Effect of nutrient kinetics and cytoarchitecture on bacterioplankter size

Expanded specific affinity theory specifies the advantages of small dry mass and dilute cytoarchitecture in impoverished systems. For marine samples, bacterioplankton mean dry mass, according to flow cytometry, was near 24.5 fg cell(-1). Conversion to volume with buoyant density gave a cell volume of 0.124 mu m(3). Total DNA was 2.9 fg cell(-1). This compared with the size of a single genome of a small extinction-culture isolate, Sphingomonas sp. RB2256, of 3.96 fg or 3.6 Mb. The genome size of such isolates and other cultures decreased with metabolic simplicity. It was found that bacterioplankton could be exposed to radiolabeled amino acids and then sorted for size and that the specific affinities of the fraction of small organisms were as great as the fraction of large cells. Size, DNA, and metabolic-complexity distributions were concordant with the concepts that cell volume approaches a minimum set by sufficient space for the smallest genome that can provide sufficient information for competitive dissolved-nutrient acquisition, and that space requirements are further alleviated by the expression of few cytoplasmic-enzyme molecules in each of the various pathways and a dilute cytoplasm. Bacterioplankton approached a minimum genome size of 1.7 Mb with a minimum cell volume of about 0.06 mu m(3) and a DNA content of 16% dry weight. The property of small dry mass with a low DNA content was common in in situ bacteria but absent from cultivated representatives, which led to speculation that failure to grow in the laboratory is related to missing regulatory information.