Iron Storage

Collaborative Research: Iron storage in diatoms and N2-fixing cyanobacteria: mechanisms, regulation and biogeochemical significance

Studies on the Fe physiology of phytoplankton have primarily focused on the induction of high affinity uptake pathways or the rearrangement of photosynthetic machinery to decrease the cellular demand, but very little attention has been given to the mechanisms of intracellular Fe storage. Proper handling and storage of Fe on timescales of generations can ensure adequate Fe nutrition in episodic environments and short term storage of Fe is essential to “buffer” the intracellular redox-labile Fe concentration and prevent Fenton production of reactive oxygen species. Our understanding of Fe storage lags far behind what is known for C, N and P, despite that sufficient Fe can be stored for at least 4 cell divisions, much more than in the cases of P, N and (especially) C. Since the biogeochemical cycles of Fe and C, N and P are linked via the Fe quotas of phytoplankton, it is critical that we understand the environmental and physiological controls on this parameter. Fe can be stored in proteins such as those of the ferritin superfamily or sequestered into intracellular vacuoles. Some marine diatoms, such as Phaeodactylum tricornutum have ferritin genes, while ferritin has not been detected bioinformatically or by evolutionary PCR methods in other diatoms such as Thalassiosira pseudonana. We have measured the Fe-dependent regulation of transcript and protein abundance of NRAMP, a protein likely involved in vacuolar Fe metabolism, which is an alternative method of Fe storage found in Arabidopsis thaliana and yeast. We propose the regulation and biogeochemical significance of ferritin and vacuole-mediated Fe storage may differ, reflecting the obvious difference in mechanism. The filamentous N2 fixing cyanobacterium, Trichodesmium erythraeum, possesses three ferritin/ bacterioferritin genes, suggesting specialization of these proteins. Both Fe storage and Fe buffering are likely critical functions in Trichodesmium, yet nothing is known of either aspect of Fe homeostasis. This proposal focuses on understanding the intracellular cycling and storage of Fe in marine diatoms and N2 fixing cyanobacteria and the relationship between Fe storage and cell quota. Specific objectives are to: 1) Determine the factors that regulate ferritin transcription, apo-protein synthesis and ferritin iron content in P. tricornutum lab cultures. We hypothesize that ferritins serve as Fe storage reservoirs over long, generational time scales. Because they are targeted to chloroplasts, we also hypothesize that ferritins may buffer Fe to prevent oxidative stress during degradation and synthesis of photosynthetic components. 2) Determine the role of storage vacuoles and NRAMP in Fe storage and mobilization in lab cultures of T. pseudonana. We hypothesize that vacuoles store Fe and NRAMP helps mobilize Fe in T. pseudonana, T. oceanica, and possibly other centric diatoms. 3) Evaluate the relationships between Fe storage proteins and cellular quota in culture and field populations of Trichodesmium. We hypothesize one or more of these proteins serve as an Fe reservoir over long, generational, time scales – in which case they may be an indicator of nutritional Fe status. We also hypothesize that one or more of these proteins are co-localized in cells specifically responsible for N2 fixation in Trichodesmium colonies as a mechanism to buffer the Fe released through the diel degradation of the Fe-rich nitrogenase proteins. We will address the above objectives using genetic, immunological, and synchrotron-based approaches applied to laboratory cultures of P. tricornutum, T. pseudonana,and Trichodesmium. We will also analyze Trichodesmium trichomes collected from the Sargasso Sea in order to determine the biogeochemical importance of (bacterio)ferritins has a storage mechanism in this group.