Iron limitation is a characteristic biogeochemical feature of the modern ocean, with phytoplankton growth and primary productivity limited by iron availability in approximately 30% of the global ocean. Understanding phytoplankton iron acquisition and its environmental regulation is therefore essential for elucidating productivity maintenance in iron-limited regions and the marine biogeochemical cycling of iron. In this study, we used the model diatom Phaeodactylum tricornutum to investigate how light regulates iron uptake, ferric reduction, and the bioavailability of organically complexed iron.

By measuring cell growth, intracellular iron quotas, iron uptake rates, and cell-surface ferric reduction rates under different light conditions, we found that light not only modulates diatom physiological status but also strongly influences iron acquisition capacity. Under iron-limited conditions, higher light enhanced cell-surface ferric reduction, promoting the conversion of Fe(III) to more readily assimilable reduced iron and thereby increasing the ability of diatoms to utilize external iron, particularly certain forms of organically complexed iron. Furthermore, using gene editing and in vitro protein purification assays, we confirmed the functions of the ferric reduction-related genes fre1 and fre2 in P. tricornutum, and demonstrated their ferric reductase activity, electron donor preference, and substrate specificity.

Together, these results reveal, at both physiological and molecular levels, a key mechanism by which light regulates iron acquisition in marine diatoms. This study provides new insight into oceanic iron–light interactions and contributes to a better understanding of the regulation of phytoplankton productivity and iron cycling in iron-limited marine environments.