Dissolved Fe in the western and central North Pacific Ocean was characterized by surface depletion, middepth maxima and, below that, a slight decrease with depth similar to the vertical distributions of nutrients, apparent oxygen utilization, Fe(III) hydroxide solubility, and humic‐type fluorescence (H‐flu) intensity. Dissolved Fe concentrations ([D‐Fe], <0.22‐μm fraction) in the deep water column were one‐half lower in the central region (0.3–0.6 nM) than the western region (0.5–1.2 nM) although the Fe(III) solubility ([Fe(III)sol], <0.025‐μm fraction) levels and distributions in deep waters were almost the same between both regions with middepth maxima (∼0.6 nM) at 500–1500‐m depth range and then a gradual decrease to ∼0.3 nM at 5000‐m depth. Higher [D‐Fe] than [Fe(III)sol] in the deep water column of the western region results from the higher production of dissolved Fe from the decomposition of sinking particulate organic matter in the western region than the central region because of the high atmospheric and/or lateral Fe inputs in the western region. Similarity between [D‐Fe] level and [Fe(III)sol] value at each deep water depth in the central region may be attributed to [D‐Fe] being nearly in the solubility equilibrium with Fe(III) hydroxide in seawater. Strong linear correlation between [D‐Fe] and H‐flu intensity in the central region and relatively similar linear relationships between [Fe(III)sol] and H‐flu intensity in the western and central regions are the first confirmation that humic‐type fluorescent dissolved organic matter may be responsible for [D‐Fe] in the deep water column as natural organic ligands complexing with Fe(III).

31Partial pressure of CO 2 (pCO 2 ) and iron availability in seawater show corresponding 32 changes due to biological and anthropogenic activities. The simultaneous change in these 33 factors precludes an understanding of their independent effects on the ecophysiology of 34 phytoplankton. In addition, there is a lack of data regarding the interactive effects of 35 these factors on phytoplankton cellular stoichiometry, which is a key driving factor for 36 the biogeochemical cycling of oceanic nutrients. Here, we investigated the effects of 37 pCO 2 and iron availability on the elemental composition (C, N, P and Si) of the diatom 38Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle by dilute batch cultures under 4 39 pCO 2 (~200, ~380, ~600, and ~800 µatm) and 5 dissolved inorganic iron (Fe′; ~5, ~10, 40 ~20, ~50, and ~100 pmol L −1 ) conditions. Our experimental procedure successfully 41 overcame the problems associated with simultaneous changes in pCO 2 and Fe′ by 42 independently manipulating carbonate chemistry and iron speciation, which allowed us 43 to evaluate the individual effects of pCO 2 and iron availability. We found that the C:N 44 ratio decreased significantly only with an increase in Fe′, whereas the C:P ratio increased 45 significantly only with an increase in pCO 2 . Both Si:C and Si:N ratios decreased with 46 increasing pCO 2 and Fe′. Our results indicate that changes in pCO 2 and iron availability 47 could influence the biogeochemical cycling of nutrients in future oceans with high CO 2 48 levels, and, similarly, during the time course of phytoplankton blooms. Morever, pCO 2 49 and iron availability may also have affected oceanic nutrient biogeochemistry in the past, 50 as these conditions have changed markedly over the Earth's history. 51 52

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