Abstract
Spatial segregation of photosystems in the thylakoid membrane (lateral heterogeneity) observed in plants and in the green algae is usually considered to be absent in photoautotrophs possessing secondary plastids, such as diatoms. Contrary to this assumption, here we show that thylakoid membranes in the chloroplast of a marine diatom, Phaeodactylum tricornutum, contain large areas occupied exclusively by a supercomplex of photosystem I (PSI) and its associated Lhcr antenna. These membrane areas, hundreds of nanometers in size, comprise hundreds of tightly packed PSI-antenna complexes while lacking other components of the photosynthetic electron transport chain. Analyses of the spatial distribution of the PSI-Lhcr complexes have indicated elliptical particles, each 14 × 17 nm in diameter. On larger scales, the red-enhanced illumination exerts a significant effect on the ultrastructure of chloroplasts, creating superstacks of tens of thylakoid membranes.
Highlights
Spatial segregation of photosystems in the thylakoid membrane observed in plants and in the green algae is usually considered to be absent in photoautotrophs possessing secondary plastids, such as diatoms
PSII and its light-harvesting system (LHC) II reside in the grana stacks, whereas photosystem I (PSI)-LHCI complexes along with ATP synthase are sterically excluded from the grana core and localized to the stroma-facing lamellae
Moderate differences in the photosystem localization have been proposed for diatom thylakoids, with the outer lamellae of stacks being enriched in PSI and ATP synthase compared to the inner parts of the stacks[13,14]
Summary
Spatial segregation of photosystems in the thylakoid membrane (lateral heterogeneity) observed in plants and in the green algae is usually considered to be absent in photoautotrophs possessing secondary plastids, such as diatoms Contrary to this assumption, here we show that thylakoid membranes in the chloroplast of a marine diatom, Phaeodactylum tricornutum, contain large areas occupied exclusively by a supercomplex of photosystem I (PSI) and its associated Lhcr antenna. The internal thylakoid membrane system of the plant chloroplast is differentiated into appressed (grana) and unappressed (stroma lamellae) regions, to which photosystems (PSI and PSII) are spatially segregated. This arrangement is termed lateral heterogeneity[4]. Evidence for a certain degree of lateral heterogeneity of distribution of PSI (but not PSII), not in relation to appressed and unappressed membranes, exists for stramenopile groups other that diatoms[15]
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