The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

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Settling particles were collected from the Ligurian Sea in the northwestern Mediterranean Sea in May 2003 and separated by elutriation into different settling velocity classes (.230, 115-230, 58-115, and ,58 m d 21 ). Particles of the different classes were incubated for 5 d to study their biodegradability. Particulate opal content and organic compound composition (amino acids, pigments, lipids, and carbohydrates) were analyzed initially and at regular time intervals during the incubation period. Most particles (48-67% of total mass) sank at greater than 230 m d 21 and were dominated by large diatom-derived aggregates produced during the spring bloom period. The initial organic composition and the biological lability of these particles varied with settling velocity. The strong phytoplankton signal was visible in all settling velocity classes, while slower settling particles carried with them a greater zooplankton and bacterial signature. As the different class particles decomposed, their compositions changed and became more similar with time, with a dominance of compounds that suggests a more degraded state: the amino acids c-aminobutyric acid and b-alanine, the pigments pyropheophorbide and pheophytin, the deoxysugars fucose and rhamnose, and lipid metabolites (diglycerides and monoglycerides, alcohols, and free fatty acids). Biogenic opal in the particles dissolved faster in more degraded particles than in fresher particles, suggesting that loss of organic matter may expose opal to dissolution. The coupling of settling velocity and decomposition rate measurements shows quantitatively that slower settling particles are quickly degraded and AcknowledgmentsThis research was part of the MedFlux and PECHE (Production and Export of Carbon: Control by Heterotrophs at small temporal scale) programs and was supported by the U.S. National Science Foundation Chemical Oceanography Program (OCE-0136370, OCE-0136318, and OCE-0113687) and the French CNRS (Centre National de la Recherche Scientifique), respectively. Participation of B.M. was funded by ORFOIS (Origin and Fate of Biogetic Particle Fluxes in the ocean) (EVK2-CT2001-00100). We thank Michael Peterson, Lynn Abramson, Jenni Szlosek, Meaghan Askea, and Isabell Putnam for shipboard and laboratory help; David Hirschberg and Michael Peterson for CHN analysis; Claude Mante for help with statistical data treatment; and the captain and crew of the RV Seward Johnson II. We wish to acknowledge the associate editor and two anonymous reviewers for very helpful comments and suggestions on the manuscript. This is MedFlux contribution 7 and MSRC contribution 1318.

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International audienceWhile a relationship between ballast and carbon in sedimenting particles has been well-documented, the mechanistic basis of this interaction is still under debate. One hypothesis is that mineral ballast protects sinking organic matter from degradation. To test this idea, we undertook a laboratory experiment using the diatom Skeletonema marinoi to study in parallel the dissolution of one of the most common mineral ballasts, biogenic silica (bSiO2), and the associated degradation of organic matter. Three different models were applied to our results to help elucidate the mechanisms driving bSiO2 dissolution and organic compound degradation. Results of this modeling exercise suggest that the diatom frustule is made up of two bSiO2 phases that dissolve simultaneously, but at different rates. In our experiments, the first phase was more soluble (View the MathML source) and made up 31% of the total bSiO2. In this phase, bSiO2 was mainly associated with membrane lipids and the amino acids glutamic acid, tyrosine, and leucine. The second phase was more refractory (View the MathML source), and contained more neutral lipid alcohols and glycine. Until it dissolved, the first bSiO2 phase effectively protected much of the organic matter from degradation: particulate organic carbon (POC) degradation rate constants increased from 0.025 to 0.082 d−1 after the total dissolution of this phase, and particulate organic nitrogen (PON) degradation rate constants increased from 0.030 to 0.094 d−1. Similar to POC and PON, the total hydrolyzable amino acids (THAA) degradation rate constant increased from 0.054 to 0.139 d−1 after dissolution of the first bSiO2 phase. The higher THAA degradation rate constant is attributed to a pool of amino acids that was produced during silicification and enclosed between the two silica phases. This pool of amino acids might come from the incorporation of silica deposition vesicles into the diatom wall and might not be directly associated with bSiO2. In contrast, most lipid degradation was not prevented by association with the more soluble bSiO2 phase, as the average lipid degradation rate constant decreased from 0.048 to 0.010 d−1 after 17 d of degradation. This suggests that most lipids were associated with rather than protected by silica, except pigments that appeared resistant to degradation, independently from silica dissolution. When the only organic compounds remaining were associated with the second bSiO2 phase, degradation rate constants decreased greatly; concentrations changed only slightly after day 25

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