The sea is home to an immense array of algal diversity that spans fully eight orders of magnitude in physical dimension, from the smallest phytoplankton to the largest seaweeds. Growth forms range from the beautiful siliceous houses of diatoms, through the delicately armored and self-propelled arrays of dinoflagellates and the heavily armored arrays of crustose algae, to the more familiarly plant-like growth plans of seaweeds. Life cycles can be even more diverse and unfamiliar, and the restless medium provides alien (to humans) challenges in fertilization and recruitment. Some algae live as symbionts in animals. Among the fundamental issues are acquisition of enough light and nutrients for photosynthesis and avoidance of too much of the most powerful, UV wavelengths.