viruses can redirect host metabolic priorities to leverage complex polymers typically resistant to microbial breakdown. Moreover, the study delineates the potential viral contributions to sulfur and nitrogen cycling by encoding enzymes involved in these elemental cycles, further underscoring their multifaceted roles in sustaining deep-sea microbial communities.

The research challenges the traditional perception of viruses merely as parasitic agents and instead portrays them as nuanced participants in microbial ecology. Viruses can act symbiotically by equipping their hosts with metabolic versatility via horizontal gene transfer of AMGs. Such genetic exchanges may facilitate microbial adaptation to fluctuating environmental conditions, particularly in nutrient-poor habitats. The long-term evolutionary implications suggest a coevolutionary arms race where viruses drive host genome innovation, enabling survival in one of Earth’s most extreme ecosystems.

From a methodological standpoint, the study underscores the power of integrative approaches combining metagenomics, viral enrichment, and metabolic reconstruction. The identification of novel viral lineages and their functional genetic payloads would be unobtainable through culture-based methods alone due to the uncultivability of many deep-sea microbes and their viral predators. These advances mark a paradigm shift in marine microbiology, pivoting towards culture-independent strategies to map and interpret microbial and viral dark matter.

Importantly, the results also resonate with applied sciences, as deep-sea viruses and their encoded enzymes might inspire biotechnological innovations. For instance, thermostable enzymes capable of breaking down complex organic matter under extreme conditions may find applications in industrial bioprocessing or bioremediation. The vast genetic reservoir held within deep-sea viral communities thus emerges as a valuable resource for bioengineering and synthetic biology endeavors.

Looking forward, the study advocates for intensified exploration of viral roles in other deep biosphere contexts, such as sub-seafloor sediments and methane hydrate deposits. There is a growing appreciation that viruses might fundamentally influence energy fluxes and elemental cycling in subsurface biospheres, with implications for understanding life’s boundaries on Earth and potential extraterrestrial habitats.

Wang and colleagues’ findings also prompt reconsideration of oceanic ecosystem models that often omit viral influences. Incorporating viral-mediated processes could refine predictions of carbon fluxes, nutrient dynamics, and ecosystem responses to environmental change. Particularly in the context of anthropogenic impacts such as deep-sea mining and climate change, understanding the viral component may be critical to forecasting ecosystem resilience.

The convergence of viral ecology and deep-sea microbiology elucidated in this work affirms that the deep ocean is not a static repository but a highly dynamic and interconnected biosphere. Viruses and their hosts engage in intimate ecological and evolutionary dialogues, driving metabolic innovation and ecosystem function. Such insights herald a new frontier in environmental microbiology, where viruses are recognized not merely as agents of mortality but as architects of microbial metabolism and geochemical transformation.

In conclusion, this pioneering study significantly advances our grasp of how deep-sea viral communities shape microbial metabolism and organic matter cycling. It highlights the necessity to broaden our ecological paradigms by integrating viral processes in deep-ocean studies. As we delve deeper into the understudied microbial dark matter, the profound contributions of viruses to sustaining life’s biochemical machinery at the ocean’s depths come into stark and exciting relief.

Subject of Research: Deep-sea viral diversity and their influence on host metabolism related to complex organic matter degradation.

Article Title: Deep-sea viral diversity and their role in host metabolism of complex organic matter.

Article References:
Wang, C., Zheng, R. & Sun, C. Deep-sea viral diversity and their role in host metabolism of complex organic matter. Nat Commun 16, 10134 (2025). https://doi.org/10.1038/s41467-025-65207-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41467-025-65207-y