The Permian-Triassic mass extinction forces the radiation of the modern marine phytoplankton

Among modern eukaryotic marine phytoplanktonic lineages possessing red algal plastids are the diatoms, dinoflagellates and haptophytes including coccolithophorids. Although origins of these host lineages and the timing of the endosymbioses leading to their ability to photosynthesize are ancient, their modern radiations are not. Molecular clocks suggest the timing of the origins and the radiations of these host lineages are different among the three groups. Dinoflagellates and haptophytes had class and order level radiations from 500 to 600 megaannum (Ma) and genus-level radiations, especially among the thecate dinoflagellates, about 250 Ma. Pigmented heterokonts, the last divergence in the heterokont tree, originated between 770 and 1000 Ma, but most planktonic unicellular groups diverged at the Permian-Triassic (P/T) boundary (250 Ma) and radiated thereafter based on a clock constrained by both fossil first appearances and biochemical evidence. Recent molecular evidence suggests that heterokont and haptophyte protistan host cells originally possessed both green and red algal plastids with endosymbiosis of the green plastid older than the red one. If one assumes that once the green algal plastid was eliminated, the red algal plastid lineages could then proliferate, then my clocks suggest that this event likely occurred at the P/T extinction. The possession of the red plastid only became advantageous at the P/T extinction when oceanic chemistry changes provided a habitat in which red algal plastids were better adapted and could out-compete green algal plastids. Prior to the P/T, red algal lineages were not dominant as judged by the fossil record, and there is an absence of a dominant green plastid lineage in the modern plankton. Therefore, expulsion of the green algal plastid in favour of a red algal plastid and divergence and subsequent radiation of the planktonic heterokont algae and major orders/families/genera in the haptophytes and the dinoflagellates were likely forced by the Permian-Triassic extinction event.