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

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. The results show that CO2 induced changes in the inorganic carbon to organic carbon ratio (PIC/POC) influence the porosity and hence the sinking velocity of aggregates of Emiliania huxleyi. Average sinking velocity was highest for low CO2 aggregates (~1292 m d−1) that also had the highest 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 an excess density about one order of magnitude lower (~4.2×10−4 g cm−3) when compared to aggregates from the medium and low CO2 treatments (~1.7×10−3 g cm−3). Thus, the amount of calcite in aggregates is crucial for the degree of ballasting effect. In the high CO2 treatment, aggregates with lower calcite content had higher bacterial abundance, suggesting enhanced bacterial degradation. Thus, our findings indicate that a CO2 induced reduction of calcite content aggregates could affect the vertical export of organic matter in the ocean, particularly in areas dominated by coccolithophores blooms.

Biermann, A. & Engel, A., 2009. Effect of CO2 on the properties and sinking velocity of aggregates of the coccolithophore Emiliania huxleyi. Biogeosciences Discussions 6(5): 9817-9848. Article.

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