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Author(s): Matthew M. Mills; Celine Ridame; Margaret Davey; Julie La Roche; Richard J. Geider Nature 429, 292-294 (2004).10.1038/nature02550 In this Letter to Nature , the chlorophyll a data presented in [...]

We have previously shown that a marine chlorophyte expressed flavodoxin under iron limitation but not under other nutrient stress conditions. Here we use polyclonal antiserum raised against the diatom Phaeodactylum tricornutum Bohlin to show that a similar response is observed in this species. Using our antibody, western blotting techniques, and standard colorimetric detection (4-chlorol-naphthol), we can detect at least a 25-50-fold increase in flovodoxin 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 occumulation of lovodoxin under iron stress is widespread among marine diatoms and that in may be possible to use the presence or absence of flovodoxin in natural marine diatom assemblages to detect iron limitation. However, our polyclonal antisera appears to be specific for diatoms and did not cross-react with Synechococcus sp., Micromonas pusilla (Butcher) Manton et Parke, Dunaliella 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 driven into iron limitation expressed flovodoxin. Flovodoxin was detected in mats of the diatom Rhizosolenia castracanie 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-chlorophyll region. [ABSTRACT FROM AUTHOR]

Identification of the proximal nutrient limiting primary production is a necessary first step toward evaluating the physiological state of phytoplankton communities and the biogeochemical constraints on the current oceanic carbon cycle. We conducted 48-h nutrient addition bioassay experiments to evaluate nitrogen, phosphorus, and iron limitation of primary productivity, net chlorophyll synthesis, and net increase in cell numbers of the dominant picophytoplankton from the tropical North Atlantic. Our results indicate that N was the proximal limiting factor for primary production during the autumn of 2002, followed by P and then Fe. Net chlorophyll synthesis was significantly stimulated by addition of N alone and further stimulated by addition of P. Analysis of picophytoplankton populations by analytical flow cytometry revealed a more complex response. Cellular red fluorescence, an index of cell chlorophyll content, increased in Prochlorococcus, Synechococcus, and picoeukaryotes in response to addition of NH4NO3 but was not affected by single or combined additions of P and Fe. In contrast, cell abundance in these picophytoplankton populations increased only after combined N and P (63% of comparisons) or N and Fe (21% of comparisons) additions. Thus, our experiments revealed that chlorophyll synthesis and primary production were limited by the availability of nitrogen alone, while net increase in cell abundance was colimited by N and P or N and Fe in the majority of these picophytoplankton populations. [ABSTRACT FROM AUTHOR]