The elemental compn. of microorganisms varies widely with respect to the N, S, P, and ash content as well as many trace elements depending on microorganism and growth conditions. As a consequence, the mass of 1 C-mol of biomass can differ considerably from 1 microorganism to another. S is completely volatilized during combustion, so that the calcn. of the mass fraction of O is affected when S is not measured. However, its neglect does not seriously affect the elemental compn. and the degree of redn. of biomass. P is found in ash after combustion at the same degree of redn. as in biomass. Therefore, O bound to it should be included in the elemental formula, so that the degree of redn. of biomass remains unchanged by inclusion of P into elemental formula. No external O is incorporated into biomass during combustion and thus the measured ash fraction is correct. [on SciFinder (R)]