L.: Geographic and phylogenetic variation in bacterial biovolume as revealed by protein and nucleic acid staining

Biovolume is an important characteristic of cells that shapes the contribution of microbes to total biomass and biogeochemical cycling. Most studies of bacterial cell volumes use DAPI (4!,6!-diamidino-2-phenylindole), which stains nucleic acids and therefore only a portion of the cell. We used SYPRO Ruby protein stain combined with fluorescence in situ hybridization to examine biovolumes of bacteria in the total community, as well in phylogenetic subgroups. Protein-based volumes varied more and were consistently larger than DNA-based volumes by 3.3-fold on average. Bacterial cells were ca. 30 % larger in the Arctic Ocean and Antarctic coastal waters than in temperate regimes. We hypothesized that geographic differences in the abundance of specific bacterial groups drove the observed patterns in biovolume. In support of this hypothesis, we found that Gammaproteobacteria and members of the Sphingobacteria-Flavobacteria group were larger in higher-latitude waters and that the mean volumes of both groups were larger than the mean bacterial volume in all environ-ments tested. The mean cell size of SAR11 bacteria was larger than the mean cell size of the total bacterial community on average, although this varied. Protein staining increases the accuracy of biovolume measure-ments and gives insights into how the biomass of marine microbial communities varies over time and space. Size is an important characteristic of microbes that affects cell physiology and trophic interactions. Cell size in combina