Response of Phytoplankton Iron Contents to Gradients in Iron Availability in the California Current System

Iron is a critical micronutrient that controls primary production in large swaths of the global ocean. Experiments with laboratory cultures indicate that phytoplankton differ in their ability to compete for and store Fe in response to varying ambient Fe concentrations. However there are very few measurements of the physiological responses of natural phytoplankton populations to gradients in Fe availability. Incubation experiments were conducted off the coast of California and Oregon at two sites characterized by a 10-fold difference in dissolved Fe (0.3 and 3 nM). In each experiment, incubation water was amended with either dissolved Fe (5-10 nM), the model siderophore desferrioxamine B (DFB; 200 nM), or left unamended. Iron contents of three abundant diatom groups (Chaetoceros sp. and large and small pennate diatoms) were monitored by synchrotron X-ray fluorescence, along with dissolved and bulk particulate trace metals and macronutrients over the course of 3 days. Transcriptomic samples were also collected at daily timepoints to assess molecular responses. Added dissolved Fe was drawn down in both experiments, while DFB appeared to solubilize a fraction of ambient particulate Fe in the high-Fe experiment. Iron quotas of unamended diatoms were nearly 10-fold higher under high-Fe conditions. Quotas increased in response to added Fe in both experiments, but the magnitude of changes varied between diatom taxa. DFB additions resulted in reduced Fe quotas in the low-Fe incubation, since cells were presumably forced to use internal Fe stores to support growth. These data demonstrate significant plasticity in the abilities of phytoplankton to take advantage of changing micronutrient inputs. Quota data will be compared to transcript abundance data to ascertain mechanisms of Fe quota maintenance.