In photosynthesis, $CO_2$ acts as a proton donor (hydrogen acceptor) in the Calvin cycle. $CO_2$ is fixed by RuBisCO and combined with hydrogen (from NADPH) to form glucose. In this reduction process, $CO_2$ receives protons (H⁺) and electrons from NADPH, being reduced to carbohydrate. So $CO_2$ is reduced — it accepts protons/electrons.