A highly pyridinic N-doped carbon with total N-species of similar to 15.5 at.% and surface area of similar to 1100m(2)/g was obtained from marine biowaste, Enteromorpha prolifera, via hydrothermal carbonization and a mild KOH activation, using melamine as nitrogen source. This offers a simple pathway for large-scale synthesis of N-doped carbon with partial spheres verified from X-ray photoelectron spectroscopy and scanning electron microscopy, showing great perspective in multifunctional activities for carbon capture, oxygen reduction reaction, and supercapacitor. The carbon shows CO2 uptake of similar to 3mmol/g under ambient conditions with isosteric heat of adsorption up to 40kJ/mol, in addition to a large capacitance of 214 F/g at 0.5 A/g in 6M KOH as electrode for supercapacitor. The supercapacitor exhibits superior cycling durability of 98% retention at 2 A/g after 10,000 cycles. Furthermore, the carbon as catalyst also exhibits good stability and resistance to methanol crossover as compared to commercial Pt/C catalyst, followed with a dominant 4e(-) transfer process.