Over-calcified forms of the coccolithophore <i>Emiliania huxleyi</i> in high-CO<sub>2</sub> waters are not preadapted to ocean acidification

Abstract

Abstract. Marine multicellular organisms inhabiting waters with natural high fluctuations in pH appear more tolerant to acidification than conspecifics occurring in nearby stable waters, suggesting that environments of fluctuating pH hold genetic reservoirs for adaptation of key groups to ocean acidification (OA). The abundant and cosmopolitan calcifying phytoplankton Emiliania huxleyi exhibits a range of morphotypes with varying degrees of coccolith mineralization. We show that E. huxleyi populations in the naturally acidified upwelling waters of the eastern South Pacific, where pH drops below 7.8 as is predicted for the global surface ocean by the year 2100, are dominated by exceptionally over-calcified morphotypes whose distal coccolith shield can be almost solid calcite. Shifts in morphotype composition of E. huxleyi populations correlate with changes in carbonate system parameters. We tested if these correlations indicate that the hyper-calcified morphotype is adapted to OA. In experimental exposures to present-day vs. future pCO2 (400 vs. 1200 µatm), the over-calcified morphotypes showed the same growth inhibition (−29.1±6.3 %) as moderately calcified morphotypes isolated from non-acidified water (−30.7±8.8 %). Under the high-CO2–low-pH condition, production rates of particulate organic carbon (POC) increased, while production rates of particulate inorganic carbon (PIC) were maintained or decreased slightly (but not significantly), leading to lowered PIC ∕ POC ratios in all strains. There were no consistent correlations of response intensity with strain origin. The high-CO2–low-pH condition affected coccolith morphology equally or more strongly in over-calcified strains compared to moderately calcified strains. High-CO2–low-pH conditions appear not to directly select for exceptionally over-calcified morphotypes over other morphotypes, but perhaps indirectly by ecologically correlated factors. More generally, these results suggest that oceanic planktonic microorganisms, despite their rapid turnover and large population sizes, do not necessarily exhibit adaptations to naturally high-CO2 upwellings, and this ubiquitous coccolithophore may be near the limit of its capacity to adapt to ongoing ocean acidification.

Calcification and distribution of extant coccolithophores across the Drake Passage during late austral summer 2016

“…Passage showed low costal coccolithophore diversity increasing towards oceanic regions (Charalampopoulou et al, 2016;Menschel et al, 2016;von Dassow et al, 2018) which contrasts with the high number of taxa recorded in this work at uppermost 100 m of the water column at the Chilean margin (Fig. 9).…”

“…In the Chilean margin, the highest coccolith masses recorded are related to the presence of E. huxleyi type A (including normal, calcified and overcalcified specimens) and type R 30 (Fig. 11), observed also by other authors at lower latitudes offshore Chile (Beaufort et al, 2008;Beaufort et al, 2011;von Dassow et al, 2018). Although it is uncommon, heavily calcified E. huxleyi morphotypes have been recorded in reduced pH and ΩCa conditions in other parts of the globe (e.g., Smith et al, 2012;Triantaphyllou et al, 2018).…”

“…Northern Hemisphere, E. huxleyi type A dominates the coccolithophore assemblage in the North Atlantic and in Norwegian coastal waters (e.g., van Bleijswijk et al, 1991;Holligan et al, 1993), but not in the Southern Ocean (e.g., Cook et al, 2011;Hagino et al, 2011). Emiliania huxleyi type R was observed for first time in the Drake Passage, although it has been previously just observed in the Eastern South Pacific (Beaufort et al, 2008;Beaufort et al, 2011;von Dassow et al, 2018). (2) The SAZ.…”

Coccolith mass and morphology of different Emiliania huxleyi morphotypes: A critical examination using Canary Islands material

“…[33,37]) and regional (e.g. [8,15,32,[38][39][40]) gradients, and might furthermore differ in their sensitivity to ocean acidification [41]. E. huxleyi morphotypes differ in robustness of distal shield elements and degree of central area closure, which is often interpreted as differences in degrees of calcite production or differences in coccolith mass between the morphotypes.…”

Particulate inorganic to organic carbon production as a predictor for coccolithophorid sensitivity to ongoing ocean acidification

“…Interestingly, in the latter study, the similarity in cellular PIC : POC between strains/morphotypes was associated with a similar OA sensitivity. This disconnect between visual and measured degree of calcification of a strain could provide an explanation of reports of “heavily calcified” morphotypes of E. huxleyi dominating in relatively low pH waters in winter in the Bay of Biscay (Smith et al ), the Benguela coastal upwelling (Henderiks et al ), and the coastal zone of central Chile (Beaufort et al ; von Dassow et al ).…”