Water, sunlight, carbon sources, phosphorous sources, nitrogen sources. And any additional think they have enzymes to break down and process. Advanced organisms could feed off complex organic molecules, but this isn't necessary.
The point is that with reverse chiral sugars, polymers, etc., they wouldn't be a meal for anything in the wild. (At least not one with nutritional value.) No predation and no competition leads to massive expansion, and they could exhaust resources that other necessary parts of the food web require.
Oh, I get it now. And then when they expire they deposit undigestable material in the soil, capturing carbon and eventually turning into coal. The dawn of the dextro-carboniferous period.
This would make a fantastic "phase one" alien bioweapon to aid in wiping out a planet's existing lifeforms and replace ecosystems with one better matching their own. Assuming post-biology aliens care about biology.
