Bioelectrochemical detection of cyanobacteria for harmful bloom management

Abstract

High levels of nutrients and increasing temperatures favour the periodic proliferation of harmful cyanobacterial blooms with catastrophic effects for aquatic ecosystems. The production of toxins that occurs naturally as secondary metabolites is a major concern for public health. Effective in situ monitoring of cyanobacterial populations is therefore essential.Various cyanobacteria species have been shown to carry out extracellular electron transfer (EET) to insoluble acceptorse.g. to manage reductive stress caused by excessive light exposureand are thus able to produce a measurable electrical current.This PhD project focused on exploring cyanobacterial EET to set foundations for the development of a cyanobacteria-monitoring device for early detection and management of harmful blooms. This technology exploits cyanobacteria's ability to produce electrical current to monitor these microorganisms in water reservoirs. Yet, occurrence of EET in cyanobacteria is not fully understood. There is little information on the redox potential of the reactions involved and their dependence on i.e. pH and illumination. Furthermore, there is no available method for efficient current collection from a diluted cyanobacteria suspension like those that inhabit water reservoirs.Hence, the following research objectives were pursued: (i) to investigate driving factors for EET occurrence in cyanobacteria, (ii) to elucidate the redox potential of the reactions involved in EET for different cyanobacteria species and their pH dependence, (iii) to develop a cyanobacteria sensor prototype. Three-electrode photoelectrochemical cells were used, with working electrodes poised at +0.6V vs. SHE for chronoamperometric measurements, and potential scans between -0.1 V vs. SHE and 1 V vs. SHE for all voltammetric techniques.The first research objective focused on exploring the effect of increased reductive stress triggered by illumination and absence of electron acceptors such as O2 and CO2, and the effect of pH on current generation. Observations confirmed that EET onto polarised electrodes is triggered by increasing reductive stress, and also by medium pH levels above 7.8 for the cyanobacteria species Microcystis aeruginosa. Current density increased with stepwise pH increases from about 5 mA m -2 at pH 7.8 to 30 mA m -2 at pH 10.5, for dense (0.4 mg mL -1 ) Microcystis aeruginosa suspensions with dissolved CO2 and O2 approaching equilibrium with atmospheric concentrations. This rise in current density was greater for suspensions subject to higher reductive stress, i.e. with negligible dissolved CO2 and O2.Current density under illumination for these suspensions increased stepwise with pH from 5 iv |