Cyclic evolution of phytoplankton forced by changes in tropical seasonality

Luc L Beaufort (1) , Clara Bolton (1) , Anta-Clarisse Sarr , Baptiste Suchéras-Marx , Yair Rosenthal , Yannick Donnadieu , Nicolas Barbarin , Samantha Bova , Pauline Cornuault , Yves Gally , Emmeline Gray , Jean-Charles Mazur , Martin Tetard

1 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement

The effect of global climate cycles driven by Earth’s orbital variations on evolution is poorly understood because of difficulties achieving sufficiently-resolved records of past evolution. The fossil remains of coccolithophores, a key calcifying phytoplankton group, enable an exceptional assessment of the impact of cyclic orbital-scale climate change on evolution because of their abundance in marine sediments, and because coccolithophores demonstrate extreme morphological plasticity in response to the changing environment1,2. Recently, evolutionary genetic analyses linked broad changes in Pleistocene fossil coccolith morphology to species radiation events3. Here, using high-resolution coccolith data, we show that during the last 2.8 million years coccolithophore evolution was forced by Earth’s orbital eccentricity with rhythms of ~100,000 years and 405,000 years - a distinct spectral signature to that of coeval global climate cycles4. Simulations with an Earth System Model5 including the marine carbon cycle6 demonstrate that eccentricity directly impacts the diversity of ecological niches occurring over the annual cycle in the tropical ocean. Reduced seasonality favours species with mid-size coccoliths that bloom year-round, increasing coccolith carbonate export and burial. We posit that eccentricity pacing of phytoplankton evolution contributed to the strong 405,000-year pacing seen in records of the global carbon cycle.


The effect of global climate cycles driven by Earth's orbital variations on evolution is poorly understood because of difficulties achieving 14 sufficiently-resolved records of past evolution. The fossil remains of coccolithophores, a key calcifying phytoplankton group, enable an 15 exceptional assessment of the impact of cyclic orbital-scale climate change on evolution because of their abundance in marine sediments, 16 and because coccolithophores demonstrate extreme morphological plasticity in response to the changing environment 1,2. Recently, 17 evolutionary genetic analyses linked broad changes in Pleistocene fossil coccolith morphology to species radiation events 3. Here, using 18 high-resolution coccolith data, we show that during the last 2.8 million years coccolithophore evolution was forced by Earth's orbital 19 eccentricity with rhythms of ~100,000 years and 405,000 years-a distinct spectral signature to that of coeval global climate cycles 4. 20 Simulations with an Earth System Model 5 including the marine carbon cycle 6 demonstrate that eccentricity directly impacts the diversity 21 of ecological niches occurring over the annual cycle in the tropical ocean. Reduced seasonality favours species with mid-size coccoliths 22 that bloom year-round, increasing coccolith carbonate export and burial. We posit that eccentricity pacing of phytoplankton evolution 23 contributed to the strong 405,000-year pacing seen in records of the global carbon cycle. 24 25 Coccolithophores are important producers of CaCO3 in the ocean and their fossil remains (calcite platelets called coccoliths) first appeared in 26 sediments during the late Triassic (~210 million years ago, Ma). Coccolithophores rose to dominance in the open ocean during the early 27 Cretaceous 7 and thereafter become a key biological modulator of the global carbon cycle, via both photosynthesis and calcification 8. In the 28 dominant Noëlaerhabdaceae family (including the cosmopolitan genera Emiliania, Gephyrocapsa and Reticulofenestra), species are defined by 29 the morphological characteristics of their coccoliths, with size being a key criterion 9. For Gephyrocapsa and Emiliania, phylogenies 30 reconstructed from gene sequence data indicate that morphology-based definitions correspond to biological species 3,10. Yet, within a given 31 Noëlaerhabdaceae population, interspecific and intraspecific changes in coccolith length and mass (the latter encoding degree of calcification as 32 well as size) occur in response to environmental parameters such as carbonate chemistry 1 and temperature 2. Existing studies of coccolithophore 33 evolution have focused on geological-timescale changes in species richness and turnover 11 , coccolith carbonate accumulation 7,12 , or calcification 34 potentially driven by carbon cycle changes 13. However, to date a lack of records that are both long-term and high-resolution has precluded an 35 understanding of the impact of orbital cycles (on timescales of tens to hundreds of thousands of years) on coccolithophore evolution and 36 carbonate production. 37 38

Domaines: Sciences de l'environnement

Luc L Beaufort, Clara Bolton, Anta-Clarisse Sarr, Baptiste Suchéras-Marx, Yair Rosenthal, et al.. Cyclic evolution of phytoplankton forced by changes in tropical seasonality. 2020. ⟨hal-03006397⟩