Microbial fuel cells have been considered as biosensor for detection of assimilable organic carbon (AOC). However, the signal production of from AOC is known to be completely suppressed by dissoved oxygen. 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 was found to have electrochemically active microbial communities on the anode, such as members from Shewanellaceae, Pseudoalteromonadaceae, and Clostridiaceae families, However, members from Desulfuromonadaceae, Desulfobulbaceae and Desulfobacteraceae families were only found in the negative anodic potential MEC-biosensor, which were strictly anaerobic. The positive anodic potential MEC-biosensor has shown several advantages, including faster start-up, significantly higher maximum current production, improved assimilable organic carbon (e.g., acetate) detection limit by 5 times, and tolerance of low dissolved oxygen, compared to those obtained from the negative anodic potential MEC-biosensor. The developed positive MEC-biosensor could be effectively used as real-time and inexpensive measure of AOC concentrations of high saline and low DO seawater.