Effect of Iron Limitation on DiatomsMariam RizkallahBiotechnology Master’s ProgramThe American University in Cairo

Outline• Diatoms, their structure, applications, and role in the

ecosystem• Iron limitation and whole-cell response (Allen et al.,

2008)• Ocean fertilization attempts (Smetacek et al., 2012)• Co-limitation of diatoms by iron and silica (Brzezinski et

al., 2011)

Diatoms diversity

Source: http://tolweb.org/images/Diatoms/21810

Diatoms structure and life cycle• Unicellular photosynthetic

“microalgae”• Inhabitants of aquatic

systems: seawater, freshwater and soil, freely or in an endosymbiotic relationship

• Of siliceous skeleton (frustule)• Between 20-200 microns in

diameter or length• Cell division (epitheca as

parent frustule) size reduction size restoration (auxospores)

Source: http://www.ucl.ac.uk/GeolSci/micropal/diatom.html

Diatoms in the Tree of Life

Source: http://chloroplast.ocean.washington.edu/organisms

Diatoms role in the ecosystem and applications• Diatoms and the global carbon cycle:- Primary producers in the aquatic food web [e.g., they

contribute with ≈40% of global oceanic organic carbon production per year (Allen et al., 2007)]

- Major contributors in global oxygen production

• Diatoms as indicators of the Earth's history:- Indicator of the past environments and climate changes

through tracking the sediments of the empty silica cell wall deposited after their death

- Living diatoms optimal growth conditions vs. that of extinct ones as way of palaeoenvironmental reconstruction

• Diatoms nanotechnology (Bradbury, 2004)

Iron limitation and whole-cell response

Iron limitation and whole-cell response 1/3• Observation:- Diatoms-dominating blooms after Fe-repletion in high nutrient low chlorophyll (HNLC) regions - HNLC regions: subarctic Pacific, equatorial Pacific, and Southern Oceans and North Atlantic)

Source: http://www.polartrec.com/expeditions/seawater-property-changes-in-the-southern-ocean/journals/2011-03-21

Iron limitation and whole-cell response 2/3• One of the species studied:- Phaeodactylum tricornutum: highly tolerant to Fe-limitation

• Methods:- Growth conditions: Cultures grown in Fe-limited media, followed by Fe

addition- Physiological measurements: Photosynthetic fitness, cell diameter and

volume, Fe reductase assay and chlorophyll (Chl) concentration- Gene expression profiling: Expressed Sequence Tags (ESTs )libraries,

partial genome microarray, Real Time quantitative Reverse transcription Polymerase Chain Reaction (qRT-PCR)

- Comparative genomics : Mapping ESTs to predicted proteins encoded by P. tricornutum genome and across lineages

- Metabolites extraction: Gas chromatography-mass spectroscopy (GC-MS)

Iron limitation and whole-cell response 3/3

Down-regulation of Fe-requiring pathways

Up-regulation of Fe-economic alternatives

- Cell volume reduction and Chl concentration reduction

- Respiration (Fe-dependent electron carriers, cytochrome restriction, accumulation of TCA intermediates)

- Photosynthesis (Fv/Fm, quantum yield of fluorescence)

- Nitrate assimilation- Fe-depended Reactive Oxygen

Species (ROS) defense (e.g., heme peroxidase and superoxide dismutase (SOD))

- Alternative shuttling system from chloroplast and cytosol to mitochondria

- Remodeling of the photosynthetic apparatus (peripheral light-harvesting antennas)

- Proteome remodeling: glycolysis and proteolysis of structural proteins to compensate for impaired nitrate metabolism in amino acid synthesis

- Fe-independent ROS defense (e.g., tocopherol and dehydroascorbate) and mitochondrial alternative oxidase (AOX)

Iron limitation and whole-cell response

Allen et al., 2008

Geoengineering and Ocean iron fertilization (OIF)

Geoengineering and Ocean iron fertilization (OIF)• The European Iron Fertilization Experiment (EIFEX):- Enhancement of CO2 sequestration from the atmosphere via

eliminating the phytoplankton growth limiting factor, Fe- Testing the “iron hypothesis” in the Southern Ocean as a

typical HNLC region (bloom were observed via satellite upon continental and volcanic iron addition)

- Measurements at different depths were taken, pre-, during and post-fertilization

- Chemical analysis of Chl, particulate organic carbon (POC), nitrogen (PON), phosphate (POP) and biogenic silica (BSi) concentrations (ratios indicated diatoms domination)

Co-limitation by iron and silica

Co-limitation by iron and silica

Concluding remarks• Diatoms have a major role in carbon cycle and oxygen

production.• Iron is crucial for growth, photosynthesis, respiration and ROS

response in diatoms.• However, diatoms show adaptation to iron limitation.• Ocean fertilization is a way for geoengineering depending on

iron hypothesis.• Iron may be the limiting factor for diatoms growth, however,

silica may be a limiting factor for diatoms silicification and division.

References• Allen, A. E., Laroche, J., Maheswari, U., Lommer, M., Schauer, N., Lopez, P. J.,

Finazzi, G., et al. (2008). Whole-cell response of the pennate diatom Phaeodactylum tricornutum to iron starvation. Proceedings of the National Academy of Sciences of the United States of America, 105(30), 10438–43

• Smetacek, V., Klaas, C., Strass, V. H., Assmy, P., Montresor, M., Cisewski, B., Savoye, N., et al. (2012). Deep carbon export from a Southern Ocean iron-fertilized diatom bloom. Nature, 487(7407), 313–319. doi:10.1038/nature11229

• Brzezinski, M. A., Baines, S. B., Balch, W. M., Beucher, C. P., Chai, F., Dugdale, R. C., Krause, J. W., et al. (2011). Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific. Deep Sea Research Part II: Topical Studies in Oceanography, 58(3-4), 493–511. doi:10.1016/j.dsr2.2010.08.005

• The Friedrich Hustedt Diatom Study Centre: http://www.awi.de/index.php?id=2366&L=0

• Diatoms and climate change – The use of diatom analysis in reconstructing Late Holocene climate for Kigoma Region, Tanzania: http://www.geo.arizona.edu/nyanza/pdf/Meeker.pdf