PARENT SESSION

Symposium S2D Marine photosynthesis and production
Monday August 30th, 2004 2:40 PM-4:40 PM Room 510B
Chair: John Raven
Co-Chair: Doug Campbell

Photosynthetic architecture differs in coastal and oceanic diatoms. Robert Strzepek^*,, Paul Harrison², ² Atmospheric, Marine & Coastal Environment (AMCE) Program, Hong Kong, China

ABSTRACT- Diatoms are a key taxa of eukaryotic phytoplankton and are one of the predominant contributors to global carbon fixation. They are ubiquitous in marine ecosystems despite marked gradients in environmental properties, such as dissolved iron concentrations, between coastal and oceanic waters. Previous studies demonstrate that offshore species have evolved lower iron requirements to subsist in high-nutrient, low chlorophyll (HNLC) waters, but the biochemical mechanisms responsible for their reduced iron demand are unknown. The cytochrome and iron-sulfur protein cofactors of the major photosynthetic complexes are believed to constitute the largest iron requirement of diatoms. To investigate the habitat-related difference in diatom iron requirements, we therefore measured the photosynthetic iron requirements of two closely-related centric diatom species that differ markedly in their cellular iron demand. We performed these measurements over an ecologically-relevant range of iron concentrations and growth irradiances. We report that changes in the photosystem stoichiometry of a model oceanic diatom, specifically two-fold reductions in Photosystem I and up to nine-fold reductions in the Cytochrome b₆f complex, relative to a model coastal diatom, markedly reduce its iron requirements. However, such architectural alterations to the photosynthetic apparatus may have come at a cost. Oceanic diatoms appear to have sacrificed their ability to acclimate to rapidly fluctuating light. Such light climates are a characteristic of more dynamic and turbid coastal waters. Thus, the evolution of a different photosynthetic architecture enables oceanic diatoms to subsist in iron-impoverished waters, but may exclude them from coastal habitats.

KEY WORDS: photosystem stoichiometry, marine diatoms, iron limitation, photoacclimation