Humborg, C. | Search bioRxiv

bioRxiv2026

Infauna selectively enhance DNA virus diversity and activity in marine sediments

Viruses regulate microbial mortality and biogeochemical cycling in marine sediments; however, the ecological drivers of sediment viral communities remain unclear. Infauna, including sediment-dwelling meiofauna and macrofauna, are major ecosystem engineers that reshape sediment structures and microbial processes, but their influence on viruses is unknown. We combined infaunal gradient incubations with metagenomic and metatranscriptomic analyses to assess viral DNA and RNA responses. DNA viruses showed increased abundance (3-fold), diversity, richness, and transcriptional activity under higher infauna abundance conditions, whereas RNA viruses remained unaffected, revealing striking selectivity. This selectivity reflects an infauna-dependent component mediated by bacterial activity that cannot be explained by host abundance alone. Infection profiling revealed increased transcription of viral replication and structural genes, and lytic viruses under high infauna conditions. These findings establish infauna as a previously overlooked regulator of DNA virus dynamics, integrating viral ecology into faunal-microbial frameworks in benthic ecosystems and suggesting potential influences on geochemical cycles. TeaserInfauna selectively shape viral communities in marine sediments, revealing an overlooked effect on DNA viruses.

microbiology↗

bioRxiv2025

Long-term euxinia hinders microbial ammonium removal in brackish coastal waters

Anthropogenic activities are key drivers of eutrophication and deoxygenation in coastal marine ecosystems. This stimulates the anaerobic degradation of organic matter and the release of reduced products, such as ammonium, methane, and hydrogen sulfide, which may, in turn, exacerbate eutrophication and deoxygenation. In this study, using a combination of chemical and microbial analyses, we assess the nitrogen dynamics in the water column of a eutrophic coastal system (Stockholm Archipelago) at three sites with contrasting redox conditions (oxic to long-term euxinic). At the oxic site, counter gradients of ammonium and oxygen in the water column, low nitrate {delta}15N values in bottom waters, and the 16S rRNA gene-based presence of nitrifiers indicate nitrification near the sediment-water interface. At the seasonally and long-term euxinic sites, nitrification, as inferred from the water column oxygen and nutrient profiles and the relative abundance of nitrifiers, primarily occurred near the oxycline. At these two sites, nitrate was removed below the oxycline through denitrification linked to sulfide oxidation by Sulfurimonas. Nitrous oxide emissions from surface waters in the archipelago reached up to 40 {micro}mol m-2 d-1 and were not directly related to water column redox conditions, indicating that multiple factors control coastal emissions of this greenhouse gas to the atmosphere. The relative abundance of 16S rRNA genes and of N-cycle genes in metagenomes was highest at the seasonally euxinic site. Importantly, nitrifiers were significantly less abundant at the long-term euxinic site. Our results highlight that prolonged euxinia promotes recycling of ammonium over its removal, likely due to sulfide inhibition of nitrification, which sustains eutrophication and deoxygenation of coastal systems.

microbiology↗