Significance The coastal Southern Ocean is a critical climate system component and home to high rates of photosynthesis. Here we show that cobalamin (vitamin B ₁₂ ) and iron availability can simultaneously limit phytoplankton growth in late Austral summer coastal Antarctic sea ice edge communities. Unlike other growth-limiting nutrients, the sole cobalamin source is production by bacteria and archaea. By identifying microbial gene expression changes in response to altered micronutrient availability, we describe the molecular underpinnings of limitation by both cobalamin and iron and offer evidence that this limitation is driven by multiple delicately balanced phytoplankton–bacterial interactions. These results support a growing body of research suggesting that relationships between bacteria and phytoplankton are key to understanding controls on marine primary productivity.