Description:
<jats:title>Abstract</jats:title><jats:p>Heme <jats:italic>b</jats:italic> is an iron-containing cofactor in hemoproteins that participates in the fundamental processes of photosynthesis and respiration in phytoplankton. Heme <jats:italic>b</jats:italic> concentrations typically decline in waters with low iron concentrations but due to lack of field data, the distribution of heme <jats:italic>b</jats:italic> in particulate material in the ocean is poorly constrained. Here we report particulate heme <jats:italic>b</jats:italic> distributions across the Atlantic Ocean (59.9°N to 34.6°S). Heme <jats:italic>b</jats:italic> concentrations in surface waters ranged from 0.10 to 33.7 pmol L<jats:sup>−1</jats:sup> (median = 1.47 pmol L<jats:sup>−1</jats:sup>, n = 974) and were highest in regions with a high biomass. The ratio of heme <jats:italic>b</jats:italic> to particulate organic carbon (POC) exhibited a mean value of 0.44 μmol heme <jats:italic>b</jats:italic> mol<jats:sup>−1</jats:sup> POC. We identified the ratio of 0.10 µmol heme <jats:italic>b</jats:italic> mol<jats:sup>−1</jats:sup> POC as the cut-off between heme <jats:italic>b</jats:italic> replete and heme <jats:italic>b</jats:italic> deficient (anemic) phytoplankton. By this definition, we observed anemic phytoplankton populations in the Subtropical South Atlantic and Irminger Basin. Comparison of observed and modelled heme <jats:italic>b</jats:italic> suggested that heme <jats:italic>b</jats:italic> could account for between 0.17–9.1% of biogenic iron. Our large scale observations of heme <jats:italic>b</jats:italic> relative to organic matter provide further evidence of the impact of changes in iron supply on phytoplankton iron status across the Atlantic Ocean.</jats:p>