THE surface waters of the equatorial Pacific have unusually high nitrate and phosphate concentrations, but relatively low phyto-plankton biomass^1–3. This 'high nitrate, low chlorophyll' (HNLC)⁴ phenomenon has been ascribed to 'top-down' grazing pressure by herbivores, which prevent the phytoplankton from fully utilizing the available nutrients⁵. In the late 1980s, however, Martin and co-workers proposed that iron, which is delivered to the remote open ocean in aeolean dust⁶, is the key factor limiting the standing crop of phytoplankton in HNLC areas^7,8. Using a sensitive fluor-escence method⁹, we have followed changes in photochemical energy conversion efficiency^9–10 of the natural phytoplankton com-munity both before and after artificial enrichment with iron of a small area (7.5 x 7.5 km) of the equatorial Pacific Ocean¹¹. Our results show that iron limits phytoplankton photosynthesis in all size classes in this region by impairing intrinsic photochemical energy conversion, thereby supporting the hypothesis of physiologi-cal ('bottom up') limitation by this element.