Coccolithophores are microalgae which produce much of the calcium carbonate in the ocean, important to making organic carbon sink to great depths, and they may be negatively affected by the decline in ocean pH as CO₂ rises. Can these important microbes adapt? This study found that coccolithophores inhabiting waters naturally low in pH may have already reached the limit of their ability to adapt. This suggests that how the ocean's biota sequester carbon will be strongly affected in the future.