SUBSTRATE UPTAKE AND UTILIZATION BY A MARINE ULTRAMICROBACTERIUM

A facultatively oligotrophic ultramicrobacterium (strain RB2256) isola ted from an Alaskan fjord by extinction dilution in seawater, was grow n in batch culture and under single- and dual-substrate-limitation of alanine and glucose in a chemostat. The nature of the uptake systems, and the uptake kinetics and utilization patterns of alanine and glucos e were investigated. Glucose uptake was inducible, the system exhibite d a narrow substrate specificity, and part of the uptake system was os motic-shock-sensitive. Half-saturation constants for glucose were betw een 7 and 74 mu M during glucose limitation. The initial step in gluco se metabolism was the synthesis of sugar polymers, even during glucose -limited growth. The alanine uptake system was constitutively expresse d and was binding-protein-dependent. In addition to L-alanine, nine ot her amino acids inhibited accumulation of [C-14]L-alanine, indicating broad substrate specificity of the alanine transporter. Half-saturatio n constants between 1.3 and 1.8 mu M were determined for alanine uptak e during alanine limitation, Simultaneous utilization of glucose and a lanine occurred during substrate-limited growth in the chemostat, and during growth in batch culture at relatively high (mM) substrate conce ntrations. However, the half-saturation constant for alanine transport during dual-substrate-limitation, i.e. in the presence of glucose, in creased almost fivefold. We conclude that mixed substrate utilization is an inherent property of this organism.