Need to clarify whether you want to quantify microbial population or microbial activity?

@ Hanan, the answer is Yes you can measure. For example, a wavelength of 600nm (A600) is used for measuring bacterial concentration.

@ Hanan, if you want to measure microbial biomass that also you can do. A rapid and inexpensive method to estimate SMB based on the near UV light absorption of certain molecules (nucleic acids/nucleotides) of microbial cells extracted from CHCl3 fumigated soils . The UV absorbance at 280 nm is significantly correlated to SMB measured by CHCl3 method as follows. SMB = 21747 x E280 nm. This is a fast and simple method to determine SMB but uses CHCl3.