Accurate ecological data on planktic foraminifera (calcifying microbes that play an important role in the carbon cycle) are important for modelling their response to climate change. We studied the species G. bulloides. A lack of algal symbionts and unusual shell chemistry suggest a different life history compared to other spinose species. We demonstrate that G. bulloides hosts cyanobacterial endobionts. This has implications for modelling this species and for understanding its shell chemistry.