In 1953, when Sylvia A. Earle began studying algae, the marine plants and related microbes were often considered weeds or worse. Boaters ridiculed them as scum that turned patches of sea into pea soup.

Today, Dr. Earle notes that just one type — Prochlorococcus, so small that millions can fit in a drop of water — has achieved fame as perhaps the most abundant photosynthetic organism on the planet. It daily releases countless tons of oxygen into the atmosphere.

The tiny organism is estimated to provide the oxygen in “one in every five breaths we take,” Dr. Earle said in an interview. And it is just one of thousands of types of marine algae and photosynthetic microbes — everything from invisible cells to plantlike growths to kelp forests.

A student of the big and the small, Dr. Earle is a co-author of “Ocean: An Illustrated Atlas,” published recently by National Geographic. Its maps and graphs, prose and pictures detail how discoveries like the surprising ubiquity of Prochlorococcus are illuminating the sea, its immense impact on the planet and its habitability.