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<p>Abstract</p> <p>Background</p> <p>Although the importance and widespread occurrence of iron limitation in the contemporary ocean is well documented, we still know relatively little about genetic adaptation of phytoplankton to these environments. Compared to its coastal relative <it>Thalassiosira pseudonana</it>, the oceanic diatom <it>Thalassiosira oceanica </it>is highly tolerant to iron limitation. The adaptation to low-iron conditions in <it>T. oceanica </it>has been attributed to a decrease in the photosynthetic components that are rich in iron. Genomic information on <it>T. oceanica </it>may shed light on the genetic basis of the physiological differences between the two species.</p> <p>Results</p> <p>The complete 141790 bp sequence of the <it>T. oceanica </it>chloroplast genome [GenBank: <ext-link ext-link-id="GU323224" ext-link-type="gen">GU323224</ext-link>], assembled from massively parallel pyrosequencing (454) shotgun reads, revealed that the <it>petF </it>gene encoding for ferredoxin, which is localized in the chloroplast genome in <it>T. pseudonana </it>and other diatoms, has been transferred to the nucleus in <it>T. oceanica</it>. The iron-sulfur protein ferredoxin, a key element of the chloroplast electron transport chain, can be replaced by the iron-free flavodoxin under iron-limited growth conditions thereby contributing to a reduction in the cellular iron requirements. From a comparison to the genomic context of the <it>T. pseudonana petF </it>gene, the <it>T. oceanica </it>ortholog can be traced back to its chloroplast origin. The coding potential of the <it>T. oceanica </it>chloroplast genome is comparable to that of <it>T. pseudonana </it>and <it>Phaeodactylum tricornutum</it>, though a novel expressed ORF appears in the genomic region that has been subjected to rearrangements linked to the <it>petF </it>gene transfer event.</p> <p>Conclusions</p> <p>The transfer of the <it>petF </it>from the cp to the nuclear genome in <it>T. oceanica </it>represents a major difference between the two closely related species. The ability of <it>T. oceanica </it>to tolerate iron limitation suggests that the transfer of <it>petF </it>from the chloroplast to the nuclear genome might have contributed to the ecological success of this species.</p>