BREAKTHROUGHS IN DECIPHERING THE ROLE OF COCCOLITHS: INSIGHTS FROM FORAMINIFERA

Although phylogenetically distant, the planktonic foraminifera and coccolithophores share significant biological and ecological characters, including being active unicellular calcifiers living in the superficial layer of the ocean. This explains well their parallel biogeographic and diversification trajectories, in particular during the Paleogene, Both reflect expansion and contraction of the trophic resources continuum in the ocean as a function of changing climates. Until recently, however, studies of the planktonic foraminifera have predominated in our understanding of the marine calcareous plankton, providing models upon which to reconstruct the history of the coccolithophores. This is because, unlike coccoliths, single tests of planktonic foraminifera are amenable to handling for isotopic analysis. This proved invaluable in demonstrating that the physiology of some Paleocene foraminifera shifted from heterotrophic to mixotrophic through the acquisition of photosymbionts as a response to increased oligotrophy during the early Paleocene (Danian). Enhanced oligotrophy through the late Paleocene resulted in the radiation of the shallow dwellers, non-spine-bearing Acarinina and Morozovella. Based on this, the late early Paleocene radiation of the coccolithophores has been interpreted as a response to increased oligotrophy although without documentation of associated adaptive morphologic characters. Phylogenetic study of the Order Discoasterales shows that these coccolithophores adapted to oligotrophy through rapid morphostructural changes allowing coccoliths to serve as food collectors. This discovery has two fundamental implications. First, these coccolithophores are not autotrophs. Instead they have retained the mixotrophic physiology of their coastal ancestors through the secretion of cococliths. Second, the categories of primary producers and first-level consumers are fuzzy and do not validly describe the calcifiers. Coccolithophores with coccoliths adapted to food collecting behave as consumers, whereas photosymbiont-bearing planktonic foraminifera behave as primary producers. Ecologic studies should take this into account.