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Deep-sea sediments are vast microbial habitats that cover almost two-thirds of the planet’s surface. Particularly, sediments in hydrothermal vents and oxygen minimum zones (OMZ) represent emblematic, and poorly understood extreme niches that pose strong selective pressures to life, representing untapped reservoirs of a unique microbial diversity. Nonetheless, the mycobiota in these systems remains relatively unknown. Here, we explored fungal diversity and community structure in deep-sea sediments collected from low- and high-temperature vent systems (Pescadero Basin, Pescadero Transform Fault, and Alarcón Rise) and an OMZ (Alfonso Basin) in the southern Gulf of California, by using high-throughput Illumina sequencing of the ITS1 region. We identified 102 fungal amplicon sequence variants (ASVs), principally affiliated to the Ascomycota and Basidiomycota. Our results also evidenced a high proportion of widely distributed, uncultured phylotypes (e.g., unknown clades closely related to the Basidiomycota such as Fungi sp. 18 and 19 formerly reported from deep-sea), evidencing the copious occurrence of novel fungal lineages with limited distribution to deep-sea sediments. Overall, the largest number of ASVs was recovered from high-temperature vent systems, corroborating these as diversity hotspots. In addition, clustering patterns across samples and the FUNGuild analysis revealed characteristic assemblages in each ecosystem, which could be linked to site-specific processes. This study provides baseline data paving the way for a better understanding of deep-sea fungal diversity, evidencing its potential importance in ecosystem functioning.

Microbial interactions play a key role in ecosystem functioning, with nutrient availability as an important determinant. Although phylogenetically distant bacteria and fungi commonly co-occur in nature, information on their cross-kingdom interactions under unstable, extreme environments remains poor. Hence, the aims of this work were to evaluate potential in vitro interactions among fungi and bacteria isolated from a phosphorous oligotrophic aquatic system in the Cuatro Ciénegas Basin, Mexico, and to test the nutrients-based shifts. We assessed growth changes in bacteria (Aeromonas and Vibrio) and fungi (Coprinellus micaceus, Cladosporium sp., and Aspergillus niger) on co-cultures in relation to monocultures under diverse nutrient scenarios on Petri dishes. Interactions were explored using a network analysis, and a metabolome profiling for specific taxa. We identified nutrient-dependent patterns, as beneficial interactions dominated in low-nutrients media and antagonistic interactions dominated in rich media. This suggests that cross-kingdom synergistic interactions might favor microbial colonization and growth under low nutrient conditions, representing an adaptive trait to oligotrophic environments. Moreover, our findings agree with the stress-gradient hypothesis, since microbial interactions shifted from competition to cooperation as environmental stress (expressed as low nutrients) increased. At a functional level consistent differences were detected in the production of secondary metabolites, agreeing with plate bioassays. Our results based on culture experiments, provides evidence to understand the complexity of microbial dynamics and survival in phosphorous-depleted environments.