Lycopene cyclases cyclize the open ends of acyclic lycopene (ψ，ψ-carotene) into β- or ε-ionone rings in the crucial bifurcation step of carotenoid biosynthesis. Among all carotenoid constituents， β-carotene (β，β-carotene) is found in all photosynthetic organisms， except for purple bacteria and heliobacteria， suggesting a ubiquitous distribution of lycopene β-cyclase activity in these organisms. In this work， we isolated a gene () encoding a lycopene β-cyclase from ， a red alga that is considered to be one of the primitive multicellular eukaryotic photosynthetic organisms and accumulates carotenoid constituents with both β- and ε-rings， including β-carotene， zeaxanthin， α-carotene (β，ε-carotene) and lutein. Functional complementation in demonstrated that BfLCYB is able to catalyze cyclization of lycopene into monocyclic γ-carotene (β，ψ-carotene) and bicyclic β-carotene， and cyclization of the open end of monocyclic δ-carotene (ε，ψ-carotene) to produce α-carotene. No ε-cyclization activity was identified for BfLCYB. Sequence comparison showed that BfLCYB shares conserved domains with other functionally characterized lycopene cyclases from different organisms and belongs to a group of ancient lycopene cyclases. Although also synthesizes α-carotene and lutein， its enzyme-catalyzing ε-cyclization is still unknown.