Meet Prochlorococcus -- it might be the most important microbe you've never heard of
Tiny creatures that inhabit the oceans’ well-lit upper waters emit gas or gaseous compounds. One algae Emiliana huxleyi emits dimethyl sulfide, which contributes to what we call the smell of the sea.
An unseen "forest" of these microscopic beings fills the upper 200 meters of ocean, exerting an influence on this planet every bit as profound as the forests on land. The diverse phytoplankton species inhabiting the ocean's surface waters -- which mainly consist of single-celled cyanobacteria, diatoms and other kinds of algae -- form the base of the marine food web. They account for roughly half the photosynthesis on the earth, remove nearly as much carbon dioxide from the atmosphere as all land plants, and supply about half to three-quarters of the oxygen we breathe. Without the activities of these free-floating plantlike organisms, atmospheric carbon dioxide levels would triple.