Iron physiology and metabolism of model phytoplankton taxa in a western boundary continental shelf ecosystem

Continental shelf ecosystems support diverse phytoplankton communities that contribute to global primary productivity. Iron is an important micronutrient controlling the growth of marine phytoplankton, but its role in influencing phytoplankton ecophysiology in western boundary continental shelf ecosystems is understudied. To examine iron physiology between shelf regions, representative diatoms ( Cylindrotheca closterium ) and coccolithophores ( Gephyrocapsa oceanica and Gephyrocapsa huxleyi ) collected from the relatively iron‐rich inner shelf and lower iron outer shelf of the South Atlantic Bight were grown under high and low iron conditions. The specific growth rates, photophysiology, and gene expression patterns were investigated. The diatoms employed strategies that enable success across shelf zones, including high maximum growth rates. Their high photoprotective capacity likely contributed to maintaining maximum potential quantum efficiency of photosystem II ( F v / F m ) and short Q a reoxidation times ( τQ a ) under iron limitation. In contrast, the coccolithophore isolates demonstrated metabolic strategies suited to conditions of the shelf region from which they originated. The growth of the inner‐shelf G. oceanica was decreased by 71% in low iron media, while that of outer‐shelf G. huxleyi was decreased by only 35%, indicating a lower sensitivity to iron limitation. The physiological and metabolic patterns reported here may help explain observed phytoplankton community composition and bloom succession patterns in western boundary continental shelf ecosystems.