Application of the plant microbial fuel cell (PMFC) in wetlands should be invisible without excavation of the soil. The preferred design is a tubular design with the anode directly between the plant roots and an oxygen reducing biocathode inside the tube. Oxygen should be passively supplied to the cathode via a gas diffusion layer. In this research silicone was successfully used as gas diffusion layer. The objective of this research is to start-up an oxygen reducing biocathode in situ in a tubular PMFC applied in a Phragmites australis peat soil and a Spartina anglica salt marsh. PMFCs with a biocathode were successfully started in the peat soil. Oxygen reduction is clearly catalysed, likely by microorganisms in the cathodes, as the over potential decreased resulting in an increased current density and cathode potential. The maximum daily average power generation of the best peat soil PMFC was 22 mW m(-2). PMFCs with a biocathode in the salt marsh only started with pure oxygen diffusion reaching a maximum daily average power generation of 82 mW m(-2). Both wetland PMFCs were successfully started with natural occurring microorganism in the anode and cathode. Calculations show that the power density can be increased by improving the PMFC design limiting crossover of oxygen and substrate. (C) 2016 Elsevier Ltd. All rights reserved.