Because Photosynthetic bacteria contain unstacked photosynthetic membranes with light harvesting pigments, which work like thylakoids of chloroplast.
The Cyanobacteria are the largest and most diverse group of photosynthetic bacteria previously known as blue green algae . These are true prokayotes having no chloroplast but still perform photosynthesis.
The reason for this is that they have chlorophylls which are dispersed in cytoplasm(not packed in chloroplast like photosynthetic eukaryotes). They carry out oxygenic photosynthesis i.e, they use water as an electron donor and generate oxygen during photosynthesis. Their photosynthetic systems closely resembles to that of eukaryotes:
These bacteria use Phycobilins as accessory pigments.
Photosynthetic pigments and electron transport chain(ETC) components are located in thylakoid membranes linked with particles called Phycobilisomes.
Phycocyanin(blue) pigment is their predominant phycobilin.
in these bacteria is assimilated through calvin cycle.
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