Contrasting Responses of Oceanic and Coastal <scp> <i>Synechococcus</i> </scp> to Iron Limitation and Warming Interactions

ABSTRACT This study explored the contrasting physiological and transcriptional responses to iron (Fe) and warming temperature interactions in two South China Sea Synechococcus isolates belonging to clade II from the open ocean and CB5 from the coastal ocean. The two picocyanobacterial strains utilised contrasting photosynthesis, Fe uptake, and nutrient acquisition strategies to cope with Fe limitation. In the oceanic strain, moderate warming under Fe limitation upregulated expression of photosynthesis and nutrient and Fe transport genes, increasing its growth and photosynthesis. In contrast, gene expression under low Fe in the coastal strain was less affected by warming. The oceanic isolate exhibited substrate regulation of Fe acquisition and preferred organic nutrient sources. The coastal strain had a much higher Fe quota, faster turnover of the D1 gene in photosystem II, and was optimised for inorganic nitrogen sources. Both strains showed multi‐tiered Fe uptake strategies and general stress responses to heat shock and oxidative stress. In general, gene regulation in the oceanic strain responded more effectively to both stressors than in the coastal isolate. Fe‐temperature interactions in both strains are complex and may lead to synergistic and antagonistic responses, potentially influencing global biogeochemical cycles in warmer oceans.