Induced Nutrient Assimilation in Oligotrophs: Not Beneficial, Not Possible, Both or Neither?

In the oligotrophic ocean bacterioplankton exploit ephemeral patches of dissolved organic matter (DOM) before they disappear. Previous studies of SAR11 alphaproteobacteria indicated these abundant cells often do not regulate expression of DOM uptake and oxidation functions, in contrast to common paradigms for the regulation of catabolic operons in heterotrophic bacteria. We postulated that the unregulated expression of DOM uptake functions could be an optimal cost vs. benefit fitness strategy for small cells lacking motility when nutrient concentrations fluctuate rapidly around low, ambient median values. To quantify the effect of regulatory mechanisms on the uptake and oxidation of organic nutrients, we compared the obligate oligotroph Cand. Pelagibacter sp. HTCC7211 and the copiotroph Alteromonas macleodii HOT1A3, measuring the uptake and oxidation of ¹⁴C-L-alanine by these cells in culture. Initial experiments confirmed previous studies: uptake and oxidation of alanine by the SAR11 cells was immediate and continued at constant rates, while HOT1A3 had an immediate but low basal level of uptake that sharply increased after several minutes. In HOT1A3, oxidation of the compound did not begin until several minutes after addition of the compound. In experiments with a range of alanine concentrations, down to picomolar levels, the behavior of SAR11 cells was unchanged at all concentrations, but alanine uptake and oxidation by HOT1A3 changed as alanine concentrations declined. These data are being used in a model that explores the costs, benefits, and fundamental limits of regulating DOM uptake and oxidation.