Dear Ilhem,

this method isn't connected with evaluation of cell size, but with recalculation of biomass.Moreover if you want to use it should separate all fractions, e.g., bacteria from pjytoplankton, from ciliates, flagellates, zooplankton etc., that is hardly possible. if you are working with energy budgets or biomass calculations of different taxa of planktonic or benthic invertebrates, protists, phytoplankton or phytobenthos (try to select separate group of phytobenthos) you should use cell-model calculations. For plankton bacteria in some cases may be could use the approach that you are talking about  (look attached paper of Direct Determination of Carbon and Nitrogen Contents of  RUMI FUKUDA et al. Natural Bacterial Assemblages in Marine Environments // APPLIED ANDENVIRONMENTALMICROBIOLOGY, Sept. 1998, p. 3352–3358.

About methods to determine volume cells you can read in my site the book "From energy budget of small hydrobionts to the flow of energy through hydroecosystem" with translated Chapter into English - An individual mass of small aquatic organisms. URL is https://www.researchgate.net/publication/301956530_An_individual_mass_of_small_aquatic_organisms

Andrey

Chapter An individual mass of small aquatic organisms.