Diatoms play a fundamental role in marine food webs, and significantly
contribute to global primary production and carbon sequestration into the deep ocean. In
many offshore areas of the open ocean, iron (Fe) input is low, and its availability often
limits phytoplankton biomass. Recently, gene sequences encoding ferritin, a nearly
ubiquitous iron storage and detoxifying protein, have been identified in pennate diatoms
such as Pseudo-nitzschia, but not in other Stramenopiles (which include centric diatoms,
brown algae and some protist plant parasites) or Cryptophyte relatives. Members of this
genus readily bloom upon addition of iron to Fe-limited waters, and are known to
produce the neurotoxin domoic acid. Until now, the reason for the success of pennate
diatoms in the open ocean was uncertain; however, expressing ferritin would allow
pennate species to store Fe after a transient input, using it to dominate Fe stimulated algal
blooms.
Here, the ferritin gene was cloned from the coastal pennate diatom Pseudonitzschia
multiseries, overexpressed in Escherichia coli, and purified using liquid
chromatography. The ferritin protein sequence appears to encode a non-heme, ferritinlike
di-iron carboxylate protein, while gel filtration chromatography and SDS-PAGE
indicate that this ferritin is part of the 24 subunit maxi-ferritins. Spectroscopically
monitoring the addition of Fe(II) to a buffered ferritin solution shows that the P.
multiseries protein demonstrates ferroxidase activity, binding iron and storing it as Fe(III)
in excess of 600 equivalents per protein shell. In keeping with the typical stoichiometry
of the ferroxidase reaction, oxygen (O₂) is consumed in a 2 Fe:O₂ratio while hydrogen
peroxide is produced concurrently.
iii
Diatoms evolved from secondary endosymbiosis involving eukaryotic red algae;
however, a broad phylogenetic comparison suggests that P. multiseries ferritin was likely
acquired via lateral gene transfer from cyanobacteria – not from its ancestral
endosymbionts. Until recently, no other ferritins have been identified in diatoms, and the
protein characterized here is unique in that it seems to be derived from a
prokaryotic organism yet it occurs in a marine eukaryote. These findings have direct
implications for the success of pennate diatoms in both Fe rich coastal waters and
upon Fe addition in the open ocean.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/950 |
| Date | 11 1900 |
| Creators | Moccia, Lauren Paul |
| Publisher | University of British Columbia |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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