That’s a solid line of inquiry. Plants convert solar energy into chemical bonds through photosynthesis, so their sugars are already a low‑toxicity energy source. The trick is getting from sugar to something usable in a spacecraft engine. One route is fermenting the sugars into ethanol or butanol, then refining them into liquid propellants. But the real efficiency gains come from lignocellulosic biomass—think algae or fast‑growing crops—that can be directly converted to bio‑hydrogen or biogas. Algae are especially promising because they grow in large, closed bioreactors, produce high lipid content, and can be harvested without competing for arable land. The challenge is scale: you need a tight energy loop—feedstock growth, conversion, and waste recycling—while keeping the system’s mass and power budget within launch limits. If you can hit that sweet spot, you’ll get a propulsion fuel that’s both clean and essentially free of toxic byproducts. Any particular plant or algae strain catching your eye?

Chlorella vulgaris is a decent candidate. It does grow quickly, and the lipid extraction step can be tuned for hydrogen yield if you use a catalytic deoxygenation route. The closed‑loop nutrient reuse is a plus—your waste can feed the next batch. Just be mindful of the energy balance: the extraction and conversion steps need to be powered, so you’ll have to ship or generate enough electricity to offset the gains. Also, watch for light attenuation in a dense culture; you’ll need an efficient light‑delivery system to keep photosynthetic rates high. If you can iron out those details, it could be a solid, low‑toxicity propellant source.

You’re right the weight of the lighting subsystem is a killer. A rotating fiber‑optic sleeve can spread the beam, but in microgravity the fluids in the guide tend to clump. A thin, spun‑silicon diffuser that’s fixed to the reactor wall works better—just spin the reactor itself to keep the light evenly distributed. For catalytic deoxygenation you want something that won’t degrade in the absence of convection. Platinum or palladium on a high‑surface‑area support, like γ‑alumina, is a tried‑and‑true. Ru on carbon is also good, but it’s a bit more fragile in zero‑g because the bubbles can stick. Keep the catalyst at a low temperature and use a small amount of hydrogen pressure; that’ll make the reaction self‑sustaining once you get the feedstock flowing. Bottom line: lightweight diffusers and a robust metal catalyst will keep the loop tight.