Molecular and physiological acclimation to low light and iron scarcity in a globally abundant oceanic pelagophyte

Abstract Pelagomonas calceolata is a widely distributed marine alga and is among the most numerous eukaryotes on Earth. It is abundant in subsurface chlorophyll maximum layer (SCML) communities where iron (Fe) and light are jointly scarce. To understand the strategies behind P. calceolata ’s success, we profile this organism’s physiology and gene expression as it experiences Fe/light co-limitation. We describe cellular changes under steady-state Fe limitation and short-term responses to Fe resupply. Culture experiments reveal that P. calceolata maintains low Fe:carbon (C) ratios and dynamically regulates iron-sparing strategies, including flavodoxin expression and substitution of metal-rich proteins. Furthermore, environmental gene expression shows that Fe- and light-responsive genes identified in culture are enriched in SCML metatranscriptomes, indicating that P. calceolata expresses these adaptations in situ. These results demonstrate low Fe tolerance as a key adaptation enabling P. calceolata to thrive in light-limited marine environments and highlight its role in oceanic carbon and nitrogen cycling.