Divergent responses of Atlantic coastal and oceanic Synechococcus to iron limitation.

Marine Synechococcus are some of the most diverse and ubiquitous phytoplankton, and iron (Fe) is an essential micronutrient that limits productivity in many parts of the ocean. To investigate how coastal and oceanic Atlantic Synechococcus strains acclimate to Fe availability, we compared the growth, photophysiology, and quantitative proteomics of two Synechococcus strains from different Fe regimes. Synechococcus strain WH8102, from a region in the southern Sargasso Sea that receives substantial dust deposition, showed impaired growth and photophysiology as Fe declined, yet used few acclimation responses. Coastal WH8020, from the dynamic, seasonally variable New England shelf, displayed a multitiered, hierarchical cascade of acclimation responses with different Fe thresholds. The multitiered response included changes in Fe acquisition, storage, and photosynthetic proteins, substitution of flavodoxin for ferredoxin, and modified photophysiology, all while maintaining remarkably stable growth rates over a range of Fe concentrations. Modulation of two distinct ferric uptake regulator (Fur) proteins that coincided with the multitiered proteome response was found, implying the coastal strain has different regulatory threshold responses to low Fe availability. Low nitrogen (N) and phosphorus (P) availability in the open ocean may favor the loss of Fe response genes when Fe availability is consistent over time, whereas these genes are retained in dynamic environments where Fe availability fluctuates and N and P are more abundant.