Direct Cell Mass Measurements Expand the Role of Small Microorganisms in Nature

摘要

Microbial biomass is a key parameter needed for the quantification of microbial turnover rates and their contribution to the biogeochemical element cycles. However, estimates of microbial biomass rely on empirically derived mass-to-volume relationships, and large discrepancies exist between the available empirical conversion factors. Here we report a significant nonlinear relationship between carbon mass and cell volume (m(carbon) = 197 x V-0.46; R-2 = 0.95) based on direct cell mass, volume, and elemental composition measurements of 12 prokaryotic species with average volumes between 0.011 and 0.705 mu m(3). The carbon mass density of our measured cells ranged from 250 to 1,800 fg of C mu m(-3) for the measured cell volumes. Compared to other currently used models, our relationship yielded up to 300 higher carbon mass values. A compilation of our and previously published data showed that cells with larger volumes (>0.5 mu m(3)) display a constant (carbon) mass-to-volume ratio, whereas cells with volumes below 0.5 mu m(3) exhibit a nonlinear increase in (carbon) mass density with decreasing volume. Small microorganisms dominate marine and freshwater bacterioplankton as well as soils and marine and terrestrial subsurface. The application of our experimentally determined conversion factors will help to quantify the true contribution of these microorganisms to ecosystem functions and global microbial biomass.