Regulation of the Phytoplankton Heme b Iron Pool During the North Atlantic Spring Bloom

• Published in: Frontiers in Microbiology
• Main Authors: Louropoulou, Evangelia, Gledhill, Martha, Browning, Thomas J., Desai, Dhwani K., Barraqueta, Jan-Lukas Menzel, Tonnard, Manon, Sarthou, Géraldine, Planquette, Hélène, Bowie, Andrew R., Schmitz, Ruth A., LaRoche, Julie, Achterberg, Eric P.
• Format: Text
• Language: English
• Published: Frontiers Media S.A. 2019
• Subjects: Microbiology; Irminger Basin; Iceland; Labrador Sea; North Atlantic
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6637849/; http://www.ncbi.nlm.nih.gov/pubmed/31354666; https://doi.org/10.3389/fmicb.2019.01566

Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L(-1)) 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(-1) (median = 2.0 pmol L(-1), 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(-1), n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L(-1), n = 25; chlorophyll a: median = 2.0 nmol L(-1), 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(-1), 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(-1), 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.