K-001

Richard G. Compton

richard.compton@chem.ox.ac.uk

M. Yang, C. Batchelor-McAuley, H. Chen

University of Oxford, Great Britain

Electrochemistry and Single Entity Plankton

Phytoplankton are microscopic, photosynthetic plant cells which are too small to be seen with the naked eye.  Although they account for less than 1% of the planet’s total biomass they are estimated to produce 50% of the atmospheric oxygen. Phytoplankton are remarkably diverse yet they all convert sunlight and dissolved carbon dioxide into particulate organic carbon (biomass). One specific group of phytoplankton, coccolithophores, additionally sequester dissolved carbon dioxide as calcium carbonate (calcite) forming a shell of inorganic ca micron-sized platelets (“coccoliths”). The shells ultimately find their way from surface water to the deep  where they may remain for millions of years.  It is estimated that more than 1015 tonnes of atmospheric CO2 per year are sequestered in this way! This rate is similar to that of the anthropogenic release of carbon dioxide. For this reason, and because phytoplankton are the basis of the oceanic food chain they are an important sensing target for oceanic health. Moreover as they are not farmed but, as a result of their short lifetimes are quick to respond to local changes in the environment they have been referred to as a ‘beacon of climate change”.

The lecture will discuss the basis and application of two different electrochemical experiments for monitoring plankton. First electrochemistry in conjunction with optical microscopy will show how the quantity of biomineralized CO2 in the form of calcite can be measured at the single coccolithophore level by imaging the cell as it is exposed to electrogenerated protons which chemically react with and remove the shell. Knowledge of the diffusion field around the electrode in a bespoke electrochemical cell coupled with a knowledge of the rate of proton induced calcite dissolution allows the determination of CaCO3 masses spanning over three orders of magnitudes, ranging from tens of pico-grams to tens of nanograms.

Second electrochemical-fluorescence measurements allow the counting and identification of plankton. By immobilizing phytoplankton cells on an electrode and applying a controlled potential step or galvanostatic sweep the chlorophyll fluorescence is monitored as a function of time to reveal a rapid species-specific decay of fluorescence with the different switch-off times and fluorescence transient shape reflecting differences in exoskeleton structure, cellular membranes and cellular biology allowing the development of a low-powered, automated fluoroelectrochemical based sensor so facilitating phytoplankton classification and identification within tens of seconds., the task being simplified by the use of a 1-D inception neural network6 to analyze the fluoro-electrochemical transients of 500 unseen phytoplankton single cell transients spanning 29 phytoplankton strains, into their taxonomic orders with a better than 95% accuracy.