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

Heme is an iron-containing co-factor in hemoproteins. Heme concentrations are low (<1 pmol L) in iron limited phytoplankton in cultures and in the field. Here, we determined heme 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 abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme concentrations ranged from 0.16 to 5.1 pmol L (median = 2.0 pmol L, = 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 levels (SML: median = 0.53 pmol L, = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L, = 25; chlorophyll : median = 2.0 nmol L, = 23) pointing to regulatory mechanisms of the heme pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L, = 11) in the Irminger Basin, but large diatoms ( sp.) dominated. Hence, heme 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 concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L, = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme provides a snapshot of the cellular activity and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.