FLAVODOXIN EXPRESSION AS AN INDICATOR OF IRON LIMITATION IN MARINE DIATOMS

We have previously shown that a marine chlorophyte expressed flavodoxi n under iron limitation but not under other nutrient stress conditions . Here we use polyclonal antiserum raised against the diatom Phaeodact ylum tricornutum Bohlin to show that a similar response is observed in this species. Using our antibody, western blotting techniques, and st andard colorimetric detection (4-chloro-1-naphthol), we can detect at least a 25-50-fold increase in flavodoxin in iron-depleted compared to iron-replete cells. In iron-limited batch cultures of P. tricornutum, flavodoxin accumulation was inversely proportional to growth rate and was not detectable in cultures containing initially more than 750 nM of iron. We demonstrated that the accumulation of flavodoxin under iro n stress is wide spread among marine diatoms and that it may be possib le to use the presence or absence of flavodoxin in natural marine diat om assemblages to detect iron limitation. However, our polyclonal anti sera appears to be specific for diatoms and did not cross-react with S ynechococcus sp., Micromonas pusilla (Butcher) Manton et Parke, Dunali ella tertiolecta Butcher, Chlorella sp., Emiliania huxleii (Lohm.) Hay et Parke, or Isochrysis galbana Parke. A reverse bioassay experiment was conducted with natural phytoplankton assemblages containing mainly diatoms from Long Island Sound and in shelf waters near Cape Hatteras , two areas not suspected to be iron-limited. Although flavodoxin was not detected in situ in these areas, natural populations of diatoms dr iven into iron limitation expressed flavodoxin. Flavodoxin was detecte d in mats of the diatom Rhizosolenia castrecanei Cleve collected from the Equatorial Pacific during a JGOFS cruise in 1992, consistent with the hypothesis that iron may be limiting in this high-nutrient, low-ch lorophyll region.