Deep-sea cold seeps are carbon-rich but nitrogen-limited environments that paradoxically support high microbial biomass. The nitrogen source sustaining this productivity remains debated due to a lack of direct in situevidence. Here, we developed a deep-sea surface-enhanced Raman scattering (D-SERS) sensor based on a flexible Au/Ag@CFC substrate, capable of withstanding extreme conditions (≥350 °C, ≥20 MPa) with a detection limit below 10⁻⁷ M. Deployed via the Jiaolongmanned submersible, the sensor enabled multi-point in situdetection at two cold seep sites in the South China Sea.

For the first time, we detected cyanide (−CN) in situwithin active cold seep microbial communities, at concentrations exceeding 5.7 μM. Spatial gradient measurements showed that the −CN signal disappeared just 15 cm above the community, indicating rapid microbial consumption. This suggests cyanide functions as a "metabolic currency," serving as an alternative nitrogen source that bypasses the energy-intensive step of N₂ fixation—a so-called "energy-saving nitrogen fixation" pathway. Metagenomic analysis further revealed widespread hydrogen cyanide synthase gene clusters (hcnABC) across diverse bacterial phyla in global cold seep sediments, indicating that cyanide is not merely a geological byproduct but an actively cycled microbial metabolite.

These findings challenge the traditional view of cyanide as solely an environmental toxin. Instead, cyanide emerges as a key molecular hub coupling carbon, nitrogen, and sulfur cycles in deep-sea cold seep ecosystems, offering new insights into how life thrives under extreme energy limitation. The D-SERS sensor technology provides a transformative tool for future in situbiogeochemical studies in the deep ocean.