A coastal Synechococcus strain is more resilient to iron limitation than an oceanic strain

Iron (Fe) is an essential micronutrient for the marine phytoplankton Synechococcus , which is globally distributed and contributes to approximately 17% of global marine net primary production. To investigate how contrasting ecotypes cope with Fe limitation, we compared growth, physiology, and elemental stoichiometry of coastal Synechococcus sp. PCC7002 and oceanic Synechococcus sp. WH7803 under Fe‐replete and Fe‐deplete conditions. The coastal strain PCC7002 maintained stable growth rates under Fe‐deplete conditions whereas the oceanic strain WH7803 demonstrated reduced growth rates. This stability in PCC7002 was supported by reduced cell size, which lowered Fe demand and enhanced surface area‐to‐volume ratios for more efficient nutrient uptake. Light harvesting capacity in PCC7002 was largely preserved under Fe depletion, as indicated by stable pigment : C ratios. Elevated N : P ratios in Fe‐deplete PCC7002 potentially reflected increased N investment in Fe‐response proteins while P demand remained relatively unchanged. In contrast, such flexible morphological and physiological adjustments were not observed in WH7803. Both strains showed shifts in trace element stoichiometry consistent with upregulated divalent metal transporters driving non‐Fe metal uptake under Fe depletion. However, under identical Fe‐replete conditions, PCC7002 exhibited stronger luxury Fe uptake than WH7803. Collectively, these results demonstrate ecotype‐specific Fe acclimation strategies shaped by adaptation to divergent environmental regimes at isolation sites, highlighting the role of Synechococcus diversity in regulating elemental cycling in the ocean.