Metagenomic Potential of Microbial Assemblages in the Surface Waters of the Central Pacific Ocean Tracks Variability in Oceanic Habitat

Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local environmental conditions. To understand the relationship between microbial assemblage composition, functional capability, and habitat, a random genome shotgun sequencing (metagenomic) survey was conducted with surface-water microbial assemblages (0.2-5-μm size fraction) collected at seven locations along a meridional transect from the northern edge of the South Pacific subtropical gyre to the southern edge of the North Pacific subtropical gyre (16°S-13.5°N). A total of 1.1 million unique sequence reads were obtained, of which ∼45% could be annotated to metabolic category. Microbial assemblages in equatorial divergence and countercurrent habitats were distinct phylogenetically from those in gyre waters. Ecotypes of dominant Cyanobacteria (Prochlorococcus and Synechococcus) had distinct distributions congruent with their physiological characteristics in cultivation. The metagenomic distribution of genes among metabolic pathways was very similar at all stations despite phylogenetic differences, but was unrelated to physicochemical habitat, suggesting that dominant microorganisms have a core suite of genes necessary for life in the open ocean. Among metabolic genes that varied across the transect, several patterns were observed. For example, phosphate ( $\text{PO}_{4}^{3-}$ ) stress response genes were more common in gyre waters than at the equator. The variability in frequency of several metabolic pathways (e.g., chlorophyll biosynthesis, $\text{PO}_{4}^{3-}$ metabolism, and transcription initiation bacterial sigma factors) was related to physicochemical conditions, most of which were related to taxonomic differences among habitats. Microbial communities in the central Pacific Ocean are phylogenetically distinct to the oceanic provinces which they inhabit.