Probable iron-sulfur flavoprotein that converts 7-hydroxymethyl chlorophyll a to chlorophyll a using ferredoxin as a reducing equivalent. Catalyzes the reduction of a hydroxymethyl group to a methyl group. chlorophyll a + H2O + 2 oxidized [2Fe-2S]-[ferredoxin] = 7(1)-hydroxychlorophyll a + 2 H(+) + 2 reduced [2Fe-2S]-[ferredoxin] Belongs to the FrhB family.