1. Crystal scintillators.
These are crystals which emit visible light when hit by gamma radiation (there are variants which also emit light when hit by neutron radiation). You can have radioactive isotopes on board to generate radiation, and crystals to turn the radiation into light for your photosynthetic bacterial colonies to eat.
This is current tech or near future tech.
2: Generated light solar sail.
This does not involve your bacterial colonies. It is a scheme for a reactionless drive so you do not have to throw mass behind you.
- Your ship has a light sail. But you are in the big dark.
- Behind your solar sail you will generate light using vacuum energy.
The speed of light in a vacuum is constant, according to Einstein's
theory of relativity, but its speed passing through any given material
depends on a property of that substance known as its index of
refraction. By varying a material's index of refraction, researchers
can influence the speed at which both real and virtual photons travel
within it. Lähteenmäki says one can think of this system as being much
like a mirror, and if its thickness changes fast enough, virtual
photons reflecting off it can receive enough energy from the bounce to
turn into real photons. "Imagine you stay in a very dark room and
suddenly the index of refraction of light [of the room] changes,"
Lähteenmäki says. "The room will start to glow."
Your generator steadily alters the refraction index of its generation chamber and by doing produces photons. Those which stream away behind the ship will propel it. Those streaming forward propel it backwards until they hit the solar sail, yielding net neutral propulsion from the forward streaming ones.
Downvoters - stay your hands! This is not a something for nothing scheme. As noted in the linked article, the energy to produce these photons from the void comes from the energy put in to alter the refraction index of the light generation chamber, and you put in more energy than you get out. But you can run your chamber through a cycle of refractive changes and not lose mass off your stern.
Vacuum, schmacuum, you may scoff. What about #1 - crystal scintillators? Cant we just use those for the light sail? We have them all nice and bright for the bacterial colonies? Hm, yes. Well, we want to use them all to grow germs, you know.
Well what about using that swanky light generation chamber to grow the algae? Photons are photons, and that rig is a lot lighter than the thorium we are toting around. Ah, true, true. The engineering folks and the bacterial colony folks don't talk to each other much, you see.