Collaborative Research: Assessing phosphorus-status in Prochlorococcus through kinetics and molecular approaches in chemostats and natural populations

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Global climate change, likely caused by human activities, is having an impact on the oceans. In the North Pacific Subtropical Gyre, a domain shift to an ecosystem dominated by prokaryotic organisms appears to be occurring due in part to climate-related increases in N2 fixation and subsequent phosphorus (P)-limitation. In this region, populations of the unicellular marine cyanobacterium Prochlorococcus have been increasing over the last several decades. It is not clear whether Prochlorococcus populations are P-limited or whether they have a competitive advantage under conditions of P limitation. Recently it has been reported that the HLII genotype of Prochlorococcus dominates in these P-limited waters, and HLI Prochlorococcus are present in very low concentrations. Studies on P utilization and P-stress response of axenic HLI and HLII Prochlorococcus isolates revealed significant differences in their ability to utilize organic P sources and in alkaline phosphatase activity, whereas a genomic comparison revealed few differences in their complement of P-related genes. This raises the question of what differentiates the P physiology of these two HL genotypes, and whether natural populations of Prochlorococcus experience P limitation and utilize organic P. The investigators will carry out a study employing physiological and genetic approaches on cultured isolates as well as natural populations. These will include P-limited uptake kinetic studies on axenic HL strains of Prochlorococcus to determine their relative competitive ability and examine expression of key P-related genes in P-limited chemostats. They will also carry out a field study to determine if, and which portions of, the Prochlorococcus populations are P-limited through examination of the genotype-specific gene expression and community response in natural populations and P-enrichment bottle experiments. The proposed work is designed to address one aspect of a larger scientific issue: what environmental factors regulate Prochlorococcus, the most abundant phytoplankton species in the world's oligotrophic oceans? The investigators will examine physiological and molecular responses of Prochlorococcus to P-limitation in cultured and natural populations. This combination of traditional physiological methods with state-of-the-art molecular techniques will allow a better understanding of the ecophysiology of Prochlorococcus and the genotype-specific physiological status of natural Prochlorococcus populations. Ultimately, the intent is to relate community structure to physiological function and possibly to changes in the marine P cycle. The project will support the work of two female investigators in the early stages of their academic careers as well as three graduate students, and multiple undergraduates at both institutions. The project will provide valuable research and field experience to undergraduates at USM, which is primarily an undergraduate institution serving mainly non-traditional students in the Southern Maine region. The cross-fertilization of ideas and techniques between USM and UW will have a direct influence on the content of course offerings at both institutions, through continued development of new, research-based laboratory assignments.