Opto‐Electrochemical Dissolution Reveals Coccolith Calcium Carbonate Content

Abstract

Coccoliths are plates of biogenic calcium carbonate secreted by calcifying marine phytoplankton; annually these phytoplankton are responsible for exporting >1 billion tonnes (1015 g) of calcite to the deep ocean. Rapid and reliable methods for assessing the degree of calcification are technically challenging because the coccoliths are micron sized and contain picograms (pg) of calcite. Here we pioneer an opto‐eletrochemical acid titration of individual coccoliths which allows 3D reconstruction of each individual coccolith via in situ optical imaging enabling direct inference of the coccolith mass. Coccolith mass ranging from 2 to 400 pg are reported herein, evidencing both inter‐ and intra‐species variation over four different species. We foresee this scientific breakthrough, which is independent of knowledge regarding the species and calibration‐free, will allow continuous monitoring and reporting of the degree of coccolith calcification in the changing marine environment.

Single-entity coccolithophore electrochemistry shows size is no guide to the degree of calcification

“…20 mM of Ca 2+ was used to prevent the dissolution of biogenic calcite prior to the start of the experiment, discussed in detail in ESI Section 3, † and hydroquinone was used as an acid precursor which releases two stoichiometric equivalents of protons when electrochemically oxidised at the electrode interface (Scheme 1). 22 Shown in Fig. 2c are optical images of a representative C. braarudii coccosphere undergoing electrochemically induced acid dissolution with 10 mM of hydroquinone.…”

“…This switch-over in the acid dissolution of calcite occurs around a particle radius of 10 mm. 22,47 It is important to note that the mass-transport limited rate is proportional to the geometric size of the particle and the surface-limited rate of reaction is proportional to the specic surface area of the particle (4pR f r 2 ). For the size range of coccolithophores in this study, the rate of the acid dissolution is neither limited by the mass-transport protons nor it is surfacelimited, but is subject to mixed kinetics.…”

“…20 mM of Ca 2+ was used to prevent the dissolution of biogenic calcite prior to the start of the experiment, discussed in detail in ESI Section 3,† and hydroquinone was used as an acid precursor which releases two stoichiometric equivalents of protons when electrochemically oxidised at the electrode interface (Scheme 1). 22…”

“…21 Single coccoliths may become detached from coccolithophores either during the lifecycle of the latter or aer cellular death. Although existing techniques allow the PIC content of individual coccoliths to be either measured [22][23][24] or estimated, 25 this, ultimately, provides calcication information about the past, not the present. To monitor and predict the imminent threat of climate change on our dominant openocean calciers we must quantify the PIC content bioaccumulated by coccolithophores in the present marine environment.…”

“…More recently, proof-of-concept electrochemical methods have shown their ability to identify, in combination with fluorescence spectroscopy, the speciation of marine phytoplankton and, 32 separately, single-entity measurement of detached coccolith CaCO 3 masses. 22 In the present study, electrochemistry is utilised to provide a high-throughput single-entity PIC content measurement of living coccolithophores. Herein, acid is generated electrochemically to dissolve away the CaCO 3 shell of the individual coccolithophores to reveal the underlying ‘naked’ cell.…”