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.
Highlights
Coccolithophores are planktonic single-celled photoautotrophs mostly in the range of 3–20 μm and characterized by bearing calcite plates (Monteiro et al, 2016) and represent one of the most abundant and widespread groups of marine eukaryotic phytoplankton (Iglesias-Rodríguez et al, 2002; Litchman et al, 2015)
In terms of carbonate chemistry, the surface waters of the ESP showed a general pattern of increasing CO2 and decreasing pH as one moves from open ocean waters to the Chilean coastal upwelling zones; as expected, waters were never corrosive for calcite (Fig. 3a)
While an increasing number of studies have focused on examining the potential for adaptation to ocean acidification (OA) through long-term laboratory experiments, this study has taken an alternative approach, to test for local adaptation to short-term high-CO2 and low-pH exposure in populations of cosmopolitan phytoplankton found in waters that experience naturally acidified conditions due to upwelling of high-CO2 water
Summary
Coccolithophores are planktonic single-celled photoautotrophs mostly in the range of 3–20 μm and characterized by bearing calcite plates (coccoliths) (Monteiro et al, 2016) and represent one of the most abundant and widespread groups of marine eukaryotic phytoplankton (Iglesias-Rodríguez et al, 2002; Litchman et al, 2015). CaCO3 precipitation in the surface is a source of CO2, i.e., the “carbonate counter pump” (Frankignoulle et al, 1994), CaCO3 may enhance sinking of organic matter by imposing a ballast effect on sinking aggregates (Armstrong et al, 2002; Sanders et al, 2010). This plankton functional group has a complex role in ocean carbon cycles. Understanding the response of coccolithophores to OA is needed for predicting how pelagic ecosystems and the relative intensity of the biological carbon pumps will change as atmospheric CO2 continues to increase
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
