Abstract Microorganisms play key roles in nutrient/carbon cycling and greenhouse gas fluxes in High-Arctic soils. However, the influence of soil conditions on tundra microbiomes remains poorly understood. Using high-throughput sequencing, we studied bacterial, fungal and micro-eukaryotic communities in surface soil-biocrust matrices and the underlying mineral soil along two environmental gradients in Svalbard. Both layers harboured highly different communities. Gravimetric water content and pH were the most important factors in explaining the variation in surface communities. Cyanobacteriota were the dominant phototrophs in wetter tundra soils, while Chlorophyta dominated drier ones. In mineral soils, soil texture explained community structure for all three groups, together with carbon-content and carbon: nitrogen-ratio for micro-eukaryotes and bacteria. Chloroflexota and Spirotrichea dominated mineral soils with high clay content, Actinobacteriota positively correlated with sand content. The effects of soil texture and water content were also reflected in co-occurrence networks. Surface communities split into two network clusters linked to either drier or wetter tundra. The mineral soil network was much less coherent, and clustering was related to soil texture. Our findings revealed the important interplay between soil layer, toposequence and soil microbiome in the High-Arctic. As precipitation patterns shift, the impact of topography on tundra microbial communities is likely to intensify.
Abstract Interactions between phytoplankton and bacteria are often classified as beneficial or harmful, yet interactions between the same partners can shift between these states. Here, we examine bacterial pathogenicity toward marine phytoplankton, focusing on how algal-bacterial interactions become harmful. In several Roseobacter-phytoplankton systems, bacteria repurpose metabolites produced or exchanged during the interaction by altering their concentration, signaling function, or metabolic fate, thereby contributing to host damage. We define this context-dependent repurposing as metabolic duality. We contrast this strategy with algicidal activity that does not require a demonstrated prior interaction with the algal partner or an algal-derived substrate. Examples include secreted proteases and small-molecule algicides that bacteria can produce independently of host-derived metabolites. Despite their different mechanisms, these routes often converge on oxidative stress and programmed cell death-like responses in the algal host. Bacterial identity alone does not determine the outcome. Host physiological state, microbial community composition, temperature, nutrients, and the chemical environment can all shift an interaction toward or away from pathogenicity. We therefore propose that phytoplankton pathogenicity is best viewed as an emergent property of the interactions among the bacterium, host, community, and environment. This perspective shifts the central question from which bacteria are pathogens to what makes an interaction pathogenic and provides a framework for understanding when and why algal-bacterial interactions become harmful in the ocean.
Abstract Understanding microbial responses to environmental gradients is important for ecosystem assessment in estuarine environments. Here, we integrated time-series samples and shotgun metagenomics with association network inference, to examine patterns of taxonomic and functional co-variation. We collected and sequenced eDNA from two sediment time series from neighbouring but environmentally contrasting Basque estuaries, and compared the resulting taxonomic and functional association networks, based on Metagenome Assembled Genomes (MAGs) and KEGG Orthologous Groups (KOs), respectively. Networks inferred using 16S rRNA gene metabarcoding data were also included in this comparison. Several topology-based network properties differed strongly between the two sites, although differences were not always consistent when comparing the networks based on MAGs, KOs and metabarcoding. KO-based networks revealed distinct sets of associated Ortholog groups between sites, consistent with known environmental differences. Contrary to our expectations, metabolically related genes rarely formed complete pathway modules in gene networks, likely due to functional redundancy, gene copy number variation, synchronized pathway dynamics, or algorithmic constraints of correlation-based inference. Overall, the results illustrate both the potential and interpretative limitations of metagenome-based time association networks for functional ecosystem assessment, and support their use as hypothesis-generating tools that require further validation across a larger range of sites following environmental gradients.
Abstract The foliar mycobiome plays an important role in tree health and stress resilience, yet factors shaping foliar fungal communities in boreal conifer seedlings remain poorly understood. We investigated how geographic region, seedling origin (planted vs. naturally regenerated), needle nitrogen status, and local stand structure influence needle-associated fungal communities of Scots pine (Pinus sylvestris) seedlings. Needle samples were collected from seedlings without visible disease symptoms across 23 forest stands in southern Finland. Fungal communities were characterized using Illumina ITS2 amplicon sequencing and clustered into operational taxonomical units (OTUs). Alpha diversity was analyzed via linear mixed models (LMMs), and community composition via PERMANOVA and NMDS using Bray–Curtis dissimilarity. Geographic region was the strongest driver of fungal community composition among stands, whereas within stands, community composition was primarily associated with seedling height and birch abundance. Alpha diversity was higher in the Southeastern region compared to the Southwest, and tended to be lower in planted than naturally regenerated seedlings. Moreover, diversity decreased with higher proportions of birch and spruce but increased with higher needle C:N ratios. These findings suggest that early foliar mycobiomes reflect regional conditions and local forest structure, highlighting potential forest management impacts on fungal communities.
Abstract Bark and ambrosia beetles are associated with various filamentous fungi, whose roles range from mutualists to antagonists. Mutualists can provide various benefits and are typically selectively dispersed by these wood-boring beetles. However, it is currently unknown how such bark and ambrosia beetles distinguish their fungal mutualists from other fungal associates, including insect pathogens. We hypothesized that volatile organic compounds might help maintain the mutualisms between beetles and specific fungal partners. Here, we compared the volatiles emitted by 30 fungi known to act as mutualists, commensals or antagonists for their beetle hosts. GC-MS analysis showed that all the fungi had characteristic volatile profiles with beneficial fungi emitting a greater diversity of volatiles than other fungal species. The volatile profiles of beneficial fungi were dominated by aliphatic and aromatic alcohols and esters in terms of both diversity and abundance. Furthermore, we demonstrated that the larvae and adults of two ambrosia beetle species can distinguish their mutualistic fungi from others based on volatile cues. Our results highlight the importance of aliphatic and aromatic alcohols and esters as recognition cues for mutualist fungi by wood-boring beetles. Volatiles likely play key roles in maintaining beetle associations with fungi that are essential to their insect host.
