Effect of pH on life? Use buffer or HCl? How about lactic acid?
That is a variable that you have the pleasure defining yourself. Just know that using a buffer will introduce other atoms that could affect the "living things". Conversely, if you don't use a buffer, it is likely that the pH of your "living things" will be easily altered by the metabolism of your living things (IMHO, this is worse).

I don't know the details of your project, but recommend:

-use a buffer like.. Tris, phosphate...
-use 10 mM Tris-Cl without any extra electrolyte (aside of the HCl or NaOH that you use to modulate the pH of your buffer to different pH's that you want to test)
-e.g. Tris-Cl pH 7, 8, 9.. or Phosphate pH 2,3,4,5,6,7,8,9..