Objective  Intensity of LED light for optimal growth of free-living conchocelis of Pyropia haitanensis (FCP) was studied.

  Method  The effects of varied light intensities (i.e., 10, 20, 40, 80 or 100 μmol·photons m^-2·s^-1) using LED on the growth, maximal quantum efficiency (Fv/Fm) on PS Ⅱ, content of photosynthetic pigments as well as the photosynthesis and antioxidant enzymes of Shenfu No.2, a new cultivar of P. haitanensis, were determined.

  Result  After 25 d of culturing, the weight of algae increased with increasing intensity of illumination and peaked when 80 μmol·photons m^-2·s^-1 was applied. As the weight reached the maximum, the Fv/Fm on PS Ⅱ was also at its highest level. On the other hand, the contents of chlorophyll a, carotenoids, and phycoerythrin decreased gradually as the light intensity increased. No significant differences on phycocyanin among the groups were observed. The greatest activities of CA and RubisCO in the algae were observed under 40 μmol·photons m^-2·s^-1 followed by 80 μmol·photons m^-2·s^-1 treatment before a decline. Upon an increase on light intensity, the activities of superoxide anion radicals and SOD decreased to the lowest level under 80 μmol·photons m^-2·s^-1 and then raised again.

  Conclusion  The vegetative filaments of FCP cultured under 80 μmol·photons m^-2·s^-1 light appeared to render the highest algae growth rate and Fv/Fm, lowest activities of superoxide anions and SOD, and strongest activities of RubisCO and CA.