Microbial Community Metagenomics in the Eastern Tropical North Pacific Oxygen Minimum Zone Reveals Functional Differences Along Biogeochemical Gradients

Oxygen Minimum Zones (OMZs) are pivotal ocean regions defined by low dissolved oxygen concentrations [DO]. However, biogeochemical variations within OMZs-both laterally and with depth-may select for fundamentally different microbial metabolisms important for ocean biogeochemistry. We used metagenome sequencing to investigate potential differences by specifically targeting biogeochemically-important features-including the primary and secondary nitrite maxima (PNM and SNM), the secondary chlorophyll maximum (SCM), and the upper edge of the OMZ (defined by 20 μM [DO]). Read-based analysis identified variations in 5389 functional genes but high similarity among SCM and SNM metagenomes at multiple stations. 690 genes showed significant differences between different features and included key functional genes involved in photosynthesis elevated in the PNM, while carbon fixation, anaerobic nitrogen cycling, and organic sulphur cycling genes increased in the SCM and SNM. Metagenome assembled genomes from a distinct upper OMZ edge sample included multiple Flavobacteriaceae and Rhodobacteraceae, with annotated functions contributing to metabolism of carbohydrates and amino acids, as well as aerobic anoxygenic photosynthesis (in Rhodobacteraceae). Our results identify functional genes and metabolic pathways that are enriched in unique SCM and SNM features, while also demonstrating sharp shifts in functional capacity in the overlying upper water column, within the ocean's largest OMZ.