Diatoms comprise a highly successful class of secondary symbiotic algae that can be found in almost any aquatic environment, including the Southern Ocean Sea Ice and extreme acidic environments (such as found in the Yellowstone springs). It is now well establishing that we can trace the plastid’s origin to a primary endosymbiotic event in which a non-photosynthetic protist engulfed and enslaved a cyanobacterium. Over a billion years ago, this eukaryote then gave rise to three different algae lineages: “red”, “green” and glaucophyte algae. In addition, some extant algae have a more complicated evolutionary history involving a secondary endosymbiotic event. Diatoms are an example of such a secondary endosymbiosis, yet distinct differences exist between their major classes. They have high photosynthetic rates and their ability to rapidly respond to environmental cues is reflected in their physiological plasticity, including chlorophyll and lipid contents, and remodeling of intermediate metabolism. My present working hypotheses are that in diatoms: 1. Environmental cues are communicated to the cell via a RST pathway(s) emanating from the plastid; and 2. RST pathways in the model diatom P. tricornutum comprise a simpler, trimmed-down version of those found in land plants, that may be a relic “starter set” of core molecules that were later elaborated upon in the evolution of land plants. Using an array of scientific approaches that include genetic modifications, bioinformatics, physiology, and biophysics, I work towards finding the key players and pathways that are at the basis of the diatoms sense and response mechanisms.