1.1 Heme b
 Oceanic phytoplankton are known to change the abundance of iron-containing proteins when nutrient concentrations or incident light levels change [Pankowski and McMinn, 2009; Peers and Price, 2006; Saito et al., 2011; Strzepek and Harrison, 2004; Wolfe-Simon et al., 2006]. The cellular abundance of such proteins influences the efficiency of photosynthesis [Bailey et al., 2008; Cardol et al., 2008; Peers and Price, 2006] and so links nutrient abundance and light to ocean productivity. Ferredoxin [Mckay et al., 1999; Pankowski and McMinn, 2009], photosystem I (PSI), cytochrome b6f (cytb6f) [Eberhard et al., 2008; Saito et al., 2011; Strzepek and Harrison, 2004], cytochrome c550 [Saito et al., 2011], soluble cytochrome c [Eberhard et al., 2008; Peers and Price, 2006], and iron superoxide dismutase [Wolfe-Simon et al., 2006] have all been shown to be reduced in abundance or replaced by noniron-containing proteins when oceanic phytoplankton are grown under low-iron or -nutrient conditions in the laboratory. Furthermore, recent work has shown downregulation of genes associated with iron proteins or iron protein synthesis [Allen et al., 2008; Lommer et al., 2012; Thompson et al., 2011] and diurnal cycling of iron protein pools in order to conserve iron use [Saito et al., 2011]. However, few studies have investigated the abundance of iron-containing proteins in the field or how their distributions are influenced by the prevailing nutrient and light regimes [Erdner and Anderson, 1999; Pankowski and McMinn, 2009]. Heme b is an iron-containing porphyrin which functions as a prosthetic group for proteins involved in electron transfer and the scavenging of reactive oxygen species [Chapman et al., 1997]. Heme b is highly toxic to cells if not incorporated into proteins [Espinas et al., 2012] and the stoichiometry of heme b within proteins is fixed [Shekhawat and Verma, 2010; Tanaka and Tanaka, 2007], so that changes in heme b abundance will reflect changes in the cellular abundance of heme b-containing proteins. In marine phytoplankton, heme b is incorporated into proteins involved in photosynthesis, respiration, and reactive oxygen scavenging [Honey et al., 2013]. Laboratory studies of marine eukaryotic phytoplankton showed that heme b concentrations extracted by ammoniacal detergent averaged 3.7 ± 0.9 μmol mol−1 C in resource replete conditions and that the cellular concentrations of heme b decreased under nutrient deplete conditions. Heme b accounted for 18 ± 14% of the total particulate iron pool when iron concentrations in the culture media were low. Decreases in heme b relative to particulate organic carbon (POC) were also observed to correspond with decreasing nitrate and phosphate concentrations across a transect of the (sub-) tropical North Atlantic. Chlorophyll a:heme b ratios were also found to vary for certain species when phytoplankton were grown under decreased iron, nitrate, or light conditions. However, results from the temperate and (sub-) tropical North Atlantic showed that chl a:heme b increased with depth and were thus driven by gradients in light rather than nutrients [Honey et al., 2013]. Determination of the concentration of heme b in particulate material and comparison with phytoplankton biomass, POC, and chl a in the ocean could thus provide information on the variation in heme b protein abundance and the response of phytoplankton to prevailing nutrient and light conditions.
 In this study, we determined heme b in particulate material in three contrasting regions of the Atlantic Ocean and Southern Ocean: the Iceland Basin (IB), the tropical northeast Atlantic (TNA), and the Scotia Sea (SS). The IB is situated in the high-latitude North Atlantic and is characterized by pronounced diatom-dominated spring blooms [Leblanc et al., 2009], which nevertheless do not result in the complete drawdown of nitrate and phosphate. Residual nutrient stocks are thus subsequently exploited by more mixed phytoplankton assemblages [Leblanc et al., 2009; Poulton et al., 2010]. The post spring bloom period is iron limited [Nielsdottir et al., 2009], a result of both low atmospheric iron inputs [Jickells et al., 2005] and suboptimal iron:nitrate ratios in winter-overturned deep waters [Nielsdottir et al., 2009].
 The region of the TNA examined in this study is influenced in the east by the highly productive upwelling system off the Northwest African Coast, in the south by the oxygen minimum zone off the Cape Verde Islands and in the northwest by the permanently stratified oligotrophic subtropical North Atlantic Gyre [Stramma et al., 2008]. Atmospheric iron inputs in this region are relatively high due to the influx of Saharan and Sahel dust [Jickells et al., 2005; Mulitza et al., 2010; Stuut et al., 2005] with maximum dust inputs in this region occurring in winter (January to March) [Chiapello et al., 1995]. The phytoplankton community in the TNA is dominated by picoplankton comprising Prochlorococcus sp., Synechococcus sp., and picoeukaryotes [Hill et al., 2010], although nitrogen-fixing Trichodesmium spp. are also observed in this region [Moore et al., 2009], with their abundance being tightly linked to dust inputs [Rijkenberg et al., 2011, 2012].
 The SS lies within the Atlantic sector of the Southern Ocean. The SS contrasts with much of the high-nutrient low-chlorophyll (HNLC) regions of the Southern Ocean as it supports extensive phytoplankton blooms, with the South Orkney Islands, the South Sandwich Islands, and South Georgia acting as potential sources of iron via the “island mass effect” [Doty and Oguri, 1956; Korb et al., 2005; Nielsdottir et al., 2012]. The waters around South Georgia are particularly productive and can support Antarctic diatom-dominated blooms of up to 12 mg m−3 chl a for periods of up to 5 months [Korb et al., 2005]. However, productivity in the area is patchy, and regions to the south of South Georgia have HNLC characteristics [Hinz et al., 2012; Korb et al., 2010].
 We compare the distribution of heme b to chl a, phytoplankton biomass, POC, and particulate organic nitrogen (PON) and interpret our findings within the context of the prevailing nutrient (dissolved iron (dFe), nitrate, phosphate, and silicate) and light environments. We assess how the relative abundance of heme b varies in the ocean, where chl a:heme b ratios increase and how changes in the relative abundance of heme b proteins and light-harvesting complexes relate to nutrient distributions.