Mechanistic insights into diatom iron uptake: In vitro siderophore characterization and in silico modelling and docking of ISIPs in iron-limiting conditions.

The low iron concentrations in the Indian sector of the Southern Ocean (ISSO) pose significant challenges to microbial life, particularly phytoplankton, and thereby drive the development of specialized adaptive strategies among microorganisms. Bacterial siderophores increase iron acquisition by chelating Fe3+ and enabling its uptake through specific receptors. Although phytoplanktons, especially diatoms, do not synthesize siderophores, they possess Iron Starvation Induced Proteins (ISIPs) capable of recognizing and internalizing bacterial siderophore-Fe3+ complexes, providing a crucial survival advantage in iron-deficient waters. Despite this functional significance, the tertiary structures of ISIPs, their binding affinities for exogenous siderophore-Fe3+ complexes, including details of the electron-donating atoms and metal-ligand bond lengths involved in Fe3+ chelation, remain poorly understood. Here, we characterized the siderophores produced by Stutzerimonas stutzeri strain RBSOE 11 using LC-qTOF-MS viz desferrioxamine D2, desferrioxamine Et1, and desferrioxamine E. Further, we employed an in-silico framework to model ISIP receptor structures in the two most common Southern Ocean diatom species, Thalassiosira oceanica and Chaetoceros tenuissimus, and studied their interaction with desferrioxamines-Fe complex. The in silico docking between the bacterial siderophore receptor (FoxA) and diatom (ISIP1) revealed stronger binding affinities in T. oceanica for desferrioxamine-bound Fe3+, predicting better iron uptake than gammaproteobacteria. ISIP1 of T. oceanica showed better affinity for the ferrioxamine-Fe complex than that of C. tenuissimus, suggesting varying affinities among diatoms. This work provides new insights into the strategies adopted by species between and within microbial kingdoms to access iron in the HNLC waters of ISSO.