Answering Charles Darwin’s prescient question has taken us nearly two centuries. Only in recent decades have methods and concepts been developed to explore the significance of microbes in the ocean’s web of life. Bacteria in aquatic ecosystems were first recognized for their role in the decomposition of organic material and the remineralization of inorganic nutrients, a role that only became fully accepted in the 1980s. Their importance as photosynthetic producers of organic matter became evident when so-called blue-green algae were acknowledged as being bacteria, and the microscopic cyanobacterium of the genus Synechococcus was discovered to be abundant in the oceans—particularly in the vast oligotrophic blue water where they are the dominant autotrophs.

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