Flavodoxin as an in situ marker for iron stress in phytoplankton

A fundamental issue in marine science is the identification of the factors controlling biological uptake of CO₂ in high-nitrate, low- chlorophyll regions. A recent in situ iron fertilization experiment demonstrated that iron limitation is responsible for low phytoplankton stocks in the equatorial Pacific. Here we show that flavodoxin, a biochemical marker of iron limitation, can be used to map the degree of iron stress in natural populations. Flavodoxin assays along a 900-km east-west transect in the northeastern subarctic Pacific revealed a pronounced increase in iron stress in the region west of the 135° W meridian. Addition of dissolved iron alleviated this stress. Immunostaining of single cells from the most western station showed that flavodoxin is present specifically within the chloroplasts of diatoms. Our approach provides a rapid means of defining the extent of iron stress in the ocean and supports the hypothesis that diatoms are iron stressed in the northeast Pacific.