| Fe limitation is well documented in oceanic high-nutrient, low chlorophyll (HNLC) regions. As a complementary approach to identify Fe deficiency in marine or freshwater systems, my research has focused on the development and characterization of biochemical diagnostics—in particular, the redox catalysts flavodoxin and ferredoxin (Fd)—to be used as probes of Fe deficiency in the natural environment. In many photoautotrophs, Fd is functionally replaced by flavodoxin during periods of Fe deficiency in order to relieve a portion of the cellular Fe burden. A Fd index (Fd/[Flvd+Fd]) then serves as a valid proxy of physiological Fe deficiency.; Polyclonal antisera were raised against flavodoxin and Fd purified from a marine cryptophyte alga Rhodomonas lens, and their utility as diagnostic reagents for determining the Fe status of freshwater and marine cryptophytes were assessed. In addition, physiological parameters were used to assess the extent of Fe deficiency in the various microalgae surveyed.; Comparing two freshwater cryptomonads (Cryptomonas sp. UTCC 337 and C. erosa UTCC 446) cultured under a range of Fe additions, we demonstrated that Cryptomonas sp. was more tolerant of Fe deprivation compared with C. erosa. Whereas Fd was detected in cells provided 100 nM Fe and 1,000 nM Fe, Fd was not detected in low Fe culture (10 nM Fe). Concomitant accumulation of flavodoxin, however, was not demonstrated using antiserum raised from the protein purified from R. lens.; To characterize our antibodies for their potential as broad taxonomic biomarkers of Fe deficiency, a species survey was performed to assess the reactivity of anti-Fd and anti-flavodoxin with other marine and freshwater cryptomonds, and additionally with two red microalgae. Anti-Fd cross-reacted with proteins of appropriate size in extracts from each of the cryptomonads tested and from one of the red algal representatives. In contrast, anti-flavodoxin did not cross-react with any of the species surveyed apart from R. lens. A more comprehensive analysis of Fd and flavodoxin accumulation in R. lens grown under various levels of Fe addition demonstrated an apparent uncoupling of flavodoxin expression from physiological Fe deficiency of in this species. We also provided evidence that flavodoxin accumulation in R. lens appears to be in response to the external Fe concentration rather than internal Fe quotas. |
