Iron availability modulates ferric reduction oxidase (FRO) gene expression in the toxic dinoflagellate Alexandrium pacificum.

Iron (Fe) is an essential element for living organisms and plays a central role in numerous biological processes. Although Fe uptake mechanisms have been extensively characterized in plants, they remain poorly understood in microalgae, particularly in dinoflagellates. In this study, we identified two novel ferric reduction oxidase genes (FROs) in the marine dinoflagellate Alexandrium pacificum (designated ApFRO1 and ApFRO2) and examined their expression under varying Fe conditions. ApFRO1 and ApFRO2 encode open reading frames of 2205 bp and 2688 bp, respectively, each containing ferric reductase family domains. Both proteins possess multiple transmembrane regions, consistent with predicted localization at the plasma membrane. Significant differences in cell growth were observed across Fe treatments. Strong upregulation of both ApFROs (up to 34.9-fold) occurred under Fe-depleted conditions over a three-day period. Both Fe deficiency and Fe excess reduced chlorophyll autofluorescence (CAF), indicating impaired photosynthetic performance. Together, these findings demonstrate that fluctuations in Fe availability markedly influence the physiology of A. pacificum, and that cells activate an Fe3+-reductase-mediated Fe acquisition pathway involving FROs.