Project: Population genomics and ecotypic divergence in the most dominant lineage of marine bacteria

In the upper water column of Earth's coastal and open oceans, roughly one million microscopic, single-celled bacteria inhabit each milliliter of seawater, where they play important roles in driving nutrient cycles and other processes that are vital to the habitability of these systems to other marine life. While some marine bacteria are similar to plants in that they use energy from the sun to transform the greenhouse gas carbon dioxide into living material and produce oxygen as a byproduct, other marine bacteria known as chemoheterotrophs are similar to humans and other animals in that they consume organic matter and oxygen, producing carbon dioxide as a consequence of their growth. Although they are limited in size and shape when observed under a microscope, genetic techniques such as DNA sequencing have revealed tremendous functional (i.e. what they are doing) and phylogenetic (i.e. how they are related) biodiversity in natural communities of marine bacteria. Despite this high genetic diversity, a single group of phylogenetically related chemoheterotrophic bacteria known as SAR11 can sometimes make up over 50% of the microscopic cells inhabiting seawater systems around the globe; it is considered one of the most abundant organisms on Earth and thus an important aspect of ocean ecology. While it is known that the SAR11 group consists of many distinct "types" that differ in abundance with location, depth and time, we know little about what genetically encoded features distinguish the different types, or how genetic characteristics are gained and lost within the group. The goal of this study is to use a genomics approach to understand the evolutionary processes that shape one of the most abundant groups of organisms on our planet, and to improve our theoretical understanding of the evolutionary processes that shape natural microbial biodiversity in general. This project will provide advanced, cross-disciplinary professional training for a postdoctoral scientist and a graduate student, and will increase the participation of underrepresented groups in scientific research by mentoring undergraduate students of native Hawaiian or Pacific Island ancestry in hands-on research and training. Results will be incorporated into a new university course offering on comparative genomics and microbial evolution. A culture collection of marine microorganisms will also be expanded and maintained, providing a valuable resource for other marine scientists.

This project will take advantage of recent advances in DNA sequencing technology and a high throughput extinction culturing approach in order to investigate the evolutionary characteristics of genomes from sympatric populations of the globally important SAR11 marine bacterial lineage. The major objectives of this project are to understand the forces that shape genomic diversity in large bacterial populations such as SAR11, and to determine the nature by which this diversity is reflected in functional differences between populations, as inferred from genomics. SAR11 cells will be isolated from similar ecosystems in the tropical North and South Pacific, as well as the coastal ocean of the subpolar North Pacific, in order to investigate the effect of geographic distance versus habitat similarity on the population genetics of free-living, planktonic marine bacteria. By opening a unique genomic window that encompasses SAR11 lineages of varying degrees of genetic divergence simultaneously, this study will facilitate the investigation of evolutionary dynamics that spans a continuum between macro- and microevolutionary processes. Quantitative information regarding the mechanisms by which genetic diversity is generated, propagated, and removed from native SAR11 populations will also help efforts to model the fate of SAR11 and other large marine bacterioplankton populations in the face of predicted climate-induced changes to the global ocean.