Publisher Summary Although algae thrive in a spectrum of habitats, including Antarctic ice, rock, and tree surfaces, animal fur, human and animal skin, and desert sand, most forms are aquatic. Algae are critical in modern aquatic ecosystems, not only in producing oxygen for other aquatic life, but also in serving as primary producers of organic matter at the base of the food web. The algae are a large informal grouping of heterogeneous, polyphyletic, or paraphyletic groups of primarily aquatic organisms ranging from tiny, flagellated unicells only a few microns in diameter to multicellular organisms up to 80 m long, such as the giant kelps. Unlike vascular plants, the algal body (thallus) lacks organ differentiation; although some forms have developed structures functionally similar to roots, shoot axes, and leaves. Most algae are photoautotrophic; some forms, however, are mixotrophic and derive energy both from photosynthesis and uptake of organic carbon by osmotrophy, myzotrophy, or phagotrophy. A few forms have reduced or lost their photosynthetic capacities and are entirely heterotrophic. Moreover, molecular, biochemical, and ultrastructural characteristics are increasingly important in algal systematics and phylogeny.