Though they may be small, microorganisms are the most abundant form of life in the ocean. Marine microbes are responsible for making roughly half of the organic carbon that’s usable by life. Many marine microbes live near the surface, depending on energy from the sun for photosynthesis. Yet between the low supply of and high competition for some key nutrients, like nitrogen, in the open ocean, scientists have puzzled over the vast diversity of microbial species found there.