# SAR11 ecotypes across ocean basins change with depth due to changes in light and oxygen

## TL;DR

SAR11 bacteria change with depth in the ocean due to factors like light and oxygen, revealing distinct ecotypes in different zones.

## Contribution

A novel method using metagenomic read placement on a phylogenetic tree reveals SAR11 ecotype distribution across ocean basins.

## Key findings

- Specific SAR11 ecotypes correlate with eukaryotic phytoplankton or picocyanobacteria in the euphotic zone.
- Group IIb.x dominates in the lower euphotic and mesopelagic zones, with new deep subecotypes identified.
- Ecotype shifts correspond to changes in light and are linked to the absence of proteorhodopsin in deep ecotypes.

## Abstract

SAR11 bacteria are ubiquitous and abundant heterotrophs that are important mediators of marine biogeochemical cycles. Within the SAR11 clade smaller ecotypes inhabit different ecological niches. Using metagenomic read placement onto a phylogenetic tree of RNA polymerase (rpoB), we were able to determine the distribution of different ecotypes both geographically and by depth. Our method avoids biases from the absence of quality sequenced genomes for deep SAR11 ecotypes. Depth profiles that range from the surface to the bathypelagic were analyzed at 30 stations in six ocean basins. In the euphotic zone, changes in the dominant primary producer from eukaryotic algae to cyanobacteria, did not cause the abundance of SAR11 to shift between stations. However, specific SAR11 ecotypes did correlate with eukaryotic phytoplankton (1a.3 and 1a.4) or picocyanobacteria (1b.2, 1b.4, and IIaB). In the lower euphotic and mesopelagic zones, group IIb.x was overwhelmingly the dominant species but group 1c was also present, and we found several new deep subecotypes of 1b. The shift between the surface SAR11 community, dominated by 1a and surface 1b subecotypes, and the mesopelagic ecotype groups, corresponded to the maximum decrease in the light-dependent proteorhodopsin/rpoB ratio, indicating that many deep ecotypes did not possess proteorhodopsin. This ecotype switch repeatedly corresponded to the maximum in Low Light I Prochlorococcus, leading to the hypothesis that changes in light motivates the ecotype switch. Environmentally abiotic factors like light and temperature appear to be determining factors in the SAR11 ecotype distribution throughout the global oceans.