Microbial fuel cells have already been used as biosensors to monitor assimilable organic carbon (AOC). However, their signal production from AOC is known to be completely suppressed by dissoved oxygen (DO). In this study, two identical microbial electrolysis cell (MEC) based biosensors were inoculated with marine sediment and operated at two different anodic potentials, namely -300 mV and +250 mV relative to Ag/AgCl. The MEC biosensor operated under positive anodic potential conditions had electrochemically active microbial communities on the anode, including members of the Shewanellaceae, Pseudoalteromonadaceae, and Clostridiaceae families. However, the strictly anaerobic members of the Desulfuromonadaceae, Desulfobulbaceae and Desulfobacteraceae families were found only in the negative anodic potential MEC biosensor. The positive anodic potential MEC biosensor showed several other advantages as well, such as faster start-up, significantly higher maximum current production, fivefold improvement in the AOC detection limit, and tolerance of low dissolved oxygen, compared to those obtained from the negative anodic potential MEC biosensor. The developed positive anodic potential MEC biosensor can thus be used as a real-time and inexpensive detector of AOC concentrations in high saline and low DO seawater.