A fundamental issue in marine science is the identification of the fac
tors controlling biological uptake of CO2 in high nitrate, low-chlorop
hyll regions(1-10). A recent in situ iron fertilization experiment dem
onstrated that iron limitation is responsible for low phytoplankton st
ocks in the equatorial Pacific(4), Here we show that flavodoxin, a bio
chemical marker of iron limitation, can be used to map the degree of i
ron stress in natural populations. Flavodoxin assays along a 900-km ea
st-west transect in the northeastern subarctic Pacific revealed a pron
ounced increase in iron stress in the region west of the 135 degrees W
meridian. Addition of dissolved iron alleviated this stress. Immunost
aining of single cells from the most western station showed that flavo
doxin is present specifically within the chloroplasts of diatoms. Our
approach provides a rapid means of defining the extent of iron stress
in the ocean(5) and supports the hypothesis that diatoms are iron stre
ssed in the northeast Pacific(6,10).