Abstract. Coccolithophores, a diverse group of phytoplankton, make important contributions to pelagic calcite production and export, yet the comparative biogeochemical role of species other than the ubiquitous Emiliania huxleyi is poorly understood. The contribution of different coccolithophore species to total calcite production is controlled by interspecies differences in cellular calcite, growth rate and relative abundance within a mixed community. In this study we examined the relative importance of E. huxleyi and two Coccolithus species in terms of daily calcite production. Culture experiments compared growth rates and cellular calcite content of E. huxleyi (Arctic and temperate strains), Coccolithus pelagicus (novel Arctic strain) and Coccolithus braarudii (temperate strain). Despite assumptions that E. huxleyi is a fast-growing species, growth rates between the three species were broadly comparable (0.16-0.85 d −1 ) under identical temperature and light conditions. Emiliania huxleyi grew only 12 % faster on average than C. pelagicus, and 28 % faster than C. braarudii. As the cellular calcite content of C. pelagicus and C. braarudii is typically 30-80 times greater than E. huxleyi, comparable growth rates suggest that Coccolithus species have the potential to be major calcite producers in mixed populations. To further explore these results we devised a simplistic model comparing daily calcite production from Coccolithus and E. huxleyi across a realistic range of relative abundances and a wide range of relative growth rates. Using the relative differences in growth rates from our culture studies, we found that C. pelagicus would be a larger source of calcite if abundances of E. huxleyi to C. pelagicus were below 34 : 1. Relative abundance data collected from North Atlantic field samples (spring and summer 2010) suggest that, with a relative growth rate of 88 %, C. pelagicus dominated calcite production at 69 % of the sites sampled. With a more extreme difference in growth rates, where C. pelagicus grows at 1 / 10th of the rate of E. huxleyi, C. pelagicus still dominated calcite production in 14 % of the field. These results demonstrate the necessity of considering interactions between inter-species differences in growth rates, cellular calcite and relative abundances when evaluating the contribution of different coccolithophores to pelagic calcite production. In the case of C. pelagicus, we find that there is strong potential for this species to make major contributions to calcite production in the North Atlantic, although estimates of relative growth rates from the field are needed to confirm our conclusions.
The Chixulub bolide impact 66 million years ago drove near-instantaneous oceanic ecosystem collapse. Devastating diversity loss at the base of ocean food-webs likely triggered cascading extinctions across all trophic levels 1-3 and caused severe disruption of ocean biogeochemical function, especially the cycling of carbon between the surface and deep sea 4,5. The absence of sufficiently detailed biotic data spanning the postextinction interval has limited our understanding of how ecosystem resilience and biochemical function was restored, with estimates of 'recovery' varying from <100 years to 10 million years 6-8. Using a 13-million-year long nannoplankton time-series we show that post-extinction communities exhibited 1.8 million years of exceptional volatility before a more stable equilibrium community emerged displaying hallmarks of resilience. The transition to this new equilibrium-state community with a broader cellsize spectrum coincides with indicators of carbon cycle restoration and a fully functioning biological pump 9. This finding implies a fundamental link between 93 perturbation (i.e., the significant linear trend between carbon isotope excursion and variance) 94 and rapid recoveries following each event (return of variance to the background state within 95 <200 kyrs of the excursion, Fig. 1c and ref 21). 96 Tantalisingly, the shift to more stable communities approximately 64.2 Myr ago (the end of 97 Regime 1) also falls towards the top of the interval of biological pump recovery 9 (Fig. 3f). 98 Ocean biogeochemical function was profoundly disrupted by the end-Cretaceous mass 99 extinction, most obviously through weakening of the biological pump 2,5,9. The scale and duration of this productivity reduction is contentious, ranging from scenarios of a lifeless Strangelove Ocean to a partially functioning Living Ocean state 4 , but the long, multimillionyear delay in restoration of the biological pump is well established 2,22 , and indicated by both the gradual increase in vertical carbon isotope gradient to pre-extinction values 9 and changing community structures of benthic primary-consumer communities (benthic foraminifera) 23. Carbon isotope gradients finally returned to pre-extinction values by ~1.77 Myrs after the event 9 providing an upper limit on full recovery of the biological pump. This broad
Past global warming events such as the Palaeocene–Eocene Thermal Maximum (PETM—56 Ma) are attributed to the release of vast amounts of carbon into the ocean, atmosphere and biosphere with recovery ascribed to a combination of silicate weathering and organic carbon burial. The phytoplanktonic nannoplankton are major contributors of organic and inorganic carbon but their role in this recovery process remains poorly understood and complicated by their contribution to marine calcification. Biocalcification is implicated not only in long-term carbon burial but also both short-term positive and negative climatic feedbacks associated with seawater buffering and responses to ocean acidification. Here, we use exceptional records of preserved fossil coccospheres to reconstruct cell size distribution, biomass production (particulate organic carbon, POC) and (particulate) inorganic carbon (PIC) yields of three contrasting nannoplankton communities (Bass River—outer shelf, Maud Rise—uppermost bathyal, Shatsky Rise—open ocean) through the PETM onset and recovery. Each of the sites shows contrasting community responses across the PETM as a function of their taxic composition and total community biomass. Our results indicate that nannoplankton PIC:POC had no role in short-term climate feedback and, as such, their importance as a source of CO2 to the environment is a red herring. It is nevertheless likely that shifts to greater numbers of smaller cells at the shelf site in particular led to greater carbon transfer efficiency, and that nannoplankton productivity and export across the shelves had a significant modulating effect on carbon sequestration during the PETM recovery.This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.
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