Unveiling the contributions and regulation of picoeukaryotic phytoplankton in oceanic environments.

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The growth rates and fate of primary producers in the oceans are key factors regulating carbon flux in the biosphere. While resource limitation is known to play a major regulatory role, the specific factors controlling growth and mortality of picophytoplankton (2 microm diameter) are poorly understood. In particular, due to the small size and low sinking quotient of these dominant primary producers, determinants of their fate and transport in the water column are critical to the development of predictive models of carbon flux and its likely perturbation due to climate change. Picophytoplankton is composed of cyanobacteria, (Prochlorococcus and Synechococcus) and small eukaryotes. While the picocyanobacteria have received much attention, little is known about the distribution and dynamics of picophytoeukaryotes, particularly in oceanic settings. The few comparative measurements made to date indicate that the productivity of picophytoeukaryotes can rival that of picocyanobacteria. Hence, knowledge regarding the role of picophytoeukaryotes in open ocean environments is urgently needed. However, this knowledge is only valuable when put in the context of the total picophytoplankton community and, therefore, quantification of picocyanobacteria must be also be included. The overall goal is to develop a method for determining underlying physiological controls of picophytoeukaryotes. A targeted approach will be used, combining flow sorting, cDNA libraries and Expressed Sequence Tags (EST), allowing real-time expressional responses to be identified. This knowledge will highlight key physiological constraints and information on molecular underpinnings of picoeukaryotic population dynamics as well as aiding future efforts to isolate open-ocean picoeukaryotes. This is an important additional benefit from the work since environmental clone library data has demonstrated that such picoeukaryotes are poorly represented in culture collections. Long-term, the environmental genomic approach developed will also provide a high throughput mechanism for profiling expressional responses (mRNA) in the field. In the proposed work, expressional responses will be evaluated in two ocean basins, the equatorial Atlantic and the South Pacific. Broader Impacts. The experimental functional genomics approach taken to elucidate real-time responses to environmental forcing factors is applicable to all eukaryotes, and of value to other microbial ecologists seeking to identify molecular underpinnings of in situ responses. Moreover, sequences derived from uncultured picoeukaryotes, in combination with advances in molecular phylogeny, have led to a dramatic reconsideration of eukaryotic evolution; the sequences generated herein should aid refinement of evolutionary studies and the "Tree of Life". The project will provide training for students at several educational levels, including at-risk minority high school students, undergraduates and two Ph.D. candidates. Information from the project will be incorporated into graduate and undergraduate level courses on Marine Microbial Ecology (taught by the PI). Project results will be available via two internet based formats, one tailored to the general public and one for the scientific community.