Effect of CO₂ on the properties and sinking velocity of aggregates of the coccolithophore Emiliania huxleyi

Abstract. Coccolithophores play an important role in organic matter export due to their production of the mineral calcite that can act as ballast. Recent studies indicated that calcification in coccolithophores may be affected by changes in seawater carbonate chemistry. We investigated the influence of CO₂ on the aggregation and sinking behaviour of the coccolithophore Emiliania huxleyi (PML B92/11) during a laboratory experiment. The coccolithophores were grown under low (~180 μatm), medium (~380 μatm), and high (~750 μatm) CO₂ conditions. Aggregation of the cells was promoted using roller tables. Size and settling velocity of aggregates were determined during the incubation using video image analysis. Our results indicate that aggregate properties are sensitive to changes in the degree of ballasting, as evoked by ocean acidification. Average sinking velocity was highest for low CO₂ aggregates (~1292 m d⁻¹) that also had the highest particulate inorganic to particulate organic carbon (PIC/POC) ratio. Lowest PIC/POC ratios and lowest sinking velocity (~366 m d⁻¹) at comparable sizes were observed for aggregates of the high CO₂ treatment. Aggregates of the high CO₂ treatment showed a 4-fold lower excess density (~4.2×10⁻⁴ g cm⁻³) when compared to aggregates from the medium and low CO₂ treatments (~1.7 g×10⁻³ cm⁻³). We also observed that more aggregates formed in the high CO₂ treatment, and that those aggregates contained more bacteria than aggregates in the medium and low CO₂ treatment. If applicable to the future ocean, our findings suggest that a CO₂ induced reduction of the calcite content of aggregates could weaken the deep export of organic matter in the ocean, particularly in areas dominated by coccolithophores.