Marine Bacteria

Zhiyi Sun, (Gigi)'s favorite organism is a tiny bacterium in the ocean, Prochlorococcus. One amazing fact of Prochlorococcus is that it has a shrinking genome. Prochlorococcus is a very small marine bacterium with an unusual pigmentation (chlorophyll b) belonging to photosynthetic picoplankton. Prochlorococcus has proven exceptional from several standpoints. It is the smallest known photosynthetic organism, with tiny size ranging from 0.5 to 0.7 μm in diameter. It is ubiquitous between 40°N and 40°S and dominates in the nutrient poor regions of the oceans, making it the most numerically abundant photosynthetic organism in the ocean and presumably on Earth. Prochlorococcus can harvest light over a very wide range of irradiance, thus being able to grow from the top down to 200-m-deep of the water column. Natural populations of Prochlorococcus can be categorized into two major groups. The high-light-adapted Prochlorococcus inhabit the upper layer of the ocean and the low-light-adapted Prochlorococcus are always found in the deep water. Prochlorococcus has been playing a very important role in the carbon dioxide fixation in the ocean, thus having potential impact on global climate change. Recently the genomes of several strains of Prochlorococcus have been sequenced. Surprisingly it has been found that the genomes of high-light Prochlorococcus were becoming smaller. Genome shrinkage is very unusual for free-living organisms in nature. So this brings up an interesting question: What's the underlying force that drives Prochlorococcus genomes shrinking? To answer this question, Gigi collects genome sequence data, measures evolutionary parameters, develops models for sequence change and tests hypotheses using the model. By studying the reductive evolution of Prochlorococcus, Gigi hopes to gain a better understanding of the diversified evolution of microorganisms.