Microbial fuel cells in coral reef sediments as indicator tools for organic carbon eutrophication

Abstract

Eutrophication with organic carbon (OC) can be harmful for corals and their reefs due to its stimulating effects on associated and sedimentary microbes, leading to oxygen deficits. Mitigation measures require real-time monitoring of wastewater pulses close to coral reefs, where biogenic carbonate sands act as biocatalytic filters for OC. Microbial fuel cells (MFCs) where the anode is deployed in sediments and the cathode is in contact with the overlying water can directly generate electricity from the microbial degradation of OC in sediments, but their application in coral reefs has not yet been tested. During a laboratory experiment, we thus investigated if MFCs, vertically deployed in coral reef sands, can be used as indicator tools for eutrophication with artificial wastewater (AW, prepared from organic compounds and chemicals) at OC concentrations of 20, 40, or 52 mg C L-1 higher than controls. AW pulses were repeated three times at every concentration, with five weeks between the first and second pulse, and six months between the second and third pulse. Results revealed significant increases in current densities in all AW treatments (means up to 11, 36, and 39 mA m−2, respectively), while controls remained stable and low (-1 to 3 mA m−2). Current densities and OC concentrations correlated significantly (rrm = 0.64), and the slope of the correlation increased with each consecutive AW pulse. This highlights the functionality of the MFC as a qualitative indicator tool for OC pulses even months after deployment. The response time of the MFCs was fast (<1 day) compared to other indicators for water quality (e.g., coral colony- or community measures that take weeks to years to respond), and they successfully detected OC concentrations expected for wastewater effluents. Measurements can be automated for continuous monitoring, and no laboratory facilities are required (as for OC analysis), making MFC sensors a suitable tool for remote locations. Overall, these findings emphasize the high potential of these low-cost (<20 € per MFC) MFCs as indicator tools for OC pulses in coral reef environments.