Bacteria and unicellular marine plants called diatoms depend on each other for some essential nutrients, but they also compete for other nutrients. So life gets complicated in the chemical soup of the ocean.
Diatoms convert carbon dioxide into organic carbon via photosynthesis.
Bacteria need organic carbon, but many cannot make it. So they rely on diatoms, which release organic carbon into the ocean when they grow, are lysed (broken apart), or are eaten by zooplankton.
Diatoms require vitamin B12, but they cannot make it. Bacteria do make it, so diatoms rely on bacteria, which release B12 into the ocean when they grow, are lysed, or are eaten.
Diatoms and bacteria both need iron, which is a scarce commodity in the ocean. So they compete for it. If bacteria use up the iron, diatoms won’t have enough to make organic carbon; if diatoms use up the iron, bacteria won’t have it to make vitamin B12. (Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)