Endosymbiotic theory is a well accepted explanation for how plants acquired chloroplasts (the organelles responsible for photosynthesis). About one and a half billion years ago, primitive single-celled organisms dined on photosynthetic bacteria (cyanobacteria). Rather than being digested, the cyanobacteria became permanent residents within the larger cells.
This is where Cymbomonas, an algae that closely resembles its early ancestors, comes in. With a bit of coaxing, or more specifically by turning the lights out to preclude the use of photosynthesis, the researchers persuaded the algae to consume bacteria to fulfill their energy requirements. Cellular stains and electron microscopy were used to confirm the process.
Caption: This transmission electron micrograph shows bacteria-feeding in the green alga Cymbomonas. In the cross-section features a large vacuole (v) containing bacterial cells. It also shows the tubular duct (d) that transports food into the vacuole. Other structures pictured are plastids/chloroplasts (p), a mitochondrion (m), and Golgi bodies (g). The scale bar represents 2 micrometers.
Credit: AMNH/E. Kim
And a clip of Kim explaining the results:
This still doesn't prove that chloroplasts originated as independent bacteria, but it does show that the kind of ingestion and capture required for that sort of event is possible. When added to all the other evidence, it makes the endosymbiotic theory all but a certainty.
Maruyama, S., & Kim, E. (2013). A Modern Descendant of Early Green Algal Phagotrophs Current Biology DOI: 10.1016/j.cub.2013.04.063.