Response of the unicellular diazotrophic cyanobacterium Crocosphaera watsonii to iron limitation.

International audienceIron (Fe) is widely suspected as a key controlling factor of N2 fixation due to the high Fe content of nitrogenase and photosynthetic enzymes complex, and to its low concentrations in oceanic surface seawaters. The influence of Fe limitation on the recently discovered unicellular diazotrophic cyanobacteria (UCYN) is poorly understood despite their biogeochemical importance in the carbon and nitrogen cycles. To address this knowledge gap, we conducted culture experiments on Crocosphaera watsonii WH8501 growing under a range of dissolved Fe concentrations (from 3.3 to 403 nM). Overall, severe Fe limitation led to significant decreases in growth rate (2.6-fold), C, N and chlorophyll a contents per cell (up to 4.1-fold), N2 and CO2 fixation rates per cell (17- and 7-fold) as well as biovolume (2.2-fold). We highlighted a two phased response depending on the degree of limitation: (i) under a moderate Fe limitation, the biovolume of C. watsonii was strongly reduced, allowing the cells to keep sufficient energy to maintain an optimal growth, volume-normalized contents and N2 and CO2 fixation rates; (ii) with increasing Fe deprivation, biovolume remained unchanged but the entire cell metabolism was affected, as shown by a strong decrease in the growth rate, volume-normalized contents and N2 and CO2 fixation rates. The half-saturation constant for growth of C. watsonii with respect to Fe is twice as low as that of the filamentous Trichodesmium indicating a better adaptation of C. watsonii to poor Fe environments than filamentous diazotrophs. The physiological response of C. watsonii to Fe limitation was different from that previously shown on the UCYN Cyanothece sp, suggesting potential differences in Fe requirements and/or Fe acquisition within the UCYN community. These results contribute to a better understanding of how Fe bioavailability can control the activity of UCYN and explain the biogeography of diverse N2 fixers in ocean

Saharan dust: a source of bioavailable iron for the marine unicellular diazotrophic cyanobacterium C. watsonii

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Correction: Response of the Unicellular Diazotrophic Cyanobacterium Crocosphaera watsonii to Iron Limitation (vol 9, e86749, 2014)

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Temporal evolution of coastal upwelling in Eastern boundary currents from before the 20th century using paleo-climate reconstructions

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Temporal evolution of coastal upwelling in Eastern boundary currents from before the 20th century using paleo-climate reconstructions

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Temporal evolution of coastal upwelling in Eastern boundary currents from before the 20th century using paleo-climate reconstructions

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Temporal evolution of coastal upwelling in Eastern boundary currents from before the 20th century using paleo-climate reconstructions

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Influence of Fe availability on the elemental composition of <i>C. watsonii</i>.

<p>Mean cellular content of C (A), N (B) and Chl <i>a</i> (C) related to dFe and Fe′ concentrations, in log scale. Error bars represent standard deviation; different letters correspond to statistically different means (<i>p</i><0.05) and the black bar indicates the region of expected Fe hydroxide precipitation.</p

Influence of Fe availability on N₂ and CO₂ fixation rates of <i>C. watsonii</i>.

<p>(A) Relative N₂ fixation rates and (B) relative CO₂ fixation rates of <i>C. watsonii</i> related to dFe and Fe′ concentrations, in log scale. Open and filled symbols correspond to the rates per cell and V-normalized rates respectively. Error bars represent standard deviation; different letters and different numbers of star correspond to statistically different means (<i>p</i><0.05) for cellular and V-normalized rates, respectively. The black bar indicates the region of expected Fe hydroxide precipitation.</p

Influence of Fe availability on <i>C. watsonii</i> biovolume.

<p>Scanning electron microscopy photography of <i>C. watsonii</i> growing in (A) Fe-replete condition (dFe = 403.3 nM) and (B) severe Fe-limited condition (dFe = 3.3 nM). (C) Mean biovolume of <i>C. watsonii</i> related to dFe and Fe′ concentrations, in log scale. Error bars represent standard deviation; different letters correspond to statistically different means (<i>p</i><0.05) and the black bar indicates the region of expected Fe hydroxide precipitation.</p