Cell size distributions of soil bacterial and archaeal taxa

Cell size is a key ecological trait of soil microorganisms that determines a wide range of life history attributes, including the efficiency of nutrient acquisition. From culture-based work, we know that microbial cell size can vary depending on the taxon in question and the metabolic state or life stage of the cells. However, because of the methodological issues associated with determining cell sizes in situ we have a limited understanding of how cell abundances vary across cell size fractions and whether certain taxonomic groups of soil microorganisms have consistently smaller cells than other groups. Here, we extracted whole cells from three distinct soil types and fractionated them into seven size ranges (5 ??m to 0.2 ??m) by filtration. Cell abundances in each size fraction were determined by direct microscopy with the taxonomic composition of each size fraction determined by high-throughput sequencing of the 16S rRNA gene. We found that most of the soil microbial cells were smaller than those cells typically grown in culture, with 59 ??? 67% of cells <1.2 ??m in diameter. Furthermore, each size fraction harbored distinct bacterial and archaeal communities in each of the three soils, but the size distribution patterns of individual bacterial and archaeal phyla did vary across the soil types. Nevertheless, many of the taxa exhibited distinct size distribution patterns with the smaller size fractions having higher relative abundances of taxa that are rare or poorly characterized (including Acidobacteria, Gemmatimonadetes, Crenarchaeota, Verrucomicrobia, Elusimicrobia, and the candidate divisions AD3, TM6, SC3, TM7, ZB2 and WS3). In general, there was a direct relationship between average cell size and culturability, with those soil taxa that are poorly represented in culture collections tending to be smaller in size. Size fractionation not only provides some insight into the life history strategies of soil microbial taxa, it is also a useful tool to enable more focused investigations into those rare phyla and candidate divisions that remain poorly characterized.