Ocean alkalinity enhancement (OAE) is one method from the marine based carbon dioxide removal (mCDR) portfolio with the potential to remove atmospheric carbon on a gigaton scale. Various approaches have been proposed to achieve OAE, however all approaches have the common theme of an intentional increase in alkalinity and subsequent change in carbonate chemistry. Although the enhancement of alkalinity does not directly affect biological processes, subsequent shifts in carbon speciation can significantly influence various biological processes. Of particular concern is the influence OAE may have on phytoplankton communities. In order to assess this, we conducted experiments on five coastal diatom species across a matrix of carbonate chemistry conditions ranging from ~1800 – 3700 µmol kg-1 of DIC and ~2010 – 3400 µmol kg-1. Within this matrix we explore the influence of a wide range of carbonate chemistry conditions, encompassing conditions which would be expected under OAE, ocean acidification, coastal and pelagic environments as well as marginal seas. Assessing the growth, photosynthetic efficiency and silicification of bloom forming diatom species under these conditions allow us to develop carbonate chemistry dependent optimum curves which help to increase our mechanistic understanding of diatom responses to changing carbonate chemistry conditions.