Microbes dominate our planet, especially the Earth’s oceans. The distinguishing feature of microorganisms is their small size, usually defined as less than 200 micrometers (μm); they are all invisible to the naked eye. As a group, sea microbes are extremely diverse, and extremely versatile with respect to their abilities to make and eat food. All marine microbes are too small to swim against the current and are therefore classified as plankton. First we will discuss several ways to classify marine microbes.