Resumen: High-altitude hydrothermal systems provide laboratories for understanding microbial adaptation to extreme conditions. Here we investigated thermodynamic controls on metabolic transitions in Lirima hydrothermal system (Chile, 4000 meters above sea level) calculating affinities of competing carbon and sulfur reactions across pools (53–75 °C). Through geochemistry, numerical thermodynamic modeling, and stable isotope analysis, we identify Energetic Convergence Nodes where chemical affinities of competing pathways become equal. Calculations reveal hydrogenotrophic and carboxydotrophic methanogenesis converge with sulfate reduction at 63–64 °C. This corresponds with responses: carbon isotopes shift from −17.2‰ to −27.7‰, sulfur isotopes from −20.5‰ to −9.1‰, and carbon-to-sulfur ratios drop from ~10 to <1, indicating transition from carbon to sulfur-dominated metabolism. Microbial activity collapses at this temperature, while metagenomics validates both pathways. Independent analysis confirms convergence at 63–64 °C, establishing Energetic Convergence Nodes as predictive frameworks for metabolic boundaries with applications for early Earth biogeochemistry and extraterrestrial biosignature detection.