There is a 2014 article by Pagnanelli and colleagues that discusses an adverse interaction of nitrogen and glucose in photoheterotrophic growth of Chlorella at certain C/N ratios. 

Thanks, but I think it is unlikely it is due to those interactions, if at all? It is definitely a pH issue, as the pH drops to 6 and below within 48 hours with a Tris buffer.

Thinking more about your problem, if you are putting in a lot of glucose and not aerating aggressively the pH might drop due to weak buffering of TAP minimal in the presence of rapid respiration of the glucose. This would generate CO2 that converts to carbonic acid. You might want to try stronger aeration/rapid mixing, and/or lower amounts of glucose. TAP doesn't have a lot of buffer capacity. Fine for Chlamydomonas on acetate where respiration isn't that high, but Chlorella on glucose might be pushing it too far. We published a paper in Plant Physiology way back showing that Chlamy on acetate in the light was primarily utilizing the acetate carbon rather than fixing carbon via CO2, even though photosynthesis rates (by O2 evolution) were high. Chlorella could be doing something similar, so a stronger buffer might also be warranted.

I see.  I thought it might push respiration too,  and that causes the acidification of the medium.  I have tried sparging the flask with filtered air using a SS air stone,  at flowrates of around 0.1 vvm,  and stirring using a magnetic flea,  yet the pH drops.  How rigorous of an air supply or mixing are we talking about here? 

Also,  when you say utilising acetate carbon for photosynthesis,  do you mean it uses acetate instead of dissolved co2 for photosynthesis?  How would that impact the pH of the medium,  for tap acetate? And do you know why the pH of tap acetate media increases to 8? If it does use acetate carbon for photosynthesis,  the dissolved co2 in the media won't be used and the pH should not rise.  Furthermore,  tap acetate gown in dark exhibits the same as well,  pH rises to 8, despite no photosynthesis 

Chlorella are pretty tough beasts so you can put a lot of mixing energy in. One other thing to do is be sure you aren't starting with too-dilute cultures. That can exacerbate issues like pH. 

Re the photoheterotrophic work: the cells are not using acetate for photosynthesis. What they are doing is using ATP and reductant generated by PSI/PSII to drive uptake and incorporation of acetate into acetyl Co-A instead of fixing CO2 into sugars. We used stable isotope-labelled acetate and CO2 to track this. Essentially, if there was acetate available the cells preferred to use that for biosynthesis rather than fix CO2. But, the energy derived from light capture was necessary for rapid assimilation of the acetate. https://www.researchgate.net/publication/275325056_Effects_of_Acetate_on_Facultative_Autotrophy_in_Chlamydomonas_reinhardtii_Assessed_by_Photosynthetic_Measurements_and_Stable_Isotope_Analyses?ev=prf_pub

I don't recall offhand what the pH status was in those experiments. The cells grew fine however. But, Chlamydomonas has an active bicarbonate pump that may also help to control medium CO2 by taking up inorganic carbon even if the carbon is not being fixed into sugars.

Article Effects of Acetate on Facultative Autotrophy in Chlamydomona...

I see, thanks a lot for this!

Inoculum is 10% v/v for all my cultures, is that too dilute?

I guess I will try again with increased agitation and esp aeration - would large bubbles be better for stripping CO2, or does the bubble size not matter? Last time I used a air stone, which produces very fine bubbles, although the flow rate I employed was very low (< 0.1 vvm).

The hexose transporter protein in Chlorella also exhanges glucose for protons, which I think might be an issue too, but can not say if that is the main problem, or if excess respiration and formation of carbonic acid was the major culprit here.

10% inoculum should be OK, but I think the low buffering capacity of TAP might be the bigger issue for you. This medium was originally developed for growth of Chlamydomonas on acetate and was not designed for glucose-utilizing photoheterotrophs.