Iron levels unexpectedly limit bacterial growth in the ocean’s twilight zone

Microbial growth at depths of 200–500 metres has been found to be limited by iron, a key micronutrient. To meet their iron requirement, bacteria inhabiting the twilight zone manufacture siderophores — molecules that scavenge trace amounts of iron from seawater. Bacteria living 200–500 metres below the ocean’s surface show signs that a key nutrient is in short supply.