Effect of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on the properties and sinking velocity of aggregates of the coccolithophore &amp;lt;I&amp;gt;Emiliania huxleyi&amp;lt;/I&amp;gt;

A Biermann,A Engel

doi:10.5194/bg-7-1017-2010

Abstract

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.

Highlights

In times of rising atmospheric CO2, the marine carbon cycle receives special attention
We investigated the influence of CO2 on the aggregation and sinking behaviour of the coccolithophore Emiliania huxleyi (PML B92/11) during a laboratory experiment
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, in areas dominated by coccolithophores

Summary

Introduction

In times of rising atmospheric CO2, the marine carbon cycle receives special attention. A small fraction of exported carbon is deposited in the worlds’ oceans sediments where it is partly conserved on a time scale of years to millennia. The major fraction of organic matter is transported to the deep ocean by aggregates, in form of marine snow or fecal pellets (Honjo, 1982; Fowler and Knauer, 1986). Recent findings suggest that the flux of organic matter at depth ∼1800 m is directly related to the fluxes of ballast minerals (Armstrong et al, 2002; Francois et al, 2002; Klaas and Archer, 2002). A comparison of sediment trap data below 1000 m from 52 locations around the world confirmed that most of the organic carbon that is transferred to the deep sea is carried by calcium carbonate (Klaas and Archer, 2002)

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