Rapid Opto-electrochemical Differentiation of Marine Phytoplankton

Abstract

The use of electro-generated oxidants in seawater facilitates the discrimination of different plankton groups via monitoring the decay in real time of their chlorophyll-a (chl-a) fluorescence signals following potentiostatic initiation of electrolysis in their vicinity (Yang, M. et al. Chem. Sci.2019, 10(34), 7988−7993). In this paper, we explore the sensitivity of phytoplankton to different chemical species produced at various potentials in seawater. At low potentials, the oxidation of ca. millimolar bromide naturally present in seawater to hypobromous acid 'switches-off' the chl-a signal of individual Chlamydomonas concordia cells (green algae) located on the electrode surface within tens of seconds of the potential onset. At higher oxidative potentials, the oxidation of chloride and water produces oxidants (Cl 2 , OH, H 2 O 2 , etc.) that are also lethal to the plankton. To deconvolute the contributions to the response from the chemical identity of the oxidant and the amount of charge delivered to 'titrate' the individual living plankton using the loss of fluorescence as the 'end point', we introduce a ramped galvanostatic method. This approach enabled the controlled injection of charge applied to a bespoke electrochemical cell in which the plankton are immobilized on an electrode surface for rapid and sensitive measurement. It is shown that the number of moles (charge) of oxidants required to react leading to chl-a switch-off is independent of the chemical identity of the electro-generated oxidant(s) among hypobromous acid, chlorine, or water-derived oxidants. Comparative experiments between C. concordia and Emiliania huxleyi (where the latter are encapsulated by extracellular plates of calcium carbonate) show that significantly different amounts of absolute charge (moles of electro-generated oxidants) are required in each case to 'switch-off' the chl-a signal. The method provides the basis for a tool that could distinguish between different plankton cells within ca. 2 min including the setup time.

A novel fluoro‐electrochemical technique for classifying diverse marine nanophytoplankton

“…The fluoro‐electrochemical measurements were made as follows ( see Fig. 1a, adapted from Yu et al 2022): A concentrated sample from a phytoplankton monoculture in exponential growth phase (as described in the Culturing for the “susceptibility library” section) was “drop cast” onto the surface of the working electrode. After ~ 1 min of allowing the cells to be deposited on the surface, excess solution was gently drawn‐off using a tissue.…”

“…( a ) a simple schematic diagram of the electrochemistry—fluorescence microscope set up adapted from Yu et al (2022); ( b ) an example of normalized Chl a fluorescence transient data for the species Phaeocystis globosa , following onset of the potential (at time 0 s) and with a current ramping of 10 μ A s −1 . Each black line represents a single cell measurement ( n = 110), and the dashed red markings highlight the time point, t 50 , where normalized fluorescence values have been reduced by 50%.…”

“…A more detailed description of the fundamental electrochemical principles and details of the underlying methodology, including specifics of the equipment used, can be found in the previously published work by Yu et al (2022). For the purposes of this study, the step-by-step method described below outlines the essential procedures that were followed to yield the underlying dataset for the susceptibility library.…”

“…Moving forward, recent developments in the field of analytical chemistry have shown that electrochemically induced oxidative stress destroys phytoplankton chlorophyll a (Chl a ) fluorescence in a manner that is idiosyncratic, allowing for differentiation of phytoplankton species from measurements that are obtained within 10s of seconds (Yang et al 2019; Yu et al 2022). When a sufficiently high potential is applied to an electrode that is immersed in seawater, a wide range of oxidants can form and diffuse from the electrode surface, for example; oxidation of water to hydrogen peroxide and hydroxyl radicals, bromide to hypobromous acid, and chloride to dichlorine (Yang et al 2019; Yu et al 2022). The subsequent reaction of these oxidants with phytoplankton is seen to cause a rapid decay of the cellular Chl a fluorescence signal.…”

AI facilitated fluoro-electrochemical phytoplankton classification

“…16 More importantly, it has been reported that although different life-stages coexisted within the population of a species of marine green algae, Chlamydomonas concordia, the size-normalized susceptibility was not affected by its life stages. 17,18 However, considering the vast number of phytoplankton species in the contemporary ocean (∼5000), 19 systematically classifying them using a single susceptibility parameter is challenging.…”