Deschampsia antarctica and Colobantus quitensis are the only two vascular plants colonized on the Antarctic continent, which is usually exposed to extreme environments. Endophytic bacteria residing within plant tissues can exhibit diverse adaptations that contribute to their ecological success and potential benefits for their plant hosts. This study aimed to characterize 12 endophytic bacterial strains isolated from these plants, focusing on their ecological adaptations and functional roles like plant growth promotion, antifungal activities, tolerance to salt and low-carbon environments, wide temperature range, and biofilm formation. Using 16S rRNA sequencing, we identified several strains, including novel species like Hafnia and Agreia. Many strains exhibited nitrogen-fixing ability, phosphate solubilization, ammonia, and IAA production, potentially benefiting their hosts. Additionally, halotolerance and carbon oligotrophy were also shown by studied bacteria. While some Antarctic bacteria remain strictly psychrophilic, others demonstrate a remarkable ability to tolerate a wider range of temperatures, suggesting that they have acquired mechanisms to cope with fluctuations in environmental temperature and developed adaptations to survive in intermediate hosts like mammals and/or birds. Such adaptations and high plasticity of metabolism of Antarctic endophytic bacteria provide a foundation for research and development of new promising products or mechanisms for use in agriculture and technology.

Climate change is reshaping Antarctic ecosystems, where the resilience of Deschampsia antarctica and Colobanthus quitensis is mediated by endophytic microbial communities assembled under strong abiotic drivers. This study explores the temperature-dependent biofilm development in two Antarctic endophytic microbial communities (ALS and LS). Multivariate analysis revealed a fundamental trade-off between planktonic expansion and biofilm matrix investment as a function of thermal cues. While moderate warming (15–25 °C) optimized cell viability and turbidity, extreme thermal stress at 37–42 °C in nutrient-rich conditions triggered a significant shift toward a matrix-rich signature, characterized by a synergistic increase in total DNA and cellulose. Crucially, at the thermal extreme of 42 °C, we observed a decoupling of optical density from culturable biomass, where high turbidity did not translate into viable cells, signaling a state of severe environmental stress. These results identify 25 °C as the quantitative threshold for optimal growth, while temperatures of 37–42 °C act as a specific trigger for protective matrix production. Such thermal plasticity suggests that Antarctic endophytes are evolutionarily primed for persistence not only in cold native niches but also during bird-mediated dispersal at endothermic host temperatures.

This study investigates the accumulation dynamics of polyhydroxyalkanoates (PHA) in synthetic microbial communities formed by Antarctic endophytic bacteria isolated from Deschampsia antarctica and Colobanthus quitensis. We evaluated the influence of biofilm architecture (air-liquid-solid [ALS] vs. liquid-solid [LS]) and wide temperature range (4, 15, 25, 37, and 42 °C) on PHA biosynthesis. Quantitative gravimetric analysis and confocal laser scanning microscopy (CLSM) demonstrated that PHA accumulation is significantly enhanced in ALS multi-species biofilms compared to monocultures, peaking at 4 °C. Higher temperatures led to rapid depletion of energy reserves. Oxygen availability and spatial organization within the biofilm matrix are critical drivers of carbon allocation toward PHA storage in cold-adapted microbial communities. Keywords: Polyhydroxyalkanoates, Antarctic endophytes, synthetic microbial communities, biofilm architecture, psychrotolerant bacteria, confocal microscopy.