Chlorine photosynthesis on exoplanets

[Matterplay1]

[snip]
While it is very hard on atmospheric ozone, chlorine absorbs light with wavelengths shorter, IIRC, than about 420 nanometers; this would, depending on the abundance of chlorine in the atmosphere, offset the lack of ozone with regard to UV absorption. A redder star, producing less UV than Sol, might not provide a noticeable advantage in this regard, while restricting the range of useful planetary orbits.
The chlorine abundance of an atmosphere with a photosynthetic flora as described above is assumed to be about one percent (by volume); the amount of light blocked by this amount mightn't be very great, though I haven't done the calculations to check this.

Radtech497

You could be right. I've not done the calculation myself, either and frankly don't know how to do it without some research. According to this source even tiny amounts of chlorine in the atmosphere would be too much and the surface would be pitch black. He references an article by Gregory Benford all the way back in a 1971 issue of Natural History. Benford's a pretty well informed individual on such things, to perhaps he is right. Another item for me to check if I can dig up the article itself (looks more and more like I'll be making that trip to the library...). If all this is right then perhaps the old science fiction trope of chlorine-breathing aliens can't be maintained in OA's 'hard SF' setting, despite all the effort I put into making it work. Hmm.