Involved in the degradation of 3,6-anhydro-L-galactose, which is the major monomeric sugar of red macroalgae. Catalyzes the oxidation of 3,6-anhydro-L-galactose (AHG) to form 3,6-anhydrogalactonate (AHGA). 3,6-anhydro-alpha-L-galactopyranose + H2O + NADP(+) = 3,6-anhydro-L-galactonate + 2 H(+) + NADPH 3,6-anhydro-alpha-L-galactopyranose + H2O + NAD(+) = 3,6-anhydro-L-galactonate + 2 H(+) + NADH Could be used for bioconversion of red macroalgal biomass into biofuels or industrial chemicals. Belongs to the aldehyde dehydrogenase family.