Bark beetle–fungus associations are essential for nutrition, detoxification, and host colonisation, but their composition and function vary across developmental stages and environmental contexts. Hence, we characterised the fungal communities associated with two pine-feeding bark beetles, Ips sexdentatus (ISX) and Ips acuminatus (IAC), across developmental stages and compared wild-collected and laboratory-bred populations using ITS2 amplicon sequencing. Both beetle species maintained a stable core mycobiome dominated by Kuraishia, Ogataea, Ophiostoma, Graphilbum, and Cyberlindnera. These taxa have been earlier reported to be associated with nutrient provisioning, detoxification of host secondary metabolites, and chemical signalling. Adult beetles showed species-specific community differences, whereas wild-collected beetles, particularly IAC, harboured higher fungal diversity than laboratory populations, indicating a strong environmental effect. Beetles shared more fungal taxa with control wood than with gallery wood, suggesting possible fungal acquisition during feeding and concurrent restructuring of the wood mycobiome during infestation. Monoterpene bioassays with selected yeast symbionts showed differential growth responses to α-pinene, 3-carene, and terpinolene, and their mixture, with the mixture producing stronger inhibition than individual compounds. These yeast symbionts further displayed selective antagonistic activity in vitro against selected filamentous fungi, including entomopathogenic taxa, along with detectable lytic and digestive enzyme activities. Together, our findings highlight a link between community structure, predicted functions, and observed interaction phenotypes, providing a strong basis for future mechanistic studies of beetle–fungus–conifer interactions.

The geomagnetic field is a persistent environmental condition under which biological systems have evolved. Its absence creates hypomagnetic conditions, potentially disrupting this equilibrium. This study examined the effects of hypomagnetic fields on Saccharomyces cerevisiae growth and metabolism across three culture volumes (10, 75, and 250 mL). Growth inhibition was most pronounced in the smallest volume, with up to 42% reduction in 10-mL cultures, decreasing to 24% in 75 mL, and none in 250-mL cultures. Budding analysis showed significant early inhibition in 10-mL cultures at 2, 4, and 6 h, and in 75-mL cultures at 6 h, whereas no significant budding inhibition was detected in 250-mL cultures. These results suggest that smaller volumes amplify the impact of hypomagnetic exposure, whereas larger volumes may buffer it. These shifts indicate that hypomagnetic exposure affects growth kinetics and extracellular volatile profiles without causing visible cell-surface damage detectable by scanning electron microscopy (SEM), although the underlying molecular mechanism remains unresolved. Our findings identify cultivation volume as an important modifier of microbial sensitivity to magnetic-field deprivation and have implications for interpreting microbial responses in magnetically altered environments, including space-biology models, shielded structures, and fermentation-based systems. IMPORTANCE: Microorganisms are commonly studied under controlled temperature, medium, and aeration conditions, but the magnetic field environment is rarely considered as an experimental variable. This study shows that short-term hypomagnetic exposure alters growth kinetics and volatile metabolic output in Saccharomyces cerevisiae . Importantly, the magnitude of the response depended on cultivation volume: growth inhibition was strongest in small static batch cultures and was not statistically detectable in the largest volume tested. These findings identify cultivation volume as an important modifier of microbial sensitivity to magnetic field deprivation, probably through its influence on culture physiology, gas exchange, metabolite accumulation, and growth phase. The study therefore has implications for the design and interpretation of microbial magnetic-field experiments, including work performed in shielded environments, space biology models, and fermentation-related systems.