Fig. 1: Oceanographic transect with abiotic water column data and conceptual mechanistic understanding of the microbial biogeography.

a The GO-SHIP P15S transect along the 170^o W meridian in the South Pacific Ocean. Gray arrows indicate the South Equatorial Current (SEC) within the upper ocean, and include a westward returning branch between 20° and 10° S (denoted by **SEC^{69, 70}). The oceanographic provinces⁵⁴ are color coded: the Southern Ocean (SO; blue), the Subtropical Frontal Zone (STF; orange), the South Pacific Subtropical Gyre Province (SPSG; green), the Pacific Equatorial Divergence Province (PED; red), the Polar Front (PF) is in addition highlighted by a black arrow. b Profiles for SST (red; bottom x-axis) and salinity (blue; top x-axis). c Conceptual mechanistic understanding of relative changes in the functional prokaryotic and microbial-eukaryotic biogeography. Blue rods represent heterotrophic archaea and bacteria, which recycle in winter the organic matter produced in the summer and autumn months in the high nutrient low chlorophyll (HNLC) region of the SO. High primary productivity (PP) driven by phytoplankton rich in chlorophyll (green discs) is expected in the STF due to turbulence and mixing (curved arrows). The oligotrophic SPSG is characterized by low PP and nutrient co-limitation, as well as by higher abundances of photosynthetic prokaryotes (yellow circles). The westward returning branch of the SEC is indicated with the gray arrow in c, d and can be the source of the increase in N:P ratios at 100 m depth. Equatorial upwelling and mixing in the PED results in an increase of the N:P ratio at the surface and, thus, in increased PP and chlorophyll concentrations. d Latitudinal plot of N:P ratios from the surface to 350 m depth. The thick white line represents the MLD; 16S rRNA sampling stations are shown in large gray circles; shotgun metagenome samples are represented by yellow stars; and CTD stations appear as small gray dots.