Temperature Modulates Coccolithophorid Sensitivity of Growth, Photosynthesis and Calcification to Increasing Seawater pCO2

Ulf Riebesell,Mario Lebrato,Lennart T. Bach,Scarlett Sett,Signe Koch-Klavsen,Kai G. Schulz

doi:10.1371/journal.pone.0088308

Ulf Riebesell, Mario Lebrato + Show 4 more

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https://doi.org/10.1371/journal.pone.0088308

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Abstract

Increasing atmospheric CO2 concentrations are expected to impact pelagic ecosystem functioning in the near future by driving ocean warming and acidification. While numerous studies have investigated impacts of rising temperature and seawater acidification on planktonic organisms separately, little is presently known on their combined effects. To test for possible synergistic effects we exposed two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, to a CO2 gradient ranging from ∼0.5–250 µmol kg−1 (i.e. ∼20–6000 µatm pCO2) at three different temperatures (i.e. 10, 15, 20°C for E. huxleyi and 15, 20, 25°C for G. oceanica). Both species showed CO2-dependent optimum-curve responses for growth, photosynthesis and calcification rates at all temperatures. Increased temperature generally enhanced growth and production rates and modified sensitivities of metabolic processes to increasing CO2. CO2 optimum concentrations for growth, calcification, and organic carbon fixation rates were only marginally influenced from low to intermediate temperatures. However, there was a clear optimum shift towards higher CO2 concentrations from intermediate to high temperatures in both species. Our results demonstrate that the CO2 concentration where optimum growth, calcification and carbon fixation rates occur is modulated by temperature. Thus, the response of a coccolithophore strain to ocean acidification at a given temperature can be negative, neutral or positive depending on that strain's temperature optimum. This emphasizes that the cellular responses of coccolithophores to ocean acidification can only be judged accurately when interpreted in the proper eco-physiological context of a given strain or species. Addressing the synergistic effects of changing carbonate chemistry and temperature is an essential step when assessing the success of coccolithophores in the future ocean.

Highlights

Increasing fossil fuel emissions and deforestation over the past 250 years have increased atmospheric CO2 concentrations at an unprecedented pace and caused a rise in global average temperatures as well as changes in ocean carbonate chemistry [1,2]
Monospecific cultures of the coccolithophores E. huxleyi and G. oceanica were grown in dilute batch cultures [32] in a broad CO2 gradient at three different temperature regimes
The coccolithophores E. huxleyi and G. oceanica were incubated in a broad CO2 gradient and three different temperatures

Summary

Introduction

Increasing fossil fuel emissions and deforestation over the past 250 years have increased atmospheric CO2 concentrations at an unprecedented pace and caused a rise in global average temperatures as well as changes in ocean carbonate chemistry [1,2]. Coccolithophores are single-celled autotrophic phytoplankton capable to precipitate calcium carbonate (CaCO3) as small scales (coccoliths) to cover the organic part of the cell. They are considered the most important calcareous primary producers contributing ,1–10% to today’s oceanic primary production [6] and ,50% to CaCO3 found in pelagic sediments [7]. They are key players in marine biogeochemical cycles [8,9]. G. oceanica is generally less abundant than E. huxleyi but its contribution to CaCO3 export into the deep is larger because its coccoliths contain significantly more CaCO3 [21]

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