The pigment composition of diatoms differs from that of green algae and plants. Diatoms contain chlorophyll (Chl(1)) c, fucoxanthin, and diadinoxanthin (DD). An intermittent light regime during growth induced a large increase in the DD content in the marine planktonic diatom Phaeodactylum tricornutum. Light-harvesting complex containing fucoxanthin (LHCF) subunits were purified on a sucrose gradient after treatment of thylakoid membranes with a mild detergent. DD was found in all the LHCF fractions: a "major" composite LHCF fraction and the two fractions where some LHCF was associated with photosystem centers. For cells enriched in DD, most of the additional DD molecules were bound to the major LHCF fraction. The DD enrichment of the major LHCF fraction was accompanied by a decrease in the fucoxanthin to Chl a ratio. Either some fucoxanthin molecules were replaced by DD or there could be a relative enrichment of subunits rich in DD at the expense of fucoxanthin/Chl c rich subunits. Under high light illumination, a higher degree of de-epoxidation of DD into DT was observed for the major LHCF of cells enriched in DD. This fraction has the higher DD content and the higher degree of de-epoxidation. These results show that the distal antennae, probably mostly isolated as the major LHCF fraction, play a crucial role in the formation of NPQ, its amplitude depending on the amount of DD bound and on the degree of de-epoxidation (Lavaud et al. (2002) Plant Physiol. 129, 1398-1406).