Over Earth history, atmospheric carbon dioxide has declined and oxygen has risen to a first order as a result of the geological cycle of organic carbon. Coincidentally, there has been a succession of different phytoplankton rising to dominate the open ocean during different eras. The process of photosynthesis by these phytoplankton, the direct route to the generation of O2 on the planet, is mediated by carbon fixation catalysed by the enzyme RuBisco. But over Earth history the increasingly oxygenated atmosphere has become more challenging for the forwards reaction of the carboxylation of RuBisco, whose activity can be confused by the competition between the similar oxygen and carbon dioxide molecules for the catalytic active site. This talk will document a range of adaptations of phytoplankton physiology in response to this aggravating O2/CO2 ratio that have allowed phytoplankton to continue to thrive in the marine realm. It will contrast different features of groups of phytoplankton including strategies for metal acquisition and and how their prevalence may act as indicators of the prevailing environment. It will further explore how these phytoplankton adaptations may have expanded the habitability of, and climate extremes of the planet.