Microbial deamination of peptides and proteins as a source of persistent marine dissolved organic matter

Refractory dissolved organic nitrogen is a major component of the ocean's nitrogen pool, yet the mechanisms producing and preserving it remain poorly understood. Here, we show that deaminated peptides (DAPs), derived from microbial processing of proteinaceous material, are ubiquitous across oxygenated marine environments—from productive coastal waters to the oligotrophic ocean—and persist from surface to bathypelagic depths. Using ultrahigh‐resolution mass spectrometry and a curated peptide database, we detected candidate DAP formulas with substantial contributions from hydrophobic amino‐acid residues and recurrent compositions containing diaminopimelic acid ( Dapa ), consistent with a contribution of bacterial cell‐wall material. Laboratory incubations demonstrate substrate control on DAP composition: simple‐substrate treatments yield narrow, source‐specific amino acid profiles, whereas complex substrates produce broader assemblages resembling field observations. Together, widespread DAP occurrence and substrate‐dependent production in microbial incubations support peptide deamination as a potential microbial nitrogen pump pathway, linking labile surface nitrogen to persistent storage in the ocean interior.