Why the pH value decreased seriously in my algal culture?

Hi Is there anyone who would be so kind as to give me some idea to
explain this phenomenon?

I cultured several flasks of one green microalgae species in TAP
medium under irradiance of 90 umol.m-2.s-1,
I added 18g/L glucose in the medium, and after 9days of cutlure, I
found the aglal culture in bad growth condition though the biomass
accumulated was much higher than the control culture without glucose,
the color of the algae showed a tendency of turning white!! I
collected the cells and tested the pH value of the culture medium, and
it was below pH4.5!! I think most of the algal cells must have died!
The pH value in the control culture without glucose was 8.0. I checked
the culture under microscope and no bacteria was found.

I think the whole thing is very confusing

The nitrogen source in TAP medium is NH4Cl, which should bring the pH
down upon algal growth. So this could be a possible reason for the pH
decrease in the above culture. But in another culture which also
contained 18g/L glucose and under a much stronger irradiance of 800
umol.m-2.s-1. , the pH was 7.0 and with even more biomass
accumulated!!! If NH4Cl is the criminal, then why the culture under
stronger irradiance which should have taken in much more NH4Cl to
produce biomass did not give a even lower pH??

Is it possible that the CO2 produced by respiration can cause a pH as
acid as below 4.5?


The following is TAP recipe:


Tris - Acetate - Phosphate (TAP) Medium

(modified from Durnford Lab)


For One Liter of Medium:


2X Filner’s Beijernicks Solution 25ml

1M Potassium Phosphate 1ml

Trace mineral solution or 5ml

P IV solution 2X (see SVM) 3ml

Tris-Base 2.42g

Glacial Acetic Acid (17.4 mM acetate) 1ml

(pH should be 7.2)

Stock solutions:


2X Filner’s Beijernicks Solution (500 ml)

NH4Cl 8g

CaCl2 * 2H2O 1g

MgSO4 *7H2O 2g


Trace Mineral Solution (500 ml)

5 g disodium EDTA – dissolve in 400 ml water by heating and stirring

Neutralize to pH 6.5 with 5N NaoH

Add each of the following in order. Allow each to dissolve completely
before adding the next.

0.5 g FeSO4*7H2O

2.2 g ZnSO4*7H2O

1.14 g H3BO3

0.51 g MnCl2*4H2O

0.016 g CuSO4*5H2O

0.073 g Na2MoO4*2H2O

0.016 g CoCl2*6H2O


1M Potassium Phosphate Stock (50 ml)

add: 20 ml 1M stock KH2PO4 (1M stock: 6.8g/50ml)

30 ml 1M stock K2HPO4 (1M stock: 8.7g/50ml)

pH 7.2 at RT