Ocean pH levels have been dropping since the Industrial Revolution due to increased carbon dioxide gas (CO2) in the atmosphere. When CO2 is absorbed by the ocean, hydrogen ions are released, resulting in a decrease in pH. Dangerous levels of carbonic acid in the ocean ecosystem as a result of CO2 absorption may have an effect on the health of marine algae, such as diatoms. Marine diatoms account for an estimated 20% of oxygen production in the atmosphere and are essential to the aquatic food chain. I hypothesized that increased ocean acidity would decrease the photosynthetic ability of Chaetoceros gracilis, a diatom prolific in Monterey Bay, because of the usually corrosive effects of carbonic acid on both seashells and cells’ internal structures. I altered pH of algae environments and measured the photosynthetic ability of diatoms over four days by spectrophotometer. Surprisingly, a decrease in pH improved the photosynthetic ability of C. gracilis, but only within a specific pH range. The diatoms grown in pH 7.5 medium had the highest average absorbance value; therefore, this pH value is a “sweet spot” that optimizes the growth of this species of diatom. These findings indicate that C. gracilis may become more abundant in Monterey Bay as the pH of the ocean continues to drop, potentially contributing to harmful algal blooms.