Apparently, the radiation emitting from the (very hot) thermal vents extends into the visible range (albeit dimly), and thus the authors hypothesize that the bacteria are using this radiation as an energy source for their photosynthetic reactions, though it's possible the bacteria are also using light from chemiluminescence. Data quoted in a The Scientist summary of the article compares the amount of light coming from the hydrothermal vents to other natural environments:
Photon flux [near hydrothermal vents] at 750 nm, which is what GSB1's [the newly isolated bacterium's] bacteriochlorophylls absorb, is about the same as the solar photon flux available for a green sulfur bacterium found in the Black Sea, Van Dover told The Scientist. That bacterium has been estimated to take two to three years to divide, Beatty said.Unfortunately I don't have access to PNAS online, and thus haven't been able to read the full article, but The Scientist has an (apparently) thorough discussion of the article, and the abstract is publicly available.
Beatty, JT, J Overmann, MT Lince, AK Manske, AS Lang, RE Blankenship, CL Van Dover, TA Martinson, and FG Plumley. 2005. An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent. Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0503674102 (abstract).
[Update: I just discovered that Carl Zimmer has also written a short summary of this paper; he even links this discovery with a proposed mechanism for the evolution of photosynthetic processes in all life. Neat stuff.]