A Novel Protein, Ubiquitous in Marine Phytoplankton, Concentrates Iron at the Cell Surface and Facilitates Uptake

Abstract

Numerous cellular functions including respiration require iron. Plants and phytoplankton must also maintain the iron-rich photosynthetic electron transport chain, which most likely evolved in the iron-replete reducing environments of the Proterozoic ocean [1]. Iron bioavailability has drastically decreased in the contemporary ocean [1], most likely selecting for the evolution of efficient iron acquisition mechanisms among modern phytoplankton. Mesoscale iron fertilization experiments often result in blooms dominated by diatoms [2], indicating that diatoms have adaptations that allow survival in iron-limited waters and rapid multiplication when iron becomes available. Yet the genetic and molecular bases are unclear, as very few iron uptake genes have been functionally characterized from marine eukaryotic phytoplankton, and large portions of diatom iron starvation transcriptomes are genes encoding unknown functions [3-5]. Here we show that the marine diatom Phaeodactylum tricornutum utilizes ISIP2a to concentrate Fe(III) at the cell surface as part of a novel, copper-independent and thermodynamically controlled iron uptake system. ISIP2a is expressed in response to iron limitation several days prior to the induction of ferrireductase activity, and it facilitates significant Fe(III) uptake during the initial response to Fe limitation. ISIP2a is able to directly bind Fe(III) and increase iron uptake when heterologously expressed, whereas knockdown of ISIP2a in P. tricornutum decreases iron uptake, resulting in impaired growth and chlorosis during iron limitation. ISIP2a is expressed by diverse marine phytoplankton, indicating that it is an ecologically significant adaptation to the unique nutrient composition of marine environments.

Diatom Transcriptional and Physiological Responses to Changes in Iron Bioavailability across Ocean Provinces

“…However, indications of molecularlevel responses to Fe and DFB additions were observed; 74 genes were differentially expressed (p < 0.05) in Pseudo-nitzschia between the Fe and DFB treatments (Supplementary Figure 2A). Fe-stress bioindicator genes (FLDA, PETE, and ISIP2A; Whitney et al, 2011;Morrissey et al, 2015;Graff van Creveld et al, 2016) increased in expression following the addition of DFB relative to the added Fe treatment, suggesting the onset of Fe stress following the addition of DFB by the end of the first time point.…”

“…Strategies employed by phytoplankton include replacement of Fe-containing proteins with Fe-independent ones to decrease cellular Fe requirements (La Roche et al, 1996;Peers and Price, 2006;Allen et al, 2008;Lommer et al, 2012), increasing Fe uptake rates through induction of high affinity Fe uptake systems (Maldonado and Price, 2001;Morrissey et al, 2015) and using Fe storage through specialized proteins or vacuoles Nuester et al, 2012). In some diatom laboratory isolates and natural communities, these low-Fe strategies are rapidly reversed when Fe concentrations increase (Kustka et al, 2007;Lommer et al, 2012), whereas in others these strategies are permanent adaptations (Lommer et al, 2010;Marchetti et al, 2012).…”

Central role for ferritin in the day/night regulation of iron homeostasis in marine phytoplankton

“…We evidenced iron-binding proteins on blue native gels, using the mild detergent digitonin to avoid iron release from proteins (31). As we previously reported (32), this is a convenient method for studying iron-containing proteins, even if we cannot rule out the possibility that some iron-containing proteins other than ferritin may be destabilized in the presence of digitonin. Autoradiography of radiolabeled proteins revealed a major band around 480 kDa in WT cells which was not detected in ΔFtn cells, suggesting that this band corresponds to a functional ferritin complex ( Fig.…”

Colony formation in &lt;i&gt;Phaeocystis antarctica&lt;/i&gt;: connecting molecular mechanisms with iron biogeochemistry

“…S1; Table S1). These ISIPs were identified based on their transcriptome response to iron stress in diatoms and most recently have been implicated in a diatom cell surface iron-concentrating mechanism (Allen et al, 2008;Morrissey et al, 2015). Interestingly in this P. antarctica experiment, these ISIPs exhibited both "high" and "low" iron responses, but specific isoforms were more abundant only under one of those respective conditions (Fig.…”