Plastid proteomics for elucidating iron limited remodeling of plastid physiology in diatoms

Diatoms are important primary producers in the world’s oceans and their growth is constrained

in large regions by low iron availability. This low iron-induced limitation of primary production

is due to the requirement for iron in components of essential metabolic pathways including key

chloroplast functions such as photosynthesis and nitrate assimilation. Diatoms can bloom and

accumulate high biomass during introduction of iron into low iron waters, indicating adaptations

allowing for their survival in iron-limited waters and rapid growth when iron becomes more

abundant. Prior studies have shown that under iron limited stress, diatoms alter plastid-specific

processes including components of electron transport, size of light harvesting capacity and

chlorophyll content, suggesting plastid-specific protein regulation. Due to their complex

evolutionary history, resulting from a secondary endosymbiosis, knowledge regarding the

complement of plastid localized proteins remains limited in comparison to other model

photosynthetic organisms. While in-silico prediction of diatom protein localization provides

putative candidates for plastid-localization, these analyses can be limited as most plastid

prediction models were developed using plants, primary endosymbionts. In order to characterize

proteins enriched in diatom chloroplast and to understand how the plastid proteome is remodeled

in response to iron limitation, we used mass spectrometry based proteomics to compare plastid-

enriched protein fractions from Thalassiosira pseudonana, grown in iron replete and limited

conditions. These analyses show that iron stress alters regulation of major metabolic pathways in

the plastid including the Calvin cycle and fatty acid synthesis. These components provide

promising targets to further characterize the plastid specific response to iron limitation.