Iron and Prochlorococcus

Anne W. Thompson, Ph.D., 2009
Sallie Chisholm and Mak Saito, Advisors

Iron and primary productivity in the oceans are linked through photosynthesis. At the global scale iron induces phytoplankton blooms during addition experiments. At the atomic scale, iron is involved in photosystem I, the most efficient light-harvesting complex in nature. Yet, we know little of how iron influences microbial diversity and distribution. Here, we assess the influence of iron on the ecology of Prochlorococcus. With its minimal genome and ubiquity in the global ocean, Prochlorococcus represents a model system for studying the link between iron and primary productivity. To this end, we tested the iron physiology of two closely-related ecotypes. We determined that MIT9313 is capable of surviving at iron concentrations that completely inhibit MED4. The molecular basis for the iron physiologies of MED4 and MIT9313 was assessed through the whole-genome transcription response to iron. Iron-responsive genes fell into two categories: those that are shared by all (Prochlorococcus core genome) and those that are not (non-core genome). We then tested the iron physiology of natural populations and found that Synechococcus is iron-stressed in samples where Prochlorococcus is not. We demonstrate that iron-metabolism influences the ecology of Prochlorococcus both by contributing to its diversity and distinguishing it from other marine cyanobacteria.