Respiratory Kinetics of Marine Bacteria Exposed to Decreasing Oxygen Concentrations

Abstract

ABSTRACT During aerobic respiration, microorganisms consume oxygen (O 2 ) through the use of different types of terminal oxidases which have a wide range of affinities for O 2 . The K m values for O 2 of these enzymes have been determined to be in the range of 3 to 200 nmol liter −1 . In this study, we examined the time course of development of aerobic respiratory kinetics of four marine bacterial species ( Dinoroseobacter shibae , Roseobacter denitrificans , Idiomarina loihiensis , and Marinobacter daepoensis ) during exposure to decreasing O 2 concentrations. The genomes of all four species have genes for both high-affinity and low-affinity terminal oxidases. The respiration rate of the bacteria was measured by the use of extremely sensitive optical trace O 2 sensors (range, 1 to 1,000 nmol liter −1 ). Three of the four isolates exhibited apparent K m values of 30 to 60 nmol liter −1 when exposed to submicromolar O 2 concentrations, but a decrease to values below 10 nmol liter −1 was observed when the respiration rate per cell was lowered and the cell size was decreased due to starvation. The fourth isolate did not reach a low respiration rate per cell during starvation and exhibited apparent K m values of about 20 nmol liter −1 throughout the experiment. The results clearly demonstrate not only that enzyme kinetics may limit O 2 uptake but also that even individual cells may be diffusion limited and that this diffusion limitation is the most pronounced at high respiration rates. A decrease in cell size by starvation, due to limiting organic carbon, and thereby more efficient diffusion uptake may also contribute to lower apparent K m values.