The likely response of coccolithophores to ongoing warming and acidification of the global oceans remains difficult to predict, because (1) we know very little about climatic adaptation by this group of marine algae and (2) experimental evidence points to complex species-specific and even strain-specific sensitivities to short-term environmental perturbations. This study takes a long-term perspective to gain insights into species-specific evolutionary rates and climatic adaptation in the past, by reconstructing the phenotypic variability within two prominent coccolithophore lineages (Reticulofenestra and Coccolithus) during the late Miocene (NN9-12).
The Cenozoic ancestors of all extant coccolithophores have experienced much higher temperatures, higher levels of CO2 and lower ocean pH than today, according to proxy reconstructions over the past 60 million years. However, different lineages display different levels of variation in coccolith size, a useful proxy for cellular volume-to-surface area ratios in the investigated lineages (Henderiks, 2008), and this could indicate that some species are more adaptable to climatic change than others. Time series of phenotypic variability in fossil Reticulofenestra populations are strikingly similar between the Atlantic, Indian and Pacific oceans, depicting global patterns of evolution during the late Miocene. An abrupt decrease in Reticulofenestra size and abundance, first recorded in detail by Young (1991) in the Indian Ocean, occurred ~9 million years ago (within NN9). Although globally evident, this ‘crash’ was most pronounced in the western equatorial Pacific, whereas larger sized Reticulofenestra populations persisted across the same interval in the North Atlantic. At the same deep-sea sites, time series of Coccolithus size variability and abundance reveal consistent differences between oceanographic regimes. The largest ranges in size are found in the temperate North Atlantic, whereas much tighter size distributions are recorded at the tropical Pacific site. Equatorial Pacific populations display very stable, unimodal size distributions, but at low relative abundances, for about two million years prior to their disappearance from the record ~6.5 million years ago (within NN11). This marks a habitat contraction towards higher latitudes (and coastal upwelling areas), which remain strongholds of modern Coccolithus populations today. Together, these data support that species-specific responses are evident also on longer timescales, and offer a first glance at how such phenotypic patterns may relate to both global and regional records of climatic change.
Henderiks, J. 2008. Coccolithophore size rules - Reconstructing ancient cell geometry and cellular calcite quota from fossil coccoliths. Marine Micropaleontology, 67: 143-154.
Young, J. 1990. Size variation of Neogene Reticulofenestra coccoliths from Indian Ocean DSDP cores. Journal of Micropaleontology, 9: 71-86.
2010. 62- p.