Acclimation and adaptation to low-iron conditions in the marine diatoms Phaeodactylum tricornutum and Thalassiosira oceanica.

Lommer, Markus (2012) Acclimation and adaptation to low-iron conditions in the marine diatoms Phaeodactylum tricornutum and Thalassiosira oceanica. (Doctoral thesis/PhD), Christian-Albrechts-Universität Kiel, Kiel, Germany, 138 pp.

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Abstract

In the open ocean phytoplankton growth is widely limited by the availability of iron, an essential element of the photosynthetic electron transport system. Survival under these conditions requires sophisti-cated strategies to maintain growth, e.g. lowering iron requirements and enhancing cellular iron affinity. In this work we used genomic and transcriptomic data to unravel acclimation (transcriptomics) and adaptation (genomics) to low-iron conditions in the two diatoms Thalassiosira oceanica and Phaeodactylum tricornutum, who both are highly tolerant to iron limitation. The acclimation response to low ambient iron concentrations is very similar in the two diatoms. Both undergo an extensive cellular retrenchment, best visible from chloroplast reduction and from the concomitant pigment loss that imposes a chlorotic phenotype on the cells. Growth rates are very low with a high degree of photosynthetic energy dissipation. The differential regulation of the genes for ferre-doxin (PETF) and flavodoxin (FLDA) and for class II and class I fructose-bisphosphate aldolases (FBA) indicates that cellular iron requirements are lowered by replacing abundant iron-rich proteins with iron-free substitutes. Genetically fixed features of T. oceanica and P. tricornutum that may represent beneficial adaptations to low-iron are the small cell sizes found for these species and the possession of class I FBA genes not present in the coastal diatom species Thalassiosira pseudonana. As adaptations specific to T. oceanica we observe the constitutive expres-sion of the plastocyanin gene (PCY) that encodes for a substitute of iron-containing cytochrome c6. The ferredoxin gene (PETF) has been transferred from the chloroplast to the nuclear genome in this species, facilitating its co-regulation with the flavodoxin gene (FLDA). The acclimation of diatoms to low-iron resembles the processes running off in green plants and algae upon low-iron stress. Thus, the response to low-iron represents an ancient cellular mechanism com-mon to most, if not all photosynthetic groups. Genetic adaptation to a persistent shortage of iron such as in the open ocean likely occurs by exploring strategies to optionally or permanently replace abundant iron-rich proteins by iron-free substitutes, thereby approximating the cellular iron requirements to a lowest possible level. The adaptive significance of substitution strategies is strengthened by their absence in coastal diatoms like T. pseudonana which is adapted to iron-rich coastal waters and is highly sensitive to low ambient iron concen-trations.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: LaRoche, Julie and Rosenstiel, Philip
Keywords: acclimation, adaptation, iron, marine, diatom, Phaeodactylum tricornutum, Thalassiosira oceanica, FBA, ferredoxin, flavodoxin, cytochrome-c6, plastocyanin, ISIP, FCP, ELIP, LI818, NPQ, LHC, photosynthesis, iron uptake, transcriptomics, genomics, qPCR, stress, substitution, genome, gene expression Akklimatisierung, Anpassung, Eisen, marin, Diatomeen, Kieselalgen, Phaeodactylum tricornutum, Thalassiosira oceanica, FBA, Ferredoxin, Flavodoxin, Cytochrom-c6, Plastocyanin, ISIP, FCP, ELIP, LI818, NPQ, LHC, Photosynthese, Eisenaufnahme, Transcriptomics, Genomics, qPCR, Stress, Substitution, Genom, Genexpression
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
Kiel University
Refereed: No
Date Deposited: 24 Feb 2009 09:18
Last Modified: 21 Mar 2019 13:22
URI: http://oceanrep.geomar.de/id/eprint/1484

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