Niche partitioning of low-light adapted Prochlorococcus subecotypes across oceanographic gradients of the North Pacific Subtropical Front

Abstract

The cyanobacterium Prochlorococcus is the most abundant photosynthetic cell on Earth and contributes to global ocean carbon cycling and food webs. Prochlorococcus is known for its extensive diversity that falls into two groups of ecotypes, the low-light (LL) and high-light (HL) adapted ecotypes. Previous work has shown niche partitioning of the very abundant HL adapted ecotypes and subecotypes across oceanographic gradients including temperature, nutrients, and day length. However, niche partitioning of subecotypes within the LL adapted ecotypes has not been studied as well because they are less abundant and less accessible than surface, HL populations. Through high-throughput, cyanobacterial-specific, sequencing of high-resolution depth profiles from three stations across the North Pacific Subtropical Front, we discovered extensive diversity and strong latitudinal partitioning of subecotype populations within LL ecotypes, in contrast to fairly consistent patterns at the broader level of LL Prochlorococcus ecotypes. These results indicate that both shallow and deep-water processes shape microbial community structure in this region despite strong connectivity in microbial communities across the North Pacific Subtropical Front, supported by a few HL and LL sequences found ubiquitously across stations. This study expands understanding of the diversity and complexity of Prochlorococcus community structure over oceanographic gradients.