Chlorella grows perfectly well in BG-11 + glucose and a lot of other organic compounds, for example glycine. It is however very difficult to keep fungi out of the cultures.

Light cycle makes no difference. Perhaps strains that live in sewage ponds may produce some compounds which prevent overgrowth by bacteria and fungi.

If you want to get the better biomass yield, theorically you should use continuous light, in autotrophy, CO2 increases the yield but in mixotrophy ? BG11 is very rich in nitrogen but not so in phosphore, the "mixotrophy molecule" for your Chlorella is probably not exclusively glucose, is it the better ? you could test acetat salts per example, as said Raymond J Richie, introducing organic molecules increases the growth of bacteria and fungi, can you work in axeny ?

Claude - you have made some good points. BG-11 actually is very high in both N & P. You would think continuous light would give a higher yield but many green algae do not like continuous light very much and there is little improvement over a 12/12 cycle. Aeration with CO2 or air in mixotrophic cultures just makes it even more difficult to maintain axenicity. If you give Chlorella a carbon source it does not really need CO2 because it will produce a lot of CO2 from the metabolism of the carbon source. Yes - acetate is a good carbon source but a lot of other bugs like it too. For Chlorella the most troublesome contaminants are fungi when trying to grow it mixotrophically. I found semi-continuous culture of mixotrophic Chlorella almost impossible because of fungi.

Thank you Raymond and Claude for your answers. We are currently culturing C. protothecoides in BG11 achieving good growth rates; next, we want to try this strain to treat wstewater (agricultural digestate)

I grow Chl. prot. in a simple organic culture medium: glucose, yeast extract and sea water. My paper about it is on Research gate for details:

Article:A symbiotic gas exchange between bioreactors enhances microalgal biomass and lipid productivities: taking advantage of complementary nutritional modes.

C A Santos, M E Ferreira, T Lopes da Silva, L Gouveia, J M Novais, A Reis