ABSTRACT

The power of a model system for advancing our understanding of the natural world has been proven repeatedly in diverse sub-disciplines of science. This approach is equally valuable for the study of marine microbial ecology, either through the sustained study of a particular ecosystem, or through the study of a particular organism at all scales of organization from the genome to the ecosystem level. In this project the investigators will do the latter through a diverse set of laboratory and field studies designed to advance the understanding of Prochlorococcus, the numerically dominant phytoplankter in the world oceans. The objective is to understand what regulates the distribution and abundance of this important primary producer. Prochlorococcus has a number of features that make it a useful model organism for understanding the forces that shape marine microbial systems, and for generating hypotheses that will advance the field of Microbial Oceanography. It is the smallest and most abundant photosynthetic cell in the oceans, it can be isolated into culture, and it can be easily enumerated and studied in situ. Furthermore, it has the smallest genome of any known photosynthetic cell -- the minimal phototroph to date. Prochlorococcus is really a collection of "ecotypes", i.e., closely related but physiologically distinct populations that co-exist with different distributions along the light, temperature, nutrient, and predator (including viruses) gradients that
shape their habitat. These distributions are determined by the relative fitness of the cells, i.e. the balance of growth rates and death rates along these gradients. The broad challenge is to understand the forces that have shaped this microdiversity over evolutionary time, and that guide the self-organization of these populations under different selective regimes. This is a multi-dimensional project designed to understand the "bottom up" (growth limitation by light, temperature, oxygen, and nutrients) and "top down" (mortality from viruses and grazing) influences on the population growth of different Prochlorococcus ecotypes through both laboratory and field studies. The project will involve high-throughput studies of the growth of ecotypes along gradients of environmental variables in the laboratory, as well as studies of the distribution of ecotypes along spatial and temporal gradients in the field (at the Hawaii and Bermuda Time Series Stations, along a longitudinal Atlantic transect, and in Oxygen Minimum Zones in the Arabian Sea and Peruvian Upwelling) using Q-PCR to assess their relative abundance. The investigators will also study the life cycle of viruses that infect Prochlorococcus and cross infect between ecotypes, and the growth and mortality rates of specific ecotypes in field samples due to grazing pressure. Another set of analyses will measure the full diversity of co-occurring Prochlorococcus in selected field samples, and work toward understanding at what level genetic diversity corresponds to ecologically meaningful diversity. Finally, the study will develop statistical methods for the characterization and analysis of Prochlorococcus ecotype "fitness spaces" established in the laboratory, and compare them with the distribution of ecotypes along environmental gradients in the field. This will allow a rigorously analysis of the degree to which different selective pressures shape the relative abundance of the different Prochlorococcus ecotypes in the oceans, and how they change over time and space.

Broader Impacts.

This project will support (in whole or in part) the research of 5 graduate students and post-docs, three of them women, as well as undergraduate researchers. Through a formal collaboration with the Museum of Science we will also develop exhibits that focus on marine microbiology. The investigators are also participating in MIT's Knowledge Update Program, which produces multi-media educational briefings designed for both professional and student audiences to provide a focused, quick immersion into emerging fields. Finally, the participants will be working with an anthropologist at MIT who is working on a book, tentatively entitled Mapping a Sea of Genes, that will offer an anthropological look at exploring the ocean in the age of genomics. The author has been interacting with the research group to get a documentary sense of work on ocean microbes and their genes. This book will transform the way scientists, policy makers and lay public understand the sea.

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