Photosymbiosis has played a key role in the diversification of foraminifera and their carbonate production throughout geologic history. However, identification of photosymbiosis in extinct taxa remains challenging, and even among the extant species the occurrence and functional relevance of photosymbiosis remain poorly constrained. Here, we investigate photosymbiosis in living planktonic foraminifera by measuring active chlorophyll fluorescence with fast repetition rate fluorometry. This method provides unequivocal evidence for the presence of photosynthetic capacity in individual foraminifera, and it allows us to characterize multiple features of symbiont photosynthesis including chlorophyll a (Chl a) content, potential photosynthetic activity (Fv/Fm), and light-absorption efficiency (σPSII). To obtain robust evidence for the occurrence and importance of photosymbiosis in modern planktonic foraminifera, we conducted measurements on 1266 individuals from 30 species of the families Globigerinidae, Hastigerinidae, Globorotaliidae, and Candeinidae. Among the studied species, 19 were recognized as symbiotic and 11 as non-symbiotic. Of these, six species were newly confirmed as symbiotic and five as non-symbiotic. Photosymbiotic species have been identified in all families except the Hastigerinidae. A significant positive correlation between test size and Chl a content, found in 16 species, is interpreted as symbiont abundance scaled to the growth of the host and is consistent with persistent possession of symbionts through the lifetime of the foraminifera. The remaining three symbiont-bearing species did not show such a relationship, and their Fv/Fm values were comparatively low, indicating that their symbionts do not grow once acquired from the environment. The objectively quantified photosymbiotic characteristics have been used to design a metric of photosymbiosis, which allows the studied species to be classified along a gradient of photosynthetic activity, providing a framework for future ecological and physiological investigations of planktonic foraminifera.
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