Endophyte-Assisted Phytoremediation by the Halophyte Halocnemum strobilaceum Coping with Extreme Salinity and Hydrocarbon Pollution

Hypersaline waters contaminated with crude oil represent a major obstacle for phytoremediation, as few plant species tolerate both high salinity and hydrocarbon toxicity. In this study, the halophyte Halocnemum strobilaceum (Pallas) M. Bieb. was grown hydroponically in hypersaline solutions (50 and 80 g L−1 NaCl) containing crude oil (600 mg L−1). The plant was inoculated with endophytic bacteria isolated in a previous step from its root and selected for salt tolerance and hydrocarbon-degrading potential. The plant behaviour was assessed through growth and photosynthetic performance, while the degradation of hydrocarbons (C < 12 and C > 12) was monitored over time. At 50 g L−1 NaCl, crude oil reduced the plant growth by 60%, but inoculation with endophytic bacteria mitigated this decline, demonstrating their positive influence under combined salt and hydrocarbon stress. At 80 g L−1 NaCl, neither plant biomass nor chlorophyll fluorescence was significantly affected by crude oil, with or without bacterial inoculation, consistent with the strong intrinsic salt tolerance of H. strobilaceum, which likely buffered additional stress inputs. Metagenomic analyses revealed distinct root-associated microbial communities under different treatments, suggesting synergistic plant–microbe interactions that enhanced photosynthetic efficiency and metabolic stability. The presence of endophytes accelerated the degradation of aliphatic hydrocarbons (C10–C40) at both salinity levels. These findings highlight the potential of endophytic bacteria to enhance resilience in H. strobilaceum and its phytoremediation capacity, offering a promising nature-based approach for the sustainable treatment of highly saline, crude oil-contaminated industrial waters.