Dark ocean archaeal and bacterial chemoautotrophs drive vitamin B1 production in oxygen minimum zones.

Vitamin B1 (thiamine) is essential for all cells, yet many marine microbes cannot synthesize B1 de novo. Dissolved thiamine and its related chemical congeners (TRCs) concentrations are not well characterized beyond the surface ocean, where they are typically low. Here, we observed unexpected enrichment of TRCs in regions of low dissolved oxygen levels (9.4 < O₂ < 12.5 μmol kg^-1) across vertical profiles in Monterey Bay and Pacific waters 145 km offshore (Station 67-70). TRC concentrations ranged from fM to pM, with 1.1 to 4.5 fold increases from near-surface waters to the mesopelagic Oxygen Minimum Zone (OMZ). Notably, at 67-70, dissolved B1 increased 3.5-fold within the mesopelagic OMZ. Paired metagenomic analysis suggests that chemoautotrophic ammonia-oxidizing Archaea (AOA) and Thioglobaceae, alongside nitrite-oxidizing Nitrospina, are important B1 producers in OMZs. Metagenome-assembled genomes indicate that Nitrospina may alternate between B1 biosynthesis and energy-preserving salvage pathways in synergy with co-occurring AOA. Re-analysis of metatranscriptomic reads from a previous study established Thioglobaceae can be dominant expressors of key de novo B1 biosynthesis genes in Monterey Bay. These findings suggest that deep ocean chemoautotrophs are B1 prototrophs in OMZs, analogous to photoautotrophs in the epipelagic ocean, and provide the foundation for B1 trafficking.