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Regulation of the phytoplankton heme b iron pool during the North Atlantic spring bloom

Louropoulou, Evangelia ; Gledhill, Martha ; Browning, Thomas J. ; Desai, Dhwani K. ; Barraqueta, Jan-Lukas Menzel ; Tonnard, Manon ; Sarthou, Geraldine ; Planquette, Helene ; Bowie, Andrew R. ; Schmitz, Ruth A. ; LaRoche, Julie ; Achterberg, Eric P. (2019-07-11)

CITATION: Louropoulou, E., et al. 2019. Regulation of the phytoplankton heme b iron pool during the North Atlantic spring bloom. Frontiers in Microbiology, 10:1566, doi:10.3389/fmicb.2019.01566.

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Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L⁻²) in iron limited phytoplankton in cultures and in the field. Here, we determined heme b in marine particulate material (>0.7 μm) from the North Atlantic Ocean (GEOVIDE cruise – GEOTRACES section GA01), which spanned several biogeochemical regimes. We examined the relationship between heme b abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme b concentrations ranged from 0.16 to 5.1 pmol L⁻² (median = 2.0 pmol L⁻², n = 62) in the surface mixed layer (SML) along the cruise track, driven mainly by variability in biomass. However, in the Irminger Basin, the lowest heme b levels (SML: median = 0.53 pmol L⁻², n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L⁻², n = 25; chlorophyll a: median = 2.0 nmol L⁻², n = 23) pointing to regulatory mechanisms of the heme b pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L⁻², n = 11) in the Irminger Basin, but large diatoms (Rhizosolenia sp.) dominated. Hence, heme b depletion and regulation is likely to occur during bloom progression when phytoplankton class-dependent absolute iron requirements exceed the available ambient concentration of DFe. Furthermore, high heme b concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L⁻², n = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme b provides a snapshot of the cellular activity in situ and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.

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