In situ deep-sea surface-enhanced Raman scattering sensor reveals cyanide as a key factor in cold seep nitrogen cycling.

Deep-sea hydrothermal vents and cold seeps, typical ecosystems characterized by abundant carbon but limited nitrogen, support thriving microbial activities. The source of nitrogen has long been a key focus in scientific research. Although cultivation experiments have confirmed the presence of diverse nitrogen-fixing microorganisms, direct in situ evidence has been lacking. Therefore, a deep-sea in situ surface-enhanced Raman scattering sensor has been developed. It can adapt to extreme environment (≥350°C and ≥2000-m depth and LOD < 10-7 M) and has excellent deep-sea application potential. Crucially, the sensor achieved the in situ detection of cyanide (-CN) (>5.7 μM) at cold seep, providing direct evidence for an energy-efficient nitrogen fixation pathway in the microbial communities. Concurrently, the gradient detection results indicated that CN was consumed as a microbial "circulating currency." This finding offers critical in situ evidence for understanding the coupling mechanisms of cold seep carbon, nitrogen, and sulfur cycles, marking a substantial breakthrough in deep-sea sensing technology.