Enzymes operating in environmental media are of immense biotechnological interest, in particular, for sustainable energy production. Here, we show that the K316 N/D500G mutant of CotA laccase from Bacillus licheniformis operates in basic media and seawater, electrocatalyzing O ₂ reduction from 0.59 V, pH 8. CotA mutant's T1Cu center exhibits a formal potential of 0.56 V, consistent with an onset of bioelectrocatalytic reduction of O ₂ . A photoelectrochemical cell recycling H ₂ O/O ₂ by a semiconductor photoanode and CotA mutant biocathode produced 41 and 11 μW cm ² , in buffer and seawater, respectively, at pH 8, (electricity production being limited solely by the photoanode performance, reaching 240 and 220 μW cm ⁻² in a Zn-biobattery design), suggesting a prospective enzyme application for clean and sustainable production of electricity from seawater and oxygen.