Responses of the Emiliania huxleyi Proteome to Ocean Acidification

Bethan M Jones,M Debora Iglesias-Rodriguez,Henry Elderfield,Paul J Skipp,Richard J Edwards,Jeremy R Young,C David O'Connor,Mervyn J Greaves

doi:10.1371/journal.pone.0061868

Abstract

Ocean acidification due to rising atmospheric CO2 is expected to affect the physiology of important calcifying marine organisms, but the nature and magnitude of change is yet to be established. In coccolithophores, different species and strains display varying calcification responses to ocean acidification, but the underlying biochemical properties remain unknown. We employed an approach combining tandem mass-spectrometry with isobaric tagging (iTRAQ) and multiple database searching to identify proteins that were differentially expressed in cells of the marine coccolithophore species Emiliania huxleyi (strain NZEH) between two CO2 conditions: 395 (∼current day) and ∼1340 p.p.m.v. CO2. Cells exposed to the higher CO2 condition contained more cellular particulate inorganic carbon (CaCO3) and particulate organic nitrogen and carbon than those maintained in present-day conditions. These results are linked with the observation that cells grew slower under elevated CO2, indicating cell cycle disruption. Under high CO2 conditions, coccospheres were larger and cells possessed bigger coccoliths that did not show any signs of malformation compared to those from cells grown under present-day CO2 levels. No differences in calcification rate, particulate organic carbon production or cellular organic carbon: nitrogen ratios were observed. Results were not related to nutrient limitation or acclimation status of cells. At least 46 homologous protein groups from a variety of functional processes were quantified in these experiments, of which four (histones H2A, H3, H4 and a chloroplastic 30S ribosomal protein S7) showed down-regulation in all replicates exposed to high CO2, perhaps reflecting the decrease in growth rate. We present evidence of cellular stress responses but proteins associated with many key metabolic processes remained unaltered. Our results therefore suggest that this E. huxleyi strain possesses some acclimation mechanisms to tolerate future CO2 scenarios, although the observed decline in growth rate may be an overriding factor affecting the success of this ecotype in future oceans.

Highlights

Anthropogenic increases in CO2 are expected to cause a reduction in ocean pH within the century [1] with unknown consequences for marine organisms
CO2 compared to the blank, indicating that a semi-constant chemistry state was maintained throughout these experiments by keeping cell densities,105 cells mL21
Physiological properties of Emiliania huxleyi NZEH under high CO2

Summary

Introduction

Anthropogenic increases in CO2 are expected to cause a reduction in ocean pH within the century [1] with unknown consequences for marine organisms. Dissolution of CO2 in seawater causes the formation of carbonic acid (H2CO3), which dissociates to form bicarbonate (HCO3 2) and H+ ions, causing a reduction in pH [2]. Liberated H+ ions react with carbonate ions (CO3 22), further increasing [HCO32] whilst simultaneously causing a decrease in [CO3 22]. This reaction results in a drop in the saturation state of the mineral form of calcite (V-cal). Ocean acidification is expected to affect calcifying marine organisms that produce external CaCO3 structures such as shells, plates and exoskeletons [3,4]

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