Diatoms are unicellular algae with silica cell walls that play an important role in the global carbon cycle and oxygen production. They are highly responsive to iron limitation, downregulating iron-dependent pathways and upregulating iron-independent alternatives. Ocean iron fertilization experiments have shown diatom blooms in high nutrient low chlorophyll regions of the ocean in response to added iron, but diatoms may also be co-limited by silica.

1. Effect of Iron Limitation
on Diatoms
Mariam Rizkallah
Biotechnology Master’s Program
The American University in Cairo

2. 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)

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

4. 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 Source:
reduction  size restoration http://www.ucl.ac.uk/GeolSci/mi
cropal/diatom.html
(auxospores)

5. Diatoms in the Tree of Life
Source: http://chloroplast.ocean.washington.edu/organisms

6. 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)

7. Iron limitation and whole-cell
response

8. 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

9. 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)

10. Iron limitation and whole-cell
response 3/3
Down-regulation of Fe-requiring Up-regulation of Fe-economic
pathways alternatives
- Cell volume reduction and Chl - Alternative shuttling system from
concentration reduction chloroplast and cytosol to
- Respiration (Fe-dependent electron mitochondria
carriers, cytochrome restriction, - Remodeling of the photosynthetic
accumulation of TCA intermediates) apparatus (peripheral light-
- Photosynthesis (Fv/Fm, quantum harvesting antennas)
yield of fluorescence) - Proteome remodeling: glycolysis
- Nitrate assimilation and proteolysis of structural
- Fe-depended Reactive Oxygen proteins to compensate for
Species (ROS) defense (e.g., heme impaired nitrate metabolism in
peroxidase and superoxide amino acid synthesis
dismutase (SOD)) - Fe-independent ROS defense (e.g.,
tocopherol and dehydroascorbate)
and mitochondrial alternative
oxidase (AOX)

11. Iron limitation and whole-cell
response
Allen et al., 2008

12. Geoengineering and Ocean iron
fertilization (OIF)

13. 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)

14. Co-limitation by iron and silica

15. Co-limitation by iron and silica

16. 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.

17. 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