I agree with the former answers that usually you get increased pH as CO2/HCO3- is taken up faster than it is replaced from the air. HCO3- -> (CO2) + OH-

If you somehow managed to have a very good aeration so CO2 stays close to equilibrium this effect will not appear. I suggest two mechanisms that might explain your finding:

Alga need other nutrients from the water which for charge reasons are exchanged with protons. In land plants (which doesn’t take up CO2 from the soil water) it is well known that plant growth acidify soils:

NH4+ -> (NH3) + H+

K+ + H2O -> (K+, OH-) + H+

Ca2+ + 2H2O -> (Ca2+, 2OH-) + 2H+

Fe2+ + 2H2O -> (Fe2+, 2OH-) + 2H+

Another possibility is that the alga excretes acidic organic products like carboxylic acids. Alga leaks significant amounts of dissolved organic matter during their growth.

What type of phytoplankton are you culturing?  If they happen to be coccolithophores, the pH can drop due to calcification, which releases CO2.  Some coccolithophores, such as Emiliania huxleyi, show a stronger signal from calcification than from photosynthesis, so there is a net drop in pH as the culture grows denser.  If your phytoplankton are not calcifying, then I don't have a good answer for you.  

Hi Rajan

in my opinion， I would like to think about the following things:1) when did you measure the pH in a 24 h period, the dark period or light period; 2) which grown phase was measured , 3) how long did you measure in your incubation. 4) did the cells grow well?

I would expect pH to increase when the culture is growing because the use of CO2 by algae. Please, provide further details.

Would be of some help:

http://www.int-res.com/articles/meps/109/m109p083

Please check  with the microbial load (except your subject phytoplankton). They are generally responsible for increase in CO2 conc. in the cultures. Give proper light exposure to your cultures. Unless there is a large amount of phytoplankton biomass growing & taking up the CO2 from the medium, there would be a likely decline in the pH condition.

Hi Rajan Babu, how are you? Nice to meet you on Research Gate.

Basically molecular CO2 (g) get dissolved in water as bicarbonate form HCO3-(90%) and a very few percentage (0,2 - 1%) of carbonic acids (H2CO3).   CO2 binds with OH- of H2O and form HCO3-. The excess H+ ion of the same H2O binds with another molecule of H2O and form H3O+ which basically increases ready to donate H+ ions within the culture media. The excess H+ ions also can bind with HCO3- and form H2CO3, but the reaction rate is very slow. So, there are always excess H3O+  as phytoplankton intake  the dissolved CO2 (HCO3-) from the media very quickly, So, with the increase of phytoplankton growth/biomass, the rate of photosynthesis as well as concentration of H+ (H3O+) within culture media also increase, which means pH decreases.