Abstract
Iron is an essential component in many protein complexes involved in photosynthesis, but environmental iron availability is often low as oxidized forms of iron are insoluble in water. To adjust to low environmental iron levels, cyanobacteria undergo numerous changes to balance their iron budget and mitigate the physiological effects of iron depletion. We investigated changes in key protein abundances and photophysiological parameters in the model cyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803 over a 120 hour time course of iron deprivation. The iron stress induced protein (IsiA) accumulated to high levels within 48 h of the onset of iron deprivation, reaching a molar ratio of ∼42 IsiA : Photosystem I in Synechococcus PCC 7942 and ∼12 IsiA : Photosystem I in Synechocystis PCC 6803. Concomitantly the iron-rich complexes Cytochrome b6f and Photosystem I declined in abundance, leading to a decrease in the Photosystem I : Photosystem II ratio. Chlorophyll fluorescence analyses showed a drop in electron transport per Photosystem II in Synechococcus, but not in Synechocystis after iron depletion. We found no evidence that the accumulated IsiA contributes to light capture by Photosystem II complexes.
Highlights
We sought to determine the effects of iron depletion on photosynthetic physiology and allocations to the major thylakoid complexes in the model fresh water cyanobacteria Synechococcus elongatus PCC 7942 and Synechocystis sp
To adjust electron transport to these downstream constraints there is often a decrease in Photosystem II (PSII) complexes under iron scarcity [6,7]; this serves to lower the rate of electrons being introduced into the photosynthetic electron transport chain [5] so that PSI turnover does not limit PSII electron flow, thereby limiting production of dangerous oxygen radicals [7]
During iron depletion cyanobacteria accumulate an alternate chlorophyll binding complex, the iron stress induced protein (IsiA), or CP439 [6], which can attach to PSI and increase its effective absorption cross section [8], partially offsetting the effects of having fewer PSI complexes [8], and/or storing chlorophyll in a down-regulated state with safe dissipation of excitation [9,10]
Summary
We sought to determine the effects of iron depletion on photosynthetic physiology and allocations to the major thylakoid complexes in the model fresh water cyanobacteria Synechococcus elongatus PCC 7942 (hereafter Synechococcus) and Synechocystis sp. In parallel we used chlorophyll fluorescence measurements to track changes in PSII function, the functional absorbance cross section serving PSII photochemistry, and changes in electron transport during iron depletion.
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